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Username Post: 1999 4L60E hard 1-2 shift by PCM command for unknown reason
letsbangout
Contributor
Posts 148
12-19-11 04:14 AM - Post#2170215    

(this is going to be a very long story) In my 1999 Yukon, I've had a hard 1-to-2 shift occur intermittently for the entire 13 months I've owned it. I've already posted once to this forum with a similarly-related question about the engine mounts being a possible source of this problem, but that was pretty much put to bed and I'd like to confront it directly this time. I'm aware of the valve body / P1870 issues, but I don't believe they apply because of the extremely intermittent nature of this. In my mind, a problem with the valve body or the plate should cause a regular occurrence of the problem; for me it's rather irregular and hard shifts happen very infrequently (but keep reading and i'll describe their frequency better). It should be noted, however, that I've not dropped and opened the tranny because I have nowhere to do it (and I'm thinking and hoping this is PCM-related too, you'll see why in a minute). The one thing I can tell you is that it loves to shift hard when it's cold. When I first start out from cold (relative to weather in South FL of course), it loves to shift hard. Seldom does it ever start shifting hard when it's warmed up and been run previously during the day.

When I first bought the truck 13 months ago, it used to shift hard as hell (but has gotten better). I ran the AC most of the time for the first 3 weeks until the compressor blew up (the AC cycling switched was bypassed by the previous owner, doh!). After not running with AC the problem got slightly better. By the time I'd replaced the MAP sensor because it had codes for it a couple of times, the problem was much better afterwards - the hard shifts happened maybe 4 or 5 times a week, driving perhaps 5 drive-cycles per day on average, in mostly city conditions. The other intermittent code I'd get was P0141, the heater circuit on one of the post-cat O2 sensors. I replaced the O2 sensors with a new pair of Densos and the problem seemed to be slightly better (and of course I haven't had any codes ever since). At that point my problem happened on probably 4 or 5 shifts in 3 weeks time. Those are the only two codes I've ever seen on the computer - the MAP sensor and the bank 1 O2 sensor 2 heating circuit. That was maybe a month ago when I replaced the O2 sensors.

But now it's back with a vengeance! The culprit? It seems to be the PCM doing this whenever I run my AC. I just dropped in a new Delco compressor, new hoses, o-tube, accumulator and new cycling switch. Whenever the compressor is activated via the AC controls, it shifts so hard you'd swear the next shift is gonna destroy second gear. It happens like clockwork.. I've done like 7 or 8 drive cycles with the AC on, and just about every single shift is hard as hell. So whereas before it was 4 or 5 hard shifts in 3 weeks, I've probably had 50 of them in only a couple of days. It's apparent to me that the PCM is doing this. But why?

A little more background on my truck, according to the mechanic I bought it from: It was used regularly for the first 4 years of its life, seemingly by a corporation who leased it, based on what I can infer by the Carfax. It was sold at 50k miles to a South FL woman who drove it for a few thousand miles and then blew the engine because it overheated. It sat for a good long while as she saved money for a new engine. When she replaced the engine some months (or years?) later, she blew a head gasket because it overheated again. She took it to the mechanic I bought it from, I guess it was more money than she was willing to spend, so he bought it from her as-is and turned it into his baby for pulling his boat. He fixed the overheating thing by dropping in a new radiator because they hadn't replaced it the first time. I bought it in late 2010 with 70k original miles, and it's a 99. Obviously it sat for a long long time, as evidenced by the paint fade and rust on the top, which was present when I bought it.

So I could have loads of problems involving original seals or sensors, on account of it sitting so long. Initially I was skeptical of the MAF sensor but my el-cheapo scan tool says it's detecting the right amount of airflow at idle, when converted from lbs/min to grams/min or whatever it reads, and compared to what Alldata says (I think 5 to 7 grams at idle). The MAP sensor is a brand new Delco as of like 2 months ago. Post-cat O2 sensors are new. Fuel trims are normal looking, usually 0 to 10% lean from what I remember. Load readings on the engine also look normal, as does the throttle position reading, though I've not given it another look ever since doing the AC job last week.

But given the fact that I don't have a GM Tech II, what should I be looking for? Maybe a vacuum leak? And where? Are there any other sensors I should be looking at? Is it possible the TPS is to blame, even though it doesn't throw codes? I've just got to believe something is out of whack with regard to the PCM. The only other thing I would know to do is buy ScanXL from Palmer Performance (you can read about it on http://scantool.net). It claims to be able to read things and log/graph them with an OBD-II interface that are proprietary to GM vehicles if you have the GM add-on, but it remains to be seen how many of those parameters are actually implemented on OBS Tahoes/Yukons, I've heard for example that it's not possible to get ABS parameters from OBS trucks, so buying ScanXL could be a waste. And I sure would like to solve this beforehand, and buy all that hardware/software later on as a maintenance investment.

Any and all advice is appreciated. Thanks!
355Cheyenne
Senior Member
Posts 3762
355Cheyenne
12-19-11 08:46 AM - Post#2170281    

"Is it possible the TPS is to blame, even though it doesn't throw codes? I've just got to believe something is out of whack with regard to the PCM. The only other thing I would know to do is buy ScanXL from Palmer Performance"

Yes it could be the TPS, not that it is but it could be and you will almost never see a TPS error code. This is because there is no data to test it against. If there was a dead range how would the ECU not know that you just didn't adjust the throttle?

There are two ways to test this, either by probing and watching the voltage slowly increase while you slowly open the throttle. from .5v to 4.5v ish give or take .5v. Or use a scan tool. I shouldn't recommend this but that tool you suggested scanxl, there are chineese knock offs online for $15. I did this personally and it works fine. I have a shift intermittent shift problem from 2~3 and thought it was a TPS because it seemed to only happen under certain throttle positions. I used the scanXL knock off to watch the TPS readout while I slowly went from idle to WOT. perfect response with no dips or drops. The more I looked into it the more I came to accept it likely has more to do with my tuning and the wrong tire ratio. In my case it isn't bad at all just was concerning I may have other issues. I am also wondering if my transmission wasn't replaced or serviced with some kind of shift kit at some point in its life because the shifts are a lot firmer then any other GM I have ever driven.

I would definitely start with the TPS. The A/C exacerbating the issue is likely because the ecu calls for additional power when the A/C is being run and maybe is making the shift harder with the additional torque?

Other than that I could see how a gummed up valve body could causing intermittent hard shift. It wouldn't be a bad idea to drop the trans pan, change the filter and inspect the valve body of obvious signs of grime. You could achieve this with some simple ride up ramps and a creeper.
98 Silverado k1500 4x4 350

letsbangout
Contributor
Posts 148
12-19-11 09:54 AM - Post#2170288    

I've looked at the TPS plenty through my scan tool, it appears normal. That would solve for your contention that there could be dead ranges in the TPS. However, what that might not show me is when there are conditions of abnormally high or low resistances (assuming they're smoothed or averaged somehow by the PCM, which is very possible). Potentiometers (a variable resistor, same thing as the TPS) can easily develop abnormal highs/lows in the range of movement - that's precisely what I would expect to throw a code - you see voltage jump from 4 volts to 0.1 in an instant - that kind of thing. Think of an audio amplifier who's gain control is sometimes unresponsive and you have to move it a little further up or down to get the correct amount of gain from it.. same thing. To test that I'd need to hook up an analog ohm meter directly to the TPS, which I have actually, just need to replace the battery. The other thing I have to figure out is how to connect probes to the TPS, given that the connector is proprietary and not easy to get to. I bought a set of alligator clips from Radio Shack which might do the trick. I just want to avoid backprobing the wires at all costs. I'll update the thread to let you guys know how that goes.

The other thing I'm looking at again is the MAP sensor. I hooked up my scan tool this morning because it started shifting hard this morning again without the AC on. I saw wild variations in the readout for barometric pressure, ranging from about 8.6 to 14.5. The 14.5 was while it was under load.. parked and revving the engine every so often yields readings from 8 to 10. Given that an increase from 8 to 10 is such a wild variation (25%), and considering that the engine is unloaded, I'm wondering why the barometric pressure seen by the PCM would change so much. I'm thinking if it did have a 25+ percent jump with an adaptable shift, that could account for a big adjustment to fuel parameters and shift pressure instantaneously.

I should also add that Alldata didn't say anything about the barometric pressure readouts, and whether they should change with RPMs. I seem to recall reading from somewhere that 8 is normal when you're close to sea-level, as I am. But it seems to me that the barometric pressure should not be changing much; that readout is designed to tell the PCM what altitude it's at, and should not vary with engine load, right??

Regarding what you said about dropping the pan, yea I could do that somehow. I haven't ruled out the possibility that I've got a transmission problem along with something else PCM-based at the same time. But the latest thing with these nail-biting shifts after fixing the AC causes me to think the problem is mostly or entirely PCM-based. I'm determined to go all over the engine if I have to, because continuing to run the AC and having the hard shifts is going to wreck my tranny real soon. Prior to fixing the AC it was usually shifting suave and acceptable, now it's nerve wracking.
355Cheyenne
Senior Member
Posts 3762
355Cheyenne
12-19-11 10:32 AM - Post#2170297    

However, what that might not show me is when there are conditions of abnormally high or low resistances (assuming they're smoothed or averaged somehow by the PCM, which is very possible). Potentiometers (a variable resistor, same thing as the TPS) can easily develop abnormal highs/lows in the range of movement - that's precisely what I would expect to throw a code - you see voltage jump from 4 volts to 0.1 in an instant - that kind of thing.

And what I am telling you is that this can happen constantly and it will not trigger a code. I have seen this many times and never seen a code develop as a result. I used to write custom tunes for my old 88 and I know for a fact that the only traps in 88 to 95 for a fault is a full closed or full open circuit. I suspect 96 to 98~99 are likely no different.


The other thing I'm looking at again is the MAP sensor. I hooked up my scan tool this morning because it started shifting hard this morning again without the AC on. I saw wild variations in the readout for barometric pressure, ranging from about 8.6 to 14.5. The 14.5 was while it was under load.. parked and revving the engine every so often yields readings from 8 to 10. Given that an increase from 8 to 10 is such a wild variation (25%), and considering that the engine is unloaded, I'm wondering why the barometric pressure seen by the PCM would change so much. I'm thinking if it did have a 25+ percent jump with an adaptable shift, that could account for a big adjustment to fuel parameters and shift pressure instantaneously.


If you goose the motor you will see a significant change in the map reading. The vacuum changes greatly when you goose the throttle under no load. What I do to test the MAP is connect the scan tool and check the MAP reading with the engine off and then go online and compare that to the actual barometric readings for my area. If the are very close of match then the next thing I do it check it at idle. I fail to recall the exact expected idle reading but I think it should be 16 to 20 inches at idle. when you open the throttle you should expect to see this drop and when you close the idle at high rpm you should see this drastically increase. Basically what you described sounds norm.


I should also add that Alldata didn't say anything about the barometric pressure readouts, and whether they should change with RPMs. I seem to recall reading from somewhere that 8 is normal when you're close to sea-level, as I am. But it seems to me that the barometric pressure should not be changing much; that readout is designed to tell the PCM what altitude it's at, and should not vary with engine load, right??

Wrong, MAP sensor is intended to provide an analog reading of engine load by watching the engine vacuum. The map should have a vacuum like coming off it that goes to a manifold vacuum source, I believe in the 96 to 98~99 the MAP bolts directly to the intake and gets its vacuum reading right off the intake w/out a hose, unlike the 88 to 95. The PCM adjust trim for changes in air density by changes in elevation etc. by watching the o2 sensor readings, not the map.

I haven't ruled out the possibility that I've got a transmission problem along with something else PCM-based at the same time. But the latest thing with these nail-biting shifts after fixing the AC causes me to think the problem is mostly or entirely PCM-based.

Highly doubtful this is a PCM failure. Everything you are describing tells me the PCM is working fine. Very often people jump to the PCM as fault and stay on it as the problem and end up spending money to make the problem worse. PCM failures are usually much more obvious and exhibit a more serious run failure. Your only issue is hard shifts, not likely a pcm is going to cause a transmission to shift hard. More likely a transmission with a problem would cause a hard shift problem. Out of likely hundreds of encounters with people proclaiming an issue with a PCM, only once can I recall that it was a PCM fault. It was a dodge ram and the PCM was fine, the problem was that some one thought the PCM was bad long ago and swapped out a correct good PCM for a v8 with one for a older body v6, gave up and sold it. We put the correct PCM back in and found a bad TPS and called it good.

EDIT: a good resource for information on MAP sensors
98 Silverado k1500 4x4 350

someotherguy
Moderator
Posts 25155
someotherguy
12-19-11 11:07 AM - Post#2170311    

bangout - BARO and MAP are two different sets of data, not to be confused. BARO is recorded as a baseline from the MAP when the key is turned on, BEFORE the engine starts. Then readings from the MAP are compared to it afterwards so it has a relative idea of MAP vs. actual barometric pressure. The early TBI years are a little lacking in how they deal with this but I think as early as the 93-up style PCM used w/the electronic transmissions is when you see this used.

A healthy stock engine should see 18~20" of vacuum when running at idle. Any time you open the throttle plate whether the engine is under load or not, you will see an instant drop in that number. This is normal.

I'm with 355 on the TPS; it can be faulty, have spikes/dead spots throughout the range and easily not set a code. The only truly effective way to "see" this is checking it with a connector breakout kit so you can use a good ANALOG meter to watch throughout the sweep of the TPS. A digital meter, and absolutely a scan tool, are not fast enough to read these flaws.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

letsbangout
Contributor
Posts 148
12-19-11 11:07 AM - Post#2170312    

Thanks for the replies, they're very insightful.

The codes I was thinking of are P0122 and P0123 for low or high voltage readings from the TPS. P0121 looks like it'd be the open-circuit code triggered for a complete failure. But like you said, who knows whether 0122 or 0123 would ever be set. Surely there is some bit of averaging going on in the computer to "smooth out" the response, even if it's only 50 msec worth of data, so the PCM might not be able to trap those events in practicality if they're only happening for a split second.

Regarding the MAP sensor, what I'm failing to understand is why the Hg barometric pressure reading changes with throttle/load. I understand fully that the MAP sensor is an analog readout of vacuum. But regardless of the vacuum, that doesn't in any way change the outside barometric forces that are acting on the engine. Which part am I failing to understand?

As for what's at fault, PCM vs Tranny, yes I know it's much easier and self-serving to simply blame the PCM. Changing sensors is a lot easier than repairing a transmission. I've read a couple of cases on forums where people have had the same shifting problem, and in one case the guy had a clogged exhaust and replaced all exhaust components and the O2 sensors to fully resolve his problem. The other case involved changing a MAF sensor. So while I don't have a heck of a lot to stand on, there is at least that much that I'm aware of. I'll be revisiting the O2 sensors also, I've been seeing a low voltage reading on the same one that threw the code (bank 1 sensor 2 - it looks like that particular sensor is very problematic on these trucks). It's quite whacky in fact.. this morning it was showing 0.9 volts while the PCM was in open-loop, and when it switched to closed loop, 0.065ish volts. I've seen it as low as 0.04 in the past, and that sensor has generally always been lower since I've started paying attention to it. From the looks of things it might have a wiring problem causing the +12v supply line to be low. I didn't bother checking the circuits initially, so that's next up on my to-do list. Or it could be a clogged cat; I know mufflers get clogged, but do the cats?

Can anyone tell me the practical effect of the post-cat O2 sensors on the way the PCM changes the fuel programming?
someotherguy
Moderator
Posts 25155
someotherguy
12-19-11 11:09 AM - Post#2170315    

The cheapo scan tool may not be smart enough to differentiate between BARO and MAP, dunno; haven't had experience with a bunch of different ones.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

letsbangout
Contributor
Posts 148
12-19-11 11:11 AM - Post#2170317    

Yep, a test with my analog voltmeter is at the top of my priorities list. In the event that I can't develop a good way to directly probe the TPS out-of-circuit, do you happen to have a part number or resource for one of the proprietary TPS connectors? Can they be bought in an auto parts store or is it more of a special-order item? I've seen something similar online but don't have a URL handy.

Thanks!
someotherguy
Moderator
Posts 25155
someotherguy
12-19-11 11:14 AM - Post#2170318    

It's easy to find the harness end of the connector - they're about $15~$20 at most parts stores. The problem of course is the sensor socket end. Dealer techs have breakout kits that let them jumper in their test leads, I don't know how hard these are to find but I'm guessing as a specialty tool they are probably not cost-effective for the DIY'er.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

letsbangout
Contributor
Posts 148
12-19-11 11:23 AM - Post#2170322    

Oh that's easy I think you're talking about making an interruptable in-line adapter to take voltage readings to/from the TPS. Probably unnecessary. A simple analog resistance reading across the TPS terminals, assuming my voltmeter is ranged correctly, should be all that's needed. If it isn't ranged correctly that's easily correctable with inline resistance. The only thing that test methodology wouldn't reveal is the possibility of voltage stability problems out of the PCM, which is possible I guess, you could always have an odd harness problem, though I think it would probably manifest itself as an open circuit and throw a code.

I'll get the harness connector if I can't probe it with alligator clips and let you guys know how it goes. Thanks for all of the advice!

someotherguy
Moderator
Posts 25155
someotherguy
12-19-11 11:27 AM - Post#2170323    

Yeah; the idea behind breaking into the circuit is to watch actual voltages on the circuit so you know 100% what's going on. For your purposes I think your plan is likely good.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

nozel
Member
Posts 109
12-19-11 02:00 PM - Post#2170357    

Didn't the 4L60E's of that vintage have trans valve body wear issues where system pressure was higher due to leakage? This would show with hard 1-2 shifts and probably a p1870 or similar code.
Any slippage when in OD (4th gear and lockup tq converter?)
Scan tool probably only way to monitor system. Open or short circuits would show as hard fault code set.
Just thinking...as had some thing similar with my old '93 which was an OBDI system but no codes when it was slipping in OD lockup.
someotherguy
Moderator
Posts 25155
someotherguy
12-19-11 02:04 PM - Post#2170359    

nozel - yep; this has already been discussed in another thread of his over in the 99-up forum. http://www.chevytalk.org/fusionbb/showtopic.php?ti...

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

letsbangout
Contributor
Posts 148
12-20-11 06:36 AM - Post#2170540    

I haven't done anything further (yet) to advance this, except pull off the TPS connector and determined that I really do need a harness connector to test it because the connector is just too small to try probing it. Right now I'm just waiting on the parts stores to open. But I want to get back to something that was previously discussed in the mean time:

"Other than that I could see how a gummed up valve body could causing intermittent hard shift. It wouldn't be a bad idea to drop the trans pan, change the filter and inspect the valve body of obvious signs of grime. You could achieve this with some simple ride up ramps and a creeper.

My research (over the course of the last year) has indicated that hard shifts can be caused by the following:

1) A sticky or incorrectly functioning 1-2 accumulator piston
2) Worn passages in the valve body causing the pumping of excessive fluid quantities
3) A worn or damaged valve body plate caused presumably by bad manufacturing in the original run, or by the #8 check ball

I'm now wondering two things:

1) Given the way you said it, are you speculating that gummy fluid in the valve body could cause this, or was that based on actual observation?
2) I'm going to take your advice and drop the pan, possibly replacing the valve body, the plate, and the 1-2 accumulator piston (if I can get to it after dropping the valve body and the plate and without yanking the entire transmission). Can you (or anyone) recommend a good book on transmission rebuilding? Maybe an ATRA or ATSG book of some kind? I'd like to have an idea of what I'm doing, and general best-practices (kind of cleaning fluids, what not to do, etc) as I've never done any kind of work inside of a transmission.

Thanks again.
letsbangout
Contributor
Posts 148
12-20-11 06:57 AM - Post#2170545    

"A healthy stock engine should see 18~20" of vacuum when running at idle."

Can you expound upon this a little - are you saying the scan tool should say 18 to 20" Hg on the live MAP reading? Or is that something completely different?

I'm still having a hard time understanding the exact significance of what comes out of the scan tool, despite having read plenty in Alldata and my Haynes manual.
someotherguy
Moderator
Posts 25155
someotherguy
12-20-11 08:30 AM - Post#2170565    

I mean an actual reading from a physical vacuum gauge. Depending on the scan tool and the piece of data being looked at, you might be seeing a number resulting from MAP minus BARO instead of the literal amount of vacuum the engine is making.

So for example if barometric pressure is 8" at your elevation, and your engine produces 18" of vacuum, it's likely that you may see -10 as the MAP reading while the engine is idling - the amount of difference in the MAP reading from BARO. That kind of thing will really send you on a goose chase if you're expecting to see the actual vacuum reading of expected 18~20". It's also a number that is useless to you unless you do know what BARO is, and your scan tool probably shows you that.

I just woke up so I may have explained that very poorly; mostly what I am saying is when we talk about vacuum numbers I mean as read by a physical gauge, not the MAP reading on the scan tool.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

355Cheyenne
Senior Member
Posts 3762
355Cheyenne
12-20-11 09:56 AM - Post#2170578    

  • someotherguy Said:
mostly what I am saying is when we talk about vacuum numbers I mean as read by a physical gauge, not the MAP reading on the scan tool.




Agreed with richard although the reading from a scan tool could provide a converted value that is Hg and that could be compared to an analog vacuum gauge. The map sensor is nothing more than an electronic vacuum sensor, the ECU uses it to read the engine vacuum to determine engine load. This information can be converted to real world numbers that could be compared to a vacuum gauge to confirm accuracy.

As far as the valve body, I will admit that my biggest limitation is in transmissions. I don't have a lot of extensive knowledge working on transmissions like I do motors so I guess I am not saying definitively anything. Your list of probable causes are correct and they all can be confirmed by dropping the trans pan and inspecting and that is what I am suggesting. I was thinking along the lines of finding a sticky valve but you get the point. I have dropped a few pans in my days and IMHO not that big a deal so it seems like the thing to do right now.


With everything you have told us, it all points to the Transmission, except for the info on the A/C but that doesn't mean it is in anyway directly related. But as in any advice it is your truck and your liability, so do what you feel you need to do and are comfortable with.
98 Silverado k1500 4x4 350

letsbangout
Contributor
Posts 148
12-21-11 03:22 AM - Post#2170849    

Thanks for the replies all. That makes sense about the MAP sensor and the way it reads, relative to the barometric pressure it senses for the split-second when you turn the key and activate the ignition voltage. So my takeway is that it's something like the inverse of what Richard said (so vacuum minus absolute pressure). That would seem to make sense given typical vacuums of 18 to 20 inches and an absolute pressure at sea level of 8".

Regarding the tranny, that's my problem.. I wouldn't know what a stuck valve looks like. I'll probably end up ordering an ATSG manual for my model year of the 4L60E. I've already seen a few pictures/diagrams of valve bodies, plates, pistons, etc. but for a relative dummy like me, the more the merrier. I figure if I'm working on the rest of my truck and I break something.. I can buy whatever I need to fix it at a 7-11.. for transmissions, not so much

That connector for TPS measurement will be here this afternoon. It was hard to find (in Advance Autoparts system). It looks like a direct replacement is Standard Motor Products S-619 for about $16, but neither Advance nor Autozone list a direct replacement in their system for my truck, and Advance doesn't have that one when searching the part number. I found online a Dorman product that Advance does have on their website (which may or may not be a direct replacement but looks exactly like it) but they have no availability what so ever, even from Dorman, according to the guy in the store. Finally we crossed the Dorman to something else and supposedly that'll be here this afternoon.

FYI, you know how you said it'd be tough to find the male end of the connector in order to make like an in-line measuring device? I'm pretty sure I've got a good method to make one of those out of epoxy, if you have the right sized pins to experiment with. If you're aware of a howto online somewhere that details this, please point me to it so I don't waste my time, but if not I might make one for the site
fritz1990
Senior Member
Posts 6389
fritz1990
12-21-11 08:13 AM - Post#2170893    

Play around in here for pigtails: http://www.acdelcotechconnect.com/pi/wiring-connec...

Regards, Jeff
1998 K1500 6.5 Coal burner
1965 C10 with 498 BBC AFR Heads
1964 C10 Ran 348W for 6 years, now SBC.

Corvettes owned: '74 '77 '78 L82 Silver Anniversary, 2002 LS1

Don't have a nervous come together!

http://picasaweb.google.com/fritz199090

letsbangout
Contributor
Posts 148
01-03-12 05:30 AM - Post#2175210    

I was able to get a good set of pigtails. FYI, the gromet on this one is purple, and it seems to have "dual holes" for each pin position, with each hole seeming to contact the same pin. I would guess it's also made to be used with Ford vehicles, as all of the pictures of 3-position TPS connectors with purple gromets that I saw seem to fit 1991-1994 Tauruses.

What I found is that the resistance never "dropped out" during my tests, as you'd expect it to with a bad potentiometer. If it were bad, the resistance should go infinite. I also wasn't able to put it in-circuit, so I couldn't do the Alldata test (to see if the pot passed more than 1.25 volts with the throttle completely closed). However, because the throttle has been seemingly hard to control at 1500 RPM while running the AC performance tests (it would jump around about +- 40 RPM), and because 1500 RPM is pretty close to where it'd be while shifting from 1 to 2, and because we're talking such small voltages, knowing that the difference between a 1.5 volt reading and a 1.55 volt reading could make the difference between a shift that damages or doesn't damage the transmission, I ordered a new Delco replacement from Rockauto for 30 bucks this morning.

I'll let you guys know whether it's solvent. If anything, I figure a new TPS will save me the headache of having the same problem in a few years, perhaps with the 2-3 shift, as bad TPSs are (probably?) better known for causing.
Allan In NE
Frequent Contributor
Posts 1294
01-03-12 07:30 AM - Post#2175235    

Very interesting thread!

Tryin' to wrap my head around your 'bad boy'.

Are you saying that you DON'T get the ratio error codes? Just those annoying, intermittent and head jerking shifts? Always just the 1-2 shift?

All other shifts are smooth and on time as they should be? No higher rpm than normal 3rd and 4th engagement?

And just to be sure here, ALWAYS with the A/C on?

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-03-12 06:46 PM - Post#2175426    

"Are you saying that you DON'T get the ratio error codes? Just those annoying, intermittent and head jerking shifts? Always just the 1-2 shift?"

Correct. I've *never* received any kind of OBD code related to the transmission, only the post-cat O2 sensors and MAP sensor, both of which have been replaced. It's worth noting that the same sensor (I believe bank 1 sensor 2) reads fairly low while in closed-loop, perhaps 0.050 volts on average, bouncing between 0.040 and 0.070. It seems a little abnormal to me, but it's good and high while the system is in open-loop, so I don't suspect a problem with the circuit, wiring, or sensor itself. The other post-cat sensor, which I replaced at the same time, doesn't suffer from that problem.

But yes, the shifts are about as "neck-snapping" as one could expect out of a 5000 pound truck, the hard shifts are enough to shake the whole truck, and apparently break my motor mount too (though it remains to be seen whether the majority of that came from hard 1-2 shifts, or from the classic slip-yoke lubrication problem sharing my whole tranny/engine and placing stress on the motor mount). Motor mount still needs to be replaced, it lets the engine jump up about 2 inches while power-braking to 1300 rpm.

"All other shifts are smooth and on time as they should be? No higher rpm than normal 3rd and 4th engagement?"

Correct, it's hard for me to tell if, for example, 2-3 is harder than it should be because these trucks/trannies are known not to be soft shifters, but it certainly doesn't feel abnormal to me. And the 3-4 engagement always acts normal as well.

"And just to be sure here, ALWAYS with the A/C on?"

After my initial posting, it seems to have gotten better about shifting hard with the AC on. It could have been an old shift-adapt table in use, which wasn't utilized previously because the AC wasn't in. Maybe someone else can comment as to whether or not there are two shift-adapt tables - one with the compressor engaged and one without. But what I said initially is no joke, I had like 40 or 50 of those hard shifts in a row when I first got the AC done.

Now it's hard to say because it's cold here, and will likely remain so for a couple of months. To complicate matters, my new Delco compressor is making noise now, so it'll take some time to swap it out and get everything right.

Thanks for your input, it's a perplexing problem indeed. But I've still got this sneaking suspicion that it has something to do with airflow, vacuum, or metering thereof, and less to do with the TPS. It's a shame I don't already have the scantool.net stuff, with the Pro version I could log in real-time whether the compressor is in/out, the speed, RPMs, and transmission shift pressure.. which would provide hard evidence that the hard shifts are by PCM command (as I suspect) and not because of something intermittent in the transmission.

Allan In NE
Frequent Contributor
Posts 1294
01-04-12 07:51 AM - Post#2175554    

Good Morning!

Hey! Can I make a few suggestions even tho I'm relatively new here? I admittedly don't know much, am extremely opinionated/bull-headed, and arguably a bit of a country bumpkin, but I guarantee you that I do know GM trannys. Hang on, this is gonna be a little long and possibly a little tough.

First off, and Sir, I say this with every ounce of respect that I can muster, but I contend that your thought processes have jumped the track here. Bear with me and don’t get mad, I’m trying to help.

You are so far down this “data stream” road that you’ve completely lost sight of the original problem. I.E. that your transmission is banging into second gear.

How in the world did you ever make the leap that this irritant is a “PCM commanded hard shift”? On just one particular shift? Always that 1-2 shift? Naw, I just don’t think so. Easy to prove me wrong tho…….hook a pressure gauge to ‘er and show me the spike at the 1-2 shift (if it does, that’s an easy fix too; just another chapter down the road).

Anyway, yes, the engine has to be running right or the transmission will never shift right, EVER, but from what you say it sounds like this has all been taken care of and all is well in that department? No codes at all and the engine runs smooth as it should?

No, the transmission didn’t break the motor mount. That thing failed all on it’s own steam. No, it isn’t going to “ruin” your transmission; it’s very hard on the downstream drivetrain tho. And lastly, again, no. These transmissions are not known for being “hard shifting”. That is pure poppycock. It should be smooth as a newborn baby’s butt; shifts barely noticeable under moderate throttle.

Okay, enough of my rough-edged banter. Let’s go repair this guy.

FIRST RULE of any GM transmission repair is that silly fuel filter. I mean it! Any restriction whatsoever in that filter will absolutely send that PCM bonkers because of the resultant change in the TPS signal. More throttle, more hydraulic pressure, that simple; it’s doing just what it’s supposed to do. Pull the filter and blow thru it in the direction of fuel flow. Any restriction at all, replace the goofy thing. (Since your problem is one particular shift however, this is probably not the case. This symptom usually shows up as all shifts being hard and delayed).

SECOND, humor me and pull the electrical connector. With the aid of a mirror and a flashlight, look down into the connector cavity in the case. We don’t want to see any fluid in there. Again, it just drives that PCM nuts.

THIRD, the real suspect. Mechanically speaking, the shift to second gear only involves the simple application of the band---that’s all. So, we’re looking at a failure either in the servo or the accumulator (hydraulic shock absorber). We know the servo is okay because it is almost flippin’ the darned truck over on the shift to second. That just leaves that silly accumulator and believe me, those things are an issue.

If this were my truck (notice how I just divorced myself of all responsibility? ), I’d drop the pan. On the right rear corner of the valve body area is the 1-2 accumulator housing held on by three little bolts (takes a 10mm socket). Pull that guy down out of there and inspect the spring, the piston and its seal. Yours is gonna be plastic so it is probably cracked. Either plastic or steel, look for the piston hole being wobbled out egg shaped.

Would just about bet my wife’s pet cat that you’re gonna find your trouble right there.

DISCLAIMER: Now, if the ‘mechanic’ PO has put a shift kit in this guy, all bets are off. Buy a SRTA and be done with it because it will never be right again.

I really, really do hope you get your truck repaired,

Allan

PS, Picture shown is out of a 4L60, but yours will be similiar.




Lifelong GM automatic transmission specialist

fritz1990
Senior Member
Posts 6389
fritz1990
01-04-12 08:55 AM - Post#2175574    

You did your homework Allan, that is my pic and bench.

Just went through this with a 4l60. Exactly the problem and I will bet this is what he finds. Read this post but it never clicked that I just went through this myself got caught up in all the other stuff.

Pull the accumulator and put this thing back on the road.

Regards, Jeff
1998 K1500 6.5 Coal burner
1965 C10 with 498 BBC AFR Heads
1964 C10 Ran 348W for 6 years, now SBC.

Corvettes owned: '74 '77 '78 L82 Silver Anniversary, 2002 LS1

Don't have a nervous come together!

http://picasaweb.google.com/fritz199090

Allan In NE
Frequent Contributor
Posts 1294
01-04-12 09:57 AM - Post#2175592    

Sorry fer stealin' your picture Pard, I was scramblin' to find a picture on the 'net and this one "jumped" out at me. Thanks!

One thing with the newer 'lectric shifts. That piston is ONE TIGHT fit on re-assembly. Should be really, really tight in the piston bore.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-04-12 11:44 AM - Post#2175619    

Thanks for the replies, very though! You're right, the engine stuff probably has very little to do with this problem, and I probably am off-base.

I'm delighted that you're knowledgeable about transmissions, because I'm not. I'll carry out all of your recommendations to the nth degree. But I've got some questions/comments:

FIRST RULE of any GM transmission repair is that silly fuel filter. I mean it! Any restriction whatsoever in that filter will absolutely send that PCM bonkers because of the resultant change in the TPS signal. More throttle, more hydraulic pressure, that simple; it’s doing just what it’s supposed to do. Pull the filter and blow thru it in the direction of fuel flow. Any restriction at all, replace the goofy thing. (Since your problem is one particular shift however, this is probably not the case. This symptom usually shows up as all shifts being hard and delayed).

Yeah I doubt I've got an issue with the fuel filter but I'll replace it anyway since it's 12 years old after all. I wouldn't want my precious fuel pump working any harder than it has to

SECOND, humor me and pull the electrical connector. With the aid of a mirror and a flashlight, look down into the connector cavity in the case. We don’t want to see any fluid in there. Again, it just drives that PCM nuts.

Which connector are you referring to specifically? From memory there are 3 or 4.

THIRD, the real suspect. Mechanically speaking, the shift to second gear only involves the simple application of the band---that’s all. So, we’re looking at a failure either in the servo or the accumulator (hydraulic shock absorber). We know the servo is okay because it is almost flippin’ the darned truck over on the shift to second. That just leaves that silly accumulator and believe me, those things are an issue.

Yeah I don't think the previous owner did anything regarding a shift kit, nor do I have reason to suspect the other two owners. He commented that it never banged into 2nd when he was pulling his boat, which is why I always attributed it to a sensor advising the PCM incorrectly. But what do I know.

Ok so the recommendation is to drop the 1-2 accumulator housing, pull out the piston and... inspect/replace? Also as I mentioned previously, I really think I need to buy an ATSG rebuild manual for my tranny because to someone like me who's never cracked one open, it seems like voodoo to me. Is ATSG recommended, or is there a better reference I should be using?

Incidentally, I also ordered a filter and gasket for my tranny along with the TPS. I figured i'd go ahead and take care of the fluid while I'm in there, as the fluid is a bit off-color for an unknown reason (possibly from the mixing of carbon dust which resulted from the old slip-yoke grease that burned up after 81000 miles, they'd mix because the rear output shaft seal leaks a little). Fluid does not smell burnt though.

Is it advisable that I drop the valve body and look for physical damage to the separator plate? I understand that the original plates were a bit weak and can easily become bent by the check balls that sit above them.
fritz1990
Senior Member
Posts 6389
fritz1990
01-04-12 12:23 PM - Post#2175631    

  • Allan In NE Said:
Sorry fer stealin' your picture Pard, I was scramblin' to find a picture on the 'net and this one "jumped" out at me. Thanks!

One thing with the newer 'lectric shifts. That piston is ONE TIGHT fit on re-assembly. Should be really, really tight in the piston bore.

Allan



No need to be sorry just glad you found it cause I was not thinkin' right and forgot all about it.

I meant that you used the search function.

I would forget about all the other stuff and just drop this and replace spring. You will need the last 8 digits of your VIN when you call GM to get one. There are some on the net but I just went GM so I got the right one. They are color coded.

Regards, Jeff
1998 K1500 6.5 Coal burner
1965 C10 with 498 BBC AFR Heads
1964 C10 Ran 348W for 6 years, now SBC.

Corvettes owned: '74 '77 '78 L82 Silver Anniversary, 2002 LS1

Don't have a nervous come together!

http://picasaweb.google.com/fritz199090

Allan In NE
Frequent Contributor
Posts 1294
01-04-12 12:47 PM - Post#2175644    

Take these suggestions one step at a time, okay? Try the fuel filter and then drive it.

If no go, look at the connector and if full of fluid, use brake clean and compressed air to clean it up good, then drive it again. You're going to need a 13mm socket to remove the heat shield from the right side of the tranny (kind of a PITA, but it's sure worth checking out.

The wiring loom comes in from the top down to this connector. It is going to have a locking collar holding it in place. Turn the collar 1/4 turn to the left, then slowly wiggle and lift up on the loom. You'll have to get your flashlite and a mirror at this point. No way you can see in that cavity otherwise.

Then if all this doesn't work and if ya have to open 'er up, have at it using a 10mm socket. Gasket is reusable. Can't remember if you said it was a K truck or a C, but if two wheel drive, you will have to lift that bugger up a bit off the tranny mount using a 15mm socket to loosen the mount so that the pan will clear the crossmember.

Then, it's just a matter of loosening the front of the pan first and letting it tip down to drain. Yes, have 5 quarts of dextron and a filter at the ready.

Now, when you get the pan down, slowly drain any remaining oil out of one corner and study the bottom of that pan. Some tiny metal and black gob around the magnet is normal.

However, if there is any 'soot' or metal in the bottom of the pan, stop. Take a picture of it and let us see. You may have more serious problems and now is the one and only time to analyze just what the heck is going on. A good eye can tell exactly what is failing by the pan residue and the inside of that plastic/paper filter.

I'm including a shot of the right side of a 4L60E. The green arrow points to the 2-4 servo area and the red to the 3-4 accumulator housing. The 1-2 accumulator lives directly below the 3-4, just the other side of the spacer plate and facing the opposite direction (up).

No, leave that valve body in place. You don't EVEN wanna drop that joker down!!!!!!! If we find absolutely nothing wrong, hollar back and we'll continue on with this jasper.

Can't advise you on the manual thing; maybe someone else on here can help you with that. I don't use 'em; just rely on my feeble old memory.

Allan

Lifelong GM automatic transmission specialist

transman618
Senior Member
Posts 839
transman618
01-04-12 04:21 PM - Post#2175713    

The accumulators are a problem on these. Specifically, the piston pin bores wear. The pin bore itself will wear into an egg shape and cause a leak. The accumulator basically cushions the shift so any leak in there will cause a harsh shift.


transman
GM MASTER TECHNICIAN
SR. TRANSMISSION BUILDER


letsbangout
Contributor
Posts 148
01-05-12 01:27 AM - Post#2175882    

Thanks for all of the replies! I will indeed carry out all of these recommendations to the nth degree, approaching things scientifically and driving on it a while in between changes.

The accumulators are a problem on these. Specifically, the piston pin bores wear. The pin bore itself will wear into an egg shape and cause a leak. The accumulator basically cushions the shift so any leak in there will cause a harsh shift.

Is the bore part of the valve body assembly, or the accumulator housing pictured above, or? What would I do in that case?
Allan In NE
Frequent Contributor
Posts 1294
01-05-12 04:54 AM - Post#2175895    

Hi Again,

Wouldn’t it be nice if the darned fuel filter cures all your ills and you don’t have to go this far? At any rate, I know this is kind of “overwhelming” for you, but believe me, it is a 15-minute repair.

After you take out the three little bolts, that housing is going to come straight down in your hand. First you will see the spring (or, what’s left of it) sitting in the recess of the piston, which lives down at the bottom of the little housing. Turn the whole she-bang over and I think that piston is just gonna fall out of the housing (from the way you describe the severity of the shift).

Inspect the housing bore to make sure that it isn’t all chewed up (doubtful). You’ll see that the piston axle is part of that housing also.

This picture is merely "representative". Hurriedly grabbed it out of my box just to show you what you’re gonna be looking at. Yours is ‘probably’ going to be plastic since your truck is a 1999 model.

The wear comes at the piston’s axle hole (arrow). Egg shaped and/or in your case, probably cracked.

I’m very interested in your project, not so much the obvious fault, but rather what’s gonna be lying in the bottom of that pan. REALLY, REALLY take note of this!

And will you post back with what you find?

Allan

Lifelong GM automatic transmission specialist

transman618
Senior Member
Posts 839
transman618
01-05-12 06:53 AM - Post#2175920    

The "Bore" I am talking about is part of the piston itself. The piston pin is inside the accumulator.


transman
GM MASTER TECHNICIAN
SR. TRANSMISSION BUILDER


letsbangout
Contributor
Posts 148
01-05-12 07:16 AM - Post#2175928    

Thanks again for all of the replies and the extreme detail. The pictures and descriptions of the accumulator/housing will be extremely helpful I'm sure. Yes I'll absolutely post back in detail and with pictures of the pan. I'll have the TPS here tomorrow, will probably install that tomorrow or over the weekend and inspect the tranny connector at the same time, run that way for a couple days, then move on to the fuel filter and later the tranny.
letsbangout
Contributor
Posts 148
01-10-12 02:15 PM - Post#2177819    

Ok so the TPS went in Saturday morning, no change what so ever.

Today I dropped in the fuel filter. I took your advice and tried blowing through the old one, taking into account the flow direction. Unlike the new filter, which seemed to pass substantially all of the air I blew into it, I could not feel the air I blew into the old one. Pre-start pressure on the old one was 60 PSI, idle pressure was 50 PSI. The idle measurement on the new one was bouncing around 52. As for solvency, of course the shifting was smooth as a baby's butt - which is almost meaningless. I say that because it did the same thing when I replaced the post-cat O2 sensors. The problem is just too intermittent to pronounce it fixed after doing a dozen or so good smooth shifts. It's been doing those firm-to-hard shifts almost constantly as of late, but only time will tell. Most likely I'll get into Allan's second and third recommendations this weekend.

That brings me to a couple of additional questions:

1) Lately I've noticed that the transmission seems to sometimes make the 1-2 shift in 2 back-to-back stages. By that I mean that typically, the shift is either one quick soft feeling, or one quick hard one. The normal "single stage" shifts are usually about 0.4 seconds in duration for a soft one and 0.2 seconds for a hard one. Occasionally when it does the "two stage" shifts, the shift will be prolonged in a manner in which it shifts harder than normal, but takes the full 0.4 seconds that a soft shift would, and it seems to "hiccup" in the middle. It's entirely possible that I'm describing slipping, but it doesn't feel like any kind of slipping that I'm accustomed to, in the sense that it doesn't have to "grab" multiple times. Think of it like a pause during the shift, whereby when it happens, I feel two jerks of the whole truck, rather than just one. Also I've never felt the tranny slipping in any gear, ever. Any idea on possible causes of this? Could it be related to the 1-2 accumulator also?

And, is there any reason I shouldn't go ahead and replace the whole accumulator assembly? I was looking at them online, the cover is like 9 bucks, the piston is 2 bucks, and I guess there is a spring in there too? Did I miss anything? I'm asking because I won't have access to a car to go get parts, so I'm limited to doing this either at home, or in the parking lot of a parts store, but of course my preference is just to be completely prepared ahead of time.

Thanks again!
someotherguy
Moderator
Posts 25155
someotherguy
01-10-12 02:19 PM - Post#2177820    

The slight increase in pressure indicates the filter was probably plugged up a bit, if you're seeing a difference at idle you'd definitely be seeing a difference under load.

However, I've never trusted the "blow through it" method of seeing if a filter seemed plugged; a new filter that is dry will not have resistance - but try this as a test, put a new filter on and prime the system a few times to soak the element with fuel, then remove it and try blowing through it. I think you'll find it's going to be noticeably resistant to blowing through it although it's absolutely not plugged up.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

Allan In NE
Frequent Contributor
Posts 1294
01-11-12 04:20 AM - Post#2178058    

Hi Banger!

Just when we were gettin' along so well, you go and quietly mention that double bump. Ya darned guy anyway!

Don't go crazy on me and start throwin' wrenches. Drive it. Keep your money in your pocket and let's see what develops. If it comes up bangin' again, then we'll dive into that acculumator.

Richard: Yeah, I know. There I go again expecting folks to read my mind.

Remember those old stone (porous pressed bronze) carb filters used back in the 60's? It took 4 pounds of pressure to overcome the spring and bypass the little filter.

Guess over the years, I've just come to judge what ‘plugged’ means by blowin' thru 'em. 4 lbs is a cheekful of air.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-11-12 09:12 AM - Post#2178113    

The hard shifts are back as of this morning. Like I said, sometimes it shifts hard when it's hot, but I can always count on it to happen to some degree when the truck hasn't been run for several hours like it did this morning (though it's not cold outside, I'm in South FL, temps here are 75 degrees lately).

I'd like to dig into the accumulator without delay. What do I need to buy to comprise a new 1-2 accumulator? And keeping in mind that I won't be able to leave until she's all put back together, what else should I have onhand?

Thanks!
Allan In NE
Frequent Contributor
Posts 1294
01-11-12 09:29 AM - Post#2178119    

Yep, that's where we gotta go next in light of the double bump thingy going on.

You'll need a new piston and it's oil ring seal. Get the heaviest accumulator spring you can find, tranny filter and 5 or 6 quarts of dextron. The accumulator housing if ya want.

If you really wanna splurge, this would be an ideal time to replace both shift solenoids, as very, very rarely a mechanical issue (worn plunger) with the solenoid in the "A" postion can contribuite to the double/pause type shift.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-11-12 09:45 AM - Post#2178124    

I'll grab the complete accumulator assembly, including the housing. I've read where people said a scored up housing could cause it to stick. I don't see how it would be possible to score it with a plastic piston (which I probably have now), but they seem to be cheap enough, about 10 bucks.

Your advice about the spring is duly noted. I'm going to hit up a transmission supply house or two, rather than the dealership, as I'm sure they have aftermarket parts that meet or exceed GMs'.

For the moment I'm going to pass on the solenoids. Not because of money or anything along those lines, but simply because I've never cracked open a transmission before. I have no doubt it'll take me twice as long as it'd take you, or more, just to do this simple task. If the solenoids give way later on, i'll just action the shifter manually to get me out of whatever situation I'm in. Hopefully by that time i'll have a set of ATSG books and the confidence to do a complete disassembly and overhaul.


transman618
Senior Member
Posts 839
transman618
01-11-12 11:29 AM - Post#2178156    

Before you assemble the accumulator take a GREEN Scotch Brite pad and scuff up the bore. This helps keep the piston lubricated and operating properly.


transman
GM MASTER TECHNICIAN
SR. TRANSMISSION BUILDER


letsbangout
Contributor
Posts 148
01-11-12 12:55 PM - Post#2178183    

Ok first of all, I'm sorry for not thoroughly reading what you said about the shift solenoids. Yes I'll definitely grab a set of solenoids and replace them. The procedure, from what I saw on Youtube, looks to be simply removing the connectors, removing the clip that holds them in, and swapping them out.

Now, I bought the complete 1-2 accumulator assembly from a local transmission supply house. It all looks real good, but I'm pretty sure these are not all new parts, as they were only 10 bucks (for everything). They came together in a bag with a decent coat of ATF on everything, or maybe it's another type of non-ATF pinkish liquid because it does seem a little thin. I took everything out of the bag and set it on a shop towel. Before doing what transman suggested, I decided to pop in the piston by itself to check for smooth, free movement. Now not only does the piston not want to move downward using the force of my two thumbs, but I also can't figure out how to take it out. I tried using a medium sized flathead jeweler screwdriver to pry upward in the little groove that goes down toward the piston, but I only managed to chip off a tiny piece of the screwdriver. I'm including a picture below:



Now, before my vivid imagination gets the best of me and I decide to do something else counter-productive:

1) Do I need to shelve these and buy new parts that come in a box, rather than a bag fulla ATF?
2) If not, how should I go about pulling out the piston? Maybe needlenose pliers? Vice-grips?
3) Is there anything else I should do, other than what transman suggested, prior to installing these?

Thanks!


Allan In NE
Frequent Contributor
Posts 1294
01-11-12 02:00 PM - Post#2178204    

No, you don't need new parts; these are good if that piston and seal are new.

Compressed air in the apply hole will pop that thing out right back out.

You're putting the piston in upsidedown, Pard.

Yes, it should be a good tight fit. You can use vaseline to lube the seal and the housing wall to help get it started down in the bore. We want 'er really snug. Then the spring sits in the recess of the piston---last to go in.

Keep those pictures coming and we'll walk you right thru this.

Allan
Lifelong GM automatic transmission specialist

Allan In NE
Frequent Contributor
Posts 1294
01-11-12 02:16 PM - Post#2178209    

Just to reinterate,

We don't want a "smooth free fit". That piston should be TIGHT in that bore.

You'll see the difference in the springs once you let yours down. Let it down easy and carefully, we don't want a torn gasket at that spacer plate.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-11-12 03:18 PM - Post#2178232    

"You're putting the piston in upsidedown, Pard.

Yes, it should be a good tight fit. You can use vaseline to lube the seal and the housing wall to help get it started down in the bore. We want 'er really snug. Then the spring sits in the recess of the piston---last to go in."


Thanks for the reply. I believe you're correct for older 4L60E's, i.e. before 96 or so. But from everything I've read, the accumulator goes in exactly as in my picture (for 99 transmissions), with the springs already installed of course. For reference, I can also point you to to a 4L60E overhaul guide that's been floating around the net in wide-spread distribution, which shows it the same way I assembled it:



Maybe someone else can chime in to set one of us straight (keeping in mind my 4L60E is 1999 vintage). Either way, i'll wait to assemble it until I've pulled the old one out to be sure.

Compressed air to get the piston out was actually the first thing I thought of, unfortunately I don't have a compressor. I guess now's as good a time as any to go buy one, as I've got some AC work to do which will require compressed air. So most likely I'll use tomorrow to buy the compressor and figure a few things out beforehand.
transman618
Senior Member
Posts 839
transman618
01-11-12 03:25 PM - Post#2178236    

I agree. Use compressed air to remove the piston. NOT TOO MUCH AIR. You dont want that piston to shoot across the room or hit you in the face when it pops out. Quick short little bursts of air will do fine. Last but not least. DONT PRY ON ANYTHING with the screwdrivers. After you scotch brite the bore, clean it thoroughly with some brake clean spray, lube it up real good with some petroleum jelly or trans jel and install the springs and piston. Torque the housing bolts down to 9'lbs or if you have an inch pound torque wrench, torque to 100" (INCH) pounds.


transman
GM MASTER TECHNICIAN
SR. TRANSMISSION BUILDER


Allan In NE
Frequent Contributor
Posts 1294
01-12-12 03:42 AM - Post#2178457    

Oh heck yeah.....sorry. Guess I still had that silly 4L60 I just did on my feeble brain.

Drop 'er down outta there.

Allan

Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-12-12 08:24 AM - Post#2178525    

Ok thanks. I guess your picture shows a gasket between the accumulator housing and the separator plate, should I buy a new gasket while I'm picking up the solenoids? And for the sake of verbosity, the shift solenoids you're referring to are the gang of two A/B shift solenoids that look something like this, right? http://www.otsparts.com/77945_p/77945.htm

Sorry for the million questions and I greatly appreciate your patience thus far. I'll be picking up the compressor and anything else last-minute that I might need this afternoon, so I can get underway tomorrow.
Allan In NE
Frequent Contributor
Posts 1294
01-12-12 09:14 AM - Post#2178534    

Doggone it!

I'm gonna absolutely be physically ill if we don't find something wrong in there after all this trouble I(we) am putting you thru. Hate spending your money like this.

Get a rubber tipped air nozzle to pop that piston out. Like TransMan said, use a short little burst of air, 'cause it's going to want to come outta there like a slug out of a shotgun otherwise. I cover the housing with a rag to catch the piston.

Do you have a wide-mouthed catch pan for the oil?

No on the gasket, as it is actually the entire valve body gasket and will just stay in place; let the housing down easy so we don't tear it.

Yes, the A/B shift solenoids. Really don't suspect the solenoid too much, but hey! We're gonna be right there in the neighborhood anyway.

No 'patience' required on my part whatsoever. I'm just sittin' here dancin' thru my ol' memory hopin' beyond hope that you get your truck straightened out.

Still say, this is the 'most likely' cause. (fingers crossed).

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-12-12 01:50 PM - Post#2178614    

Eh no worries.. when you buy a truck with only 70,000 miles for half of what it's worth, you expect to have to fix a thing or two. I'm just delighted that the tranny still works after 14 months of having this problem intermittently, and that I'm (hopefully) closing in on a solution without having paid a transmission shop a dime. Aamco wants something like $130 just to change the filter/fluid/gasket alone.. surely this work would add up to 500 bucks one way or another.

I bought the shift solenoids this afternoon. Also went to Harbor Freight to grab a middle-of-the-road compressor. I found that one of the two I had been eyeing online will be on sale tomorrow, $100 versus the sticker price of $120. Yes it's a small compressor (2 HP, 4.5 SCFM@90 PSI), but by the time I need something bigger, i'll be ready to build a monster with a 5 HP motor. The $20 price difference will more than finance another trip and my lunch So today I just grabbed a few fittings and a blowgun attachment with the rubber nozzle, and will get the pan and ATF tomorrow, along with the compressor and a few other miscellaneous doodads from Harbor Freight, and defer all of the work by another day or two. (as an aside, they're having one heck of a sale this weekend, on not only tools but consumable items like 30 cent wire brushes).

One more thing that's on my mind - is it necessary to clean in any way those connectors going into the shift solenoids? They're probably dry, but if the solenoid connector housing was cracked for any reason, it could cause leakage into the connector. Just thinking out loud. If you've got any other final recommendations, I'd love to hear 'em. Thanks again.
Allan In NE
Frequent Contributor
Posts 1294
01-12-12 03:16 PM - Post#2178643    

No Sir,

Nothin' to 'em as they live buried in oil anyway.

You're going to do just fine!

I gotta be gone all day Saturday, so I'm hopin' that TransMan is monitoring to help you out if need be. Just don't think you're gonna need any help tho. You're a pretty through guy and have really planned ahead.

Take lots of pictures. I sure wanna see what you find in there!

If you find absolutely nothing wrong, never, ever bring this subject up again.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-15-12 05:30 PM - Post#2179636    

So I don't have much to report. I wanted to do this today but I got started a little too late. I got the compressor out, assembled, oiled up, etc. and was able to get the piston out with the help of 100 PSI of air. I followed Transman's advice and scored up the accumulator piston bore and the pin with a green Scotch Brite pad. I'm not a big believer in Vaseline because I think it breaks down too easily, so I used about 2 raindrops worth of Valvoline general-purpose bearing grease, it has an operating range from 0 to 340 degrees and a minimum drop point of 500 - same stuff I used to grease my slip yoke. I worked it into the bore and the pin good, then reinstalled the piston by itself and was able to get good firm travel all the way down. I blew out the piston again and reassembled it with the springs.

Now I'm wondering if those springs are the right height. If that piston/housing came out of an early 90s 4L60E, maybe the springs are a little shorter because they have less space to travel to the top, since the piston is installed in the opposite direction for early 90s versus late 90s 4L60E's?

See the picture below. For reference, the piston surface seems to be sitting right at 1 centimeter below the top. That seems to be where it's "comfortable". Is that normal??




transman618
Senior Member
Posts 839
transman618
01-15-12 05:51 PM - Post#2179644    

STOP what you are doing. Disassemble that accumulator and CLEAN OUT THAT GREASE!!!!!!!!!!!!!

"I'm not a big believer in Vaseline because I think it breaks down too easily,"

Thats EXACTLY why we told you to use vaseline or Trans Jel because you need for it to break down and wash out of the bore. That accumulator is lubricated by ATF. all that grease is going to do is stay there and attract any particle it can get ahold of. One small particle of trash, even a piece of lint from a shop rag can wreak havoc inside an automatic transmission. This is why I always dry all my parts with compressed air only.


transman

GM MASTER TECHNICIAN
SR. TRANSMISSION BUILDER


letsbangout
Contributor
Posts 148
01-15-12 05:55 PM - Post#2179648    

Will do. My line of thinking was that heavier grease wouldn't break down and wind up in another part of the transmission that might be sensitive to it. I'll replace w/ vaseline and take careful note to follow your advice more closely, as you're the expert, not me

I guess the resting height of the piston looks ok to you?
someotherguy
Moderator
Posts 25155
someotherguy
01-15-12 05:59 PM - Post#2179652    

Sometimes the most well-meaning improvisations are bad...guys like transman, Allan, Chevytech, etc. are giving you advice based on years and years of experience - best to follow 'em to the letter.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects

letsbangout
Contributor
Posts 148
01-16-12 01:43 PM - Post#2179969    

Lacking a reply about the piston's position at rest, im proceeding anyway. Just dropped the pan. Fluid was a half brown half reddish color, maybe even slightly more brown than red. Oddly my hand was covered in a pitch black substance that seemed to come down with the fluid, as the fluid color on my hands was like transparent black, with clumps of solid black fluid on my hands too. Will have pics shortly.
Allan In NE
Frequent Contributor
Posts 1294
01-16-12 02:46 PM - Post#2179979    

Waiting, waiting......

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-16-12 03:52 PM - Post#2180010    

I haven't even looked in the pan, been dealing with tomfoolery on this new (old) accumulator. The one I pulled out (which came down incredibly easy with no pulling or tearing of the gasket) - the piston did indeed fall out when I turned it upside down. It's a metal piston with a 98 stamped on it. When I got into it, it looked to me like my existing small spring was sturdier than the "new" one I bought from the transmission shop. So I stopped everything, called them back to ask if they had new springs and a new seal. They had the big spring but nothing else new in the box. So apparently I'm going to reassemble this thing with the best combination of parts - the existing small spring, the new big spring (which is about 5mm longer, same diameter, and a heckuva lot stronger), and the existing piston/seal/housing. Nobody around here has a seal that I can get ahold of today, and I'd really rather not leave my car as-is (it's parked in the parking lot of an auto parts store in a not-so-nice area of town).

With the new setup, the piston surface is roughly level with the top surface of the accumulator housing, and the eight "tits" on the top of the piston actually sit slightly above the accumulator surface. It will compress down upon installation, hopefully it won't harm the gasket. Unless one of you is adamantly against this, I'll proceed as-is with this combination of parts. See below for photos of the fluid and the combination of my existing accumulator/piston/seal/s mall spring, with the new big spring instead of the old big spring.

By the way, on the outboard shift solenoid, the one closest to the passenger side, the bracket that the solenoid sits in is a little loose, just like so many other people have remarked and posted videos of. The other one is firm.




Allan In NE
Frequent Contributor
Posts 1294
01-16-12 04:15 PM - Post#2180019    

I like strong springs and a tight fitting piston to bore feel. Your choice of parts looks good to me.

Waiting to see if the old piston axle bore was wobbled out?

Oh, and that 'shaky' solenoid is a non-issue. It is just rocking on it's o-ring seal. Nothing to worry about.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-16-12 11:14 PM - Post#2180181    

"Waiting to see if the old piston axle bore was wobbled out?

Oh, and that 'shaky' solenoid is a non-issue. It is just rocking on it's o-ring seal. Nothing to worry about."


Remember that I reused the entire 1-2 accumulator, except for the larger of the two springs which is new and beefier, so what you see in the picture is what I had in my truck (with the new spring installed). I could not see any visible damage to the piston's axle bore. I didn't think to test that, which I could've easily done by installing the piston with only the small spring and feeling for play from side-to-side. But even if I did, I can't imagine that I would have any detectable amount of wobble since the hole in the piston looks flawless. I was more concerned about the re-used seal with 88k miles on it. It's a darned shame I didn't insist on new-in-the-box accumulator parts.

I came to find out while swapping in the new solenoid that the reason the passenger's side solenoid is loose and the other firm, is because the driver's side solenoid has some kind of a spring-loaded pipe that juts out of the valve body when you uninstall it. The passenger's side solenoid does not have this. So the spring-loaded pipe is what seems to apply pressure against the face of the solenoid to keep it steady. Like you said, no big deal since it is that way by design, but prior to swapping it, it just seemed a little strange.

I finished right at 8 hours in total, which is sans 30 minutes for lunch, and an hour to run to the transmission shop for the new spring, and back home to figure out and assemble the best combination of accumulator parts and to grab my trusty little compressor in case I needed it. I told you it'd take me double what it'd take you! The pan was clean btw. Give me a couple of days to flex out the tranny real good, especially from cold, and i'll come back and give you the two-day-old anecdotal evidence and all of my pictures.

Thank you very much for your help thus far, and thanks to everyone else who contributed as well. Win loose or draw, believe me when I say that I'll pay forward the knowledge you guys have imparted to me on this topic and others. Thanks again!
Allan In NE
Frequent Contributor
Posts 1294
01-17-12 03:43 AM - Post#2180198    

Oh, oh.

I just don't like the sound of it and don't think we accomplished very much on this repair.

No, sorry, I didn't grasp the fact that you were planning on using the old piston and seal; thought those parts were being replaced with new?

The stronger spring is going to help some, but the mere fact that, "The piston did indeed fall out when I turned it upside down" tells me that this "shock absorber" was indeed leaking by hydraulically and needed tightening up.

Sure hope we didn't waste a day here.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-19-12 04:52 AM - Post#2180914    

First of all, I should publicly thank Allan for all of his interest, dedication and insight into this repair. Not only has he contributed as you all have seen in this thread, but he private messaged me several times to check in and lend assistance. I cannot thank you enough for your help!

I just don't like the sound of it and don't think we accomplished very much on this repair."

Well neither did I. At the very least, I should have bought up the new piston that the supply house offered me along with that new spring. I was very caught up by the fact that I wouldn't be able to get a new seal regardless (unless I wanted to leave my truck as-is for several days). FYI, the seals are apparently very hard to get ahold of from transmission supply houses, they typically sell them only as part of an overhaul/rebuild kit. The dealer is another story, though I'm sure it would've taken 1+ business days to get one from the dealer.

However, I'm exceedingly pleased with the results overall. More on that later.

No, sorry, I didn't grasp the fact that you were planning on using the old piston and seal; thought those parts were being replaced with new?

I thought what I had was a 93 piston/accumulator with used springs and a new seal. Apparently it was all used. That's the trouble with buying from people in Miami - everything is stupid here. You've got to be smarter than the guy who's trying to rip you off. Not that the supply house did anything wrong - I asked them for an accumulator assembly and automagically assumed it would be something better-than-used. As in, if it were actually used, that they'd take the time to make sure it was within "new tolerance" spec. How wrong was I! I've been preaching how stupid these people are for 3+ years now.. how much longer will it be before I learn my lesson? Anyway, a side-by-side comparison of what they sold me versus what I pulled off of my truck quickly revealed that they sold me used and out-of-tolerance parts. The seal on their 93 piston was noticeably more worn than my 98.

The stronger spring is going to help some, but the mere fact that, "The piston did indeed fall out when I turned it upside down" tells me that this "shock absorber" was indeed leaking by hydraulically and needed tightening up.

Sure hope we didn't waste a day here.


That's what I was thinking too. But surprisingly, the work I did has made a significant impact. Immediately I noticed that when I launched it hard and wound it up above 2k RPM for the 1-2 shift, it was incredibly easy. I've had it up to 2500, 3000, 3500, 3800, all easy shifts. Not yet has it shifted hard at higher RPMs, which is something that would've already happened by now given the previous performance. The repair to the accumuator was a massive success!

Where it was unsuccessful, or perhaps I should say not-yet-successful, is on those shifts that happen at 1700 RPM. I've had 4 chances to drive it from cold (with the CTS reading < 100 degrees according to the scan tool, and the vehicle having sat for more than 5 hours without being run, in 65 degree ambient temps). The best way I can describe it is.. (assume everything I'll say here is with the fuel system in "closed" mode with the AC off).. when I drive it as-normal, taking off so that it shifts at 11 MPH/1700 RPM, those shifts are often-times very sudden. They happen in the 0.2 second time frame that I mentioned previously, they're never nail-biting like they have been in times past (and more-so with the AC running, though I haven't turned the AC on for any of these tests, yet). But I think they're harder than they need to be. That could be related to the fact that the piston and seal should have been replaced. I wanted to replace them, I really did, but apparently what I had was a 1993 vintage piston and seal. The replacement piston had obviously wobbled out some, as evidenced by the fact that I couldn't get it out without air, yet its seal was thinner than the one on my 98 piston which did indeed come out without air, just by turning it upside down (see below for pics). Anyway, to recap, shifts at higher RPMs (above 1800 lets say) are never nail-biting, they're never harder than they need to be, they're always crisp and quick, and they don't jolt the truck unlike times past. Shifts between 1500 and 1800 are very sudden and crisp, but (in my opinion) harder than they need to be. If I can control the engine torque adequately with my foot and cause it to shift closer to 1500 RPM, the shifts aren't as sudden or as hard. Either way, I'm of the opinion that the issue of sudden and firm shifts between 1500 and 1800 is a function of PCM programming. It's possible that a new piston and seal would mostly take care of this too, but so would a gentler PCM program. There isn't any logical explanation as to why some shifts between 1500-1800 should be "normal" feeling and others should be "harder than normal", unless there's some other physical deficiency in the transmission in play here. I should also note that when it's cold, it'd hard to make the engine do 1500 RPMs to get a gentler shift. It always wants to shoot up to 1700ish, which results in the harder shift.

Regardless, the reason I can say this was absolutely not a waste is 1) I can now say with reasonable certainty that I haven't done any major damage to my tranny by operating this way for the last 13 months, as evidenced by a lack of debris in the pan, and 2) that with the AC off, the problem has gone away above 1800 RPM and is noticeable but probably not harmful between 1500 and 1800 RPM. And should I ever be bothered by the 1500-1800 shifts (assuming I haven't already had the PCM custom-tuned), I can crack into the tranny again and replace everything in the 1-2 accumulator that didn't get replaced this time. I would think I could do the whole thing, start-to-finish, in less than an hour.

Now for the pics I promised (and some play-by-play on how the repair went, hopefully for the benefit of some poor soul who happens to have the same problem):

The first hour I spent getting the truck on wheel ramps at the front, and jacking the passenger side a little to accommodate my gut and make it more comfortable to work under. Then I pulled the pan bolts and drained the pan as best I could without making a mess. The pan came down and I went to lunch.

When I came back, I encountered the problem with the accumulator piston falling out, so I immediately wanted to (at the very least) compare what I had to the parts I'd bought, because it looked like the small 98 spring I pulled out of my tranny was harder than the 93 I was about to install. It was then that I called the tranny supply house, found out that they had new big springs, so I went and got one. Then I set everything up side-by-side to compare. Note below the red indicating what I rejected, and the green indicating what I used:



Also note how the 93 piston is considerably more worn than my 98, as evidenced by the fact that you can't even see the 93 stamp! (the 98 is on the right/top)



So I put the best combination back together and took it with me back to my truck, along with my trusty little air compressor. I swapped the shift solenoids, then took the following 3 pictures of the under-side of the tranny:







Then I installed the newly assembled accumulator, then the filter. I had a heckuva time getting the old filter collar out of the hole. I'm not sure what the professionals would use in this case, maybe a pick with a screwdriver type handle? I grabbed a hex wrench and gripped it with a set of vice-grips to pry down on the upper lip, with no success. I ended up getting it out by tapping various screw drivers into the collar body from the bottom, bunching it up so I could eventually pry it out enough to grab it with needle-nose pliers and pull it down. Of course I scored up the hole pretty good in the process. Filter went in, then I had to photograph and clean the pan:



Disregard the smudges on the left of the magnet, and on the right side of the magnet between the magnet and the "drain hump", those would be my fingerprints as I touched a few spots in the bottom of the pan to make sure they weren't metal fragments. I saw no sign of metal fragments of any kind. Though the humps of metal shavings on the magnet appeared to be higher than normal - the one I touched seemed to be about 7 or 8 millimeters in height, versus the "normal" 4 or 5mm that I've seen from transmissions that have gone 85k miles without service. That's to be expected from the hard shifting, methinks. I'm just elated that I didn't see any signs of metal in the pan.

I assigned the pan cleaning part to my girlfriend while I was torqing down the accumulator, placing the filter/collar, etc. Then came the most agonizing part - scraping off that old gasket. It was basically rubberized and completely melted to the pan. I can only hypothesize that this was the result of the original owners running the truck hot twice, the first time resulted in a blown engine and the second time blown head gaskets. Whatever the case, it wasn't coming up by hand. I had to hit it with a box cutter for a solid hour! Then I hit it with a combination of a big flathead, a small flathead, and more boxcutter. Then a wire brush, more boxcutter, more small flathead, and more wire brush. All in all, it probably took an hour and a half, start-to-finish. I hit it with brake cleaner several times throughout the process, and a few shots of compressed air to dry it out.

The pan installed with relative ease. I remember what you said about needing to take the transmission mount loose and lift up on the tranny to get the rear pan bolts in - that didn't happen. They came out with a regular 3/8" drive socket. But to get them in, I would've misthreaded them going directly. I reckoned that I could get it with a 1/4" drive socket and a U-joint attachment. And although I did misthread it again a couple of times that way, I finally got it in using that method. Of course I had to start it by hand rather than using a ratchet.

6.5 hours of work, non-inclusive of lunch and making the run to the tranny shop. I'll post back when it's time to run the AC again.. right now I've got a bunch of air in the system so I need to suck it all out and re-do the refrigerant charge. I might even disconnect everything and blow it out, who knows.

That's all for now I guess. If you have any comments on things that would affect my ability to control RPMs out of the engine in the 1500 to 1800 range, I'd love to hear them. Keep in mind I've already replaced the TPS and fuel filter. Thanks again for your help, and to everyone else who contributed.
transman618
Senior Member
Posts 839
transman618
01-19-12 06:09 AM - Post#2180930    

"You're Welcome"


transman
GM MASTER TECHNICIAN
SR. TRANSMISSION BUILDER


Allan In NE
Frequent Contributor
Posts 1294
01-19-12 08:24 AM - Post#2180983    

Hey Banger!

Thanks for that nice detailed report. I really enjoyed your description of the repair (especially the part about you getting the GF to help clean up the pan ).

HOWEVER, I still say we've just put a band-aid on this little guy. Read on.

This transmission started life in 1968 as the TH350 and all was merrily going along just fine until the powers that be threw another gear in the mix.

Neither here nor there, but this guy uses a combination of "stacked and timed" shifts. 1-2 is mechanically stacked, 2-3 is timed, and the 3-4 is stacked once again. In other words, one gear isn't just laid in on top of another. ('Tol ya that was a wasted paragraph).

Anyhoo, think of that 1-2 shift as a balancing act. On the servo side is a 'harsh' shift; on the accumulator side is a 'soft' feeling shift.

This 'balancing' of the two has been very carefully calculated and engineered to give a nice smooth "acceptable" shift at all conceivable conditions (hard throttle/soft throttle--heavy load/light load). If the servo side is stronger, she's gonna shift stronger; if the accumulator side is tougher, the shifts gonna feel "mushy".

Apply oil is applied to both the accumulator and the servo at the same time. This causes the accumulator to 'move' in it's bore against it's spring pressure FIRST and makes the band engagement lag behind just a little bit (stretches/smoothes out the shift).

You toughened up your accumulator piston spring so you've moved the 'balance' point back in favor of the shock absorber. See what I mean?

Trouble is, with a leaky seal, the function of the accumulator is compromised. Oil is going around that piston to a degree and we're not getting a nice smooth apply action of that 1-2 servo. In other words, it's still too darned "fast".

Yes, you're right. The more throttle you apply, the stronger the shift is due to the electrical/hydraulic strategy of the computer. It's gotta be that way because a 'cushy/slow' shift under heavy load/hard acceleration would very soon fry that band.

I'm including a picture of a piston and it's seal. See that center, 'raised' portion of the seal? That thing should go in the bore REALLY, REALLY difficult insuring a TIGHT fit....making it nothing more than a hydraulic ram against that set of springs.

If you ever decide to dance thru this thing again, hollar and I'll mail you a seal/piston; they're lying about all over the place 'round here.

Oopps! Now I'm starting to blabber. Glad your truck works okay,

Allan




Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-19-12 08:46 AM - Post#2180990    

Thanks for the additional explanation, that added a bit of missing depth to my understanding of what the accumulator does. I will absolutely take you up on your offer for the seal at some point in the future, as the one thing I still have trouble with that probably isn't going away until I fix the accumulator right, is those 1-2 shifts from absolute cold, when the PCM is in open-loop. Though it's better now, it's always been a problem for me historically. Also I know the pressures will be upped by the PCM when it comes time to run the AC again. Like I said, I can't imagine it taking more than an hour or so to repeat the process, now that I've got a better idea of what I'm doing. I'm guessing I could reuse whatever amount of ATF I can salvage in the pan, right?
letsbangout
Contributor
Posts 148
01-19-12 08:59 AM - Post#2180998    

One more thing - can you think of any reason I'd want to reset the Adaptive Shift tables back to default? I know the GM literature says to do it when you've made transmission repairs, and some Cadillac owners I've encountered say that resetting it improved some drivability issues, though I'm not so inclined to believe that. Thoughts?
Allan In NE
Frequent Contributor
Posts 1294
01-19-12 08:59 AM - Post#2180999    

Absolutely! You bet you can.

If I recall, that repair pays something like 20 minutes. But, on the line we've got all the 'quick' air tools and do (did) 'em every blessed day, day in and day out.

Allan
Lifelong GM automatic transmission specialist

Allan In NE
Frequent Contributor
Posts 1294
01-19-12 09:14 AM - Post#2181008    

Not on a repair like this.

These strategies are really detailed any more (like the addition of that sweet engine braking for instance).

The computer constantly monitors the transmission for wear and makes adjustments to the shift strategy so it is always optimum (it does this by calculating/adjusting shift time parameters).

They are kind of like the IAC motor in that regard.

I never bothered resetting the strategies after an overhaul, because the darned computer did it's own adjustment after a short time anyway. After 10-15 miles around town and a few key cycles, it already knows what is what.

Allan
Lifelong GM automatic transmission specialist

letsbangout
Contributor
Posts 148
01-25-12 03:21 PM - Post#2183388    

Just an update on this, as predicted, shifts have gotten much harder with the AC running. I'd say when the AC is running and they're hard, they're about 80% of the firmness as those hard neck-snapping shifts before the transmission work. Also it seems like the effects of having the AC running for a couple of days reverberated into the PCM programming without the AC because now the shifts are a little harder, even with the AC having been turned off for a couple of days now. I'm thinking it has something to do with bank 1 of my exhaust. Let me explain.

Recall that I said at the top of this thread that I'd replaced both of my post-cat O2 sensors because of (if memory serves me correctly) several P0120 and P0121 codes (bank 1 sensor 2 heater circuit malfuctions, and sensor values out of tolerance). Without having ever looked at the voltage of those old sensors, or checking the circuits, I just copped a set of new Denso sensors and replaced them. Now the bank 1 sensor 2 voltage is way down, it starts at like 0.15 in open-loop and gradually works its way down to around 0.05 and i've seen it read as low as 0.04 on the scan tool. However it's never thrown a code because of the voltages.

But the day after I charged up the AC, the computer illuminated the check engine light twice with P0420 (catalyst bank 1 inefficiency) while I had the AC running. I cleared it once immediately, drove it another 10 or 15 miles and the code came back. This was with the AC on. Since then, I cleared the code and left the AC off. I've been driving ever since, probably another 70 or 80 miles and 4 drive cycles, and haven't seen the code come back. I just checked it and it's not pending either. Do you have any recommendations in that regard? I'm guessing I should probably check exhaust pressure at the sensor fitting (if I recall, you look for 2 to 3 PSI with a vacuum/pressure gauge, right?), check the circuit to make sure it supplies 12 volts, and if everything looks good, perhaps swap the 2 sensors to see if the problem follows the sensor?

Also the "double jolt" is back on the 1 to 2 shift, I've noticed that over the course of the last week.

Finally, I've noted that whenever my truck is ran from cold to warm, it will often times continue to shift harder than normal on the 1-2 shift even after it's in closed-loop and is good and hot. But if I were to turn the key off, wait a few minutes (or 30, or 45) and restart it, the shifts will be not so hard. Is this just anecdotal, or is this a real behavior with a real explanation?

(as an aside, I wish I could somehow prove to you that I'm not "full of it" with regard to this PCM stuff.. I understand there are scan tools out there like Drew's Mongoose and EFIlive that can meter the commanded shift pressure from the PCM, and I look forward to the day when I can afford to buy one, though hopefully I'll already have the shifting issue resolved by then).
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