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Username Post: IAC Not working 1989 V2500 5.7 TBI L05        (Topic#205541)
jason8902 
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jason8902
Age: 45
Loc: San Diego CA
Reg: 12-18-08
12-19-08 11:34 AM - Post#1588281    

Hello.. I have a 1989 V2500 Suburban that doesn't have a responsive IAC. I have replaced the IAC and still no different. I placed jumpers between the harness and the IAC so I could probe the voltages while running. I haven't place a scope on it yet. The high and low side voltage seem appropriate... less than 1 volt across the low side and about 10.8 on the high. When probing one of the high side voltages with the other connector to ground I get between 2.9 and 3.5. It does appear that the computer is trying to control the idle but the IAC for some reason isn't responding. The truck idles too high, so I tried pulling the pintle out a little bit before I put it back in and that got it to idle lower but you have to babysit the throttle when you get in (cause the IAC isn't doing anything). However this hasn't thrown any codes yet. I did just fail an emissions test with high NOx. High NOx is from high combustion chamber temps which happen when you have a lean mixture. Since I believe my IAC to not be working I think it's the culprit of the high combustion chamber temps (since it's basically a controlled vacuum leak). HELP!!! Thanks in advance.



 
someotherguy 
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someotherguy
Loc: Texas
Reg: 08-01-03
12-19-08 11:45 AM - Post#1588288    
    In response to jason8902

Hi Jason and welcome to the forum.

Is the IAC in the full open position, causing the lean condition? If you look down at the TBI unit with the air cleaner off, you may be able to see inside the passage whether the IAC pintle is seated or not. In this picture you can't see the tip of the pintle at all; the IAC is opened.



Have you tried doing a reset on the IAC? One method I know of for this is to disconnect the battery long enough to clear codes/memory (about 30 seconds), then reconnect it. Jumper the A&B terminals on the ALDL connector as if you were going to read the codes by watching the check engine light flashing. Turn the key on and leave it on for about 30 seconds. Turn it off, remove the jumper.

If you have a helper turn the key for you, you should be able to watch the IAC move during this time.

Have you checked all your sensor grounds at and next to the thermostat housing? Clean, tight, undamaged?

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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jason8902
Age: 45
Loc: San Diego CA
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12-19-08 12:07 PM - Post#1588312    
    In response to someotherguy

No, the one I pulled out of there was open enough to make the idle around 1800. I removed this and added a new one. I manually adjusted the pintle out to get the truck to have a reasonable idle. However as you know this requires some bypass air and it should close when you start driving down the road. so even though it's not a tremendous vacuum leak it's still enough. I'm surprised that it's enough of a leak that the computer can't compensate for it though. So the short answer to your question is that the pintle is only slightly off the seat right now.

I haven't tried resetting it yet. I didn't know about this method.. I'll try it ASAP (which won't be until after Christmas unfortunately).

I will also recheck the grounds. Although I think they're fine I think everyone has been burned by a faulty ground before. I bought the truck and a non-running vehicle with a bag-o-parts so there is a possibility that I missed a ground somewhere. I know the ground by the temp sensor are fine.

Thanks for the reply!!



 
someotherguy 
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someotherguy
Loc: Texas
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12-19-08 12:15 PM - Post#1588321    
    In response to jason8902

If you manually adjusted the pintle, the ECM has no idea where it's at. Even if you didn't manually adjust it and just installed a different IAC, you need to do the reset. If you turn the key on while the IAC is unplugged, you need to do a reset, etc. It's a "dumb" system - doesn't know where the IAC is, has to make an assumption - so it has to have some kind of baseline to operate from.

Something I saw mentioned here recently, and it bears repeating - if you remove the IAC and plug the hole, the engine should just *barely* be able to idle. Any more than that and you have a vacuum leak elsewhere.

Don't overlook the TBI base gasket, MAP sensor vacuum line at the rear of the TBI, and possible worn TBI shaft bushings.

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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12-19-08 12:20 PM - Post#1588325    
    In response to someotherguy

Okay, great info about the 'dumbness' of the system. I should have thought of that since there is no feedback from the stepper motor.

I'll do the reset and see what happens. I also like the IAC plugging idea to see if the car has a high idle. I doubt it will though because when I pulled the pintle out it WOULD NOT idle cold, you had to babysit it for about 30 seconds. I think I can rule a vacuum leak out but I'll definitely do the check.



 
Chevytech 
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Chevytech
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
12-19-08 01:56 PM - Post#1588402    
    In response to jason8902

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

Here are the only testing specs I know of for use with a meter.

IAC testing:

Terminals A to B should be 40 to 80 ohms

Terminals C to D should be 40 to 80 ohms

Terminals A to D should be infinite

Terminals B to C should be infinite


If the IAC sticks as does not move as it for each pulse the IAC count get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumed it has moved every time it has been sent a pulse. As Richard said it is a dumb system.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.

Have you driven the vehicle at highway speeds after replacing the IAC? Specific criteria must be met for the system to self relearn the IAC position, unless you manually invoke a relearn.

To manually invoke a relearn, you can try this with all accessories OFF.
It is best if this is done with the vehicle at normal operating temperature.

Using a watch or timer to make sure you hit the threshold times on step 3 and 4:
1) Disconnect battery for at least ten seconds. (with the Key OFF)
2) Reconnect the battery.
3) Start engine letting it run for five seconds.
4) Turn key off for ten seconds.
5) Then start it and see how it idles.

  • someotherguy Said:


Something I saw mentioned here recently, and it bears repeating - if you remove the IAC and plug the hole, the engine should just *barely* be able to idle. Any more than that and you have a vacuum leak elsewhere.

Richard



Yes, I wrote this recently in another thread and I can’t find it.

With the engine warmed up, and all accessories off, the engine should idle very slowly with the IAC air passage plugged. If the engine is not in good running condition it will stall when attempting this.
If it still idles fast there is a vacuum leak somewhere.


For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
Hilandr452 
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Hilandr452
Loc: Colorado Springs
Reg: 10-24-08
12-21-08 02:09 PM - Post#1589819    
    In response to Chevytech

Very nice, detailed and technical post Chevytech.

Colorado Springs, Co.
GMC Pics
1971 GMC Custom Camper 3500 454, TH400
1973 Chevy Mitchell 25 foot Motorhome 454

Happiness is a perfectly tuned big block.


 
Chevytech 
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Chevytech
Loc: Twin Cities, Minnesota, U...
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12-21-08 03:45 PM - Post#1589899    
    In response to Hilandr452

  • Hilandr452 Said:
Very nice, detailed and technical post Chevytech.


Thank you for the kind words.

For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
jason8902 
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jason8902
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01-04-09 07:12 PM - Post#1599428    
    In response to Chevytech

Okay so I'm back in town now and started messing around with this again. Somewhat frustrating day so far.

First I disconnected the 4 wire connector from the IAC and installed 4 jumper wires between the connector and the IAC so I could probe them. At first the car idled fine and then shot up. This was promising because it told me the IAC was actually doing something. I started wiggling the wires around and was able to get the car to idle correctly. Then I popped the weatherpack apart because it looked old and jumpered straight off the wires and the same thing happened.

so then I chopped off the 4 molex terminals and installed new ones and a new weatherpack connector. Now it idles high again and wiggling the wires doesn't help. I checked all the crimps and it's fine. I was sure it was a connection problem because I could vary the idle by pulling individual wires out before but not after reterminating the wires.

Now I popped the weatherpack apart again and did the jumper wires. all had solid connections. I double checked the resistances across the 2 stepper motors and got around 54 Ohms. It seems like I was on to something but it got away from me. For a small while the computer was telling the motor how to idle but then it stopped. I trust my connector is fine now. very weird.

I've tried all the reset techniques and still no dice. bottom line- it was running as if it had a poor connection, I fixed the connection and now it's idling super high again. No amount of wire wiggling does anything.

Thanks again for all the help so far.



 
jason8902 
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jason8902
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01-05-09 07:52 PM - Post#1600295    
    In response to jason8902

Just hooked up the computer... IAC is commanded completely closed (step 0). Idle never fluctuates at all. Seems like the computer is telling it what it wants to do but it isn't happening.



 
someotherguy 
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someotherguy
Loc: Texas
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01-05-09 08:22 PM - Post#1600322    
    In response to jason8902

Have you already gone over the grounds to be sure they're all in good shape? I know the grounds up by the thermostat housing and next to it can wreak havoc on sensors if they're not making a good connection; I haven't studied the wiring diagram to see if the IAC is included.



Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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jason8902
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01-05-09 10:40 PM - Post#1600386    
    In response to someotherguy

I double checked those earlier. There is one that comes off of the coolant temp sensor and they are all solid. Battery is grounded solid to the block and block ground straps look good. I was trying to think if something weird like the IAC not getting a solid ground through the TBI or something but it shouldn't require a ground anyway since current just passes through the inductors. I don't feel good about it but just in case I took the sensor back and got a Borg Warner sensor (should come in Wednesday). I really doubt that's it though. I am so frustrated that it actually did respond to a wire jiggle for a few minutes but now seems to be completely dead. I'll be bringing a scope home from work to look at the signal being sent to it... does anyone know what the signal should look like wrt the step number? I would like to compare where it should be to where it is being commanded to be.

There are so many IAC designs. My Corvette book says the IAC on the L98 (which looks like this) uses two coils, one to close and one to open. The higher (average) voltage on the blue wires compared to the green wires suggests that the green wire side is for smaller steps than the blue wire side (and thus not a push pull system like the L98). I don't really know. I'd like to try to figure out what sort of square wave form corresponds to what step number so I can probe it and figure out if the correct signal is making it to the IAC. Maybe the reason I see a low voltage on the green side is due to a short? Again, I know it's not a constant voltage but a square wave but your DC voltmeter will show the average and that is what I'm referring to.

Watching the ECM commands on WinALDL did make me feel confident that the computer is working correctly. I'm not sure what else to look for yet.

This has got to be the most ridiculous problem I've had on a car yet... can't wait to figure it out!!



 
355Cheyenne 
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355Cheyenne
Loc: Northern MN
Reg: 10-27-04
01-06-09 11:29 AM - Post#1600676    
    In response to jason8902

I would go with someotherguy, check your grounds. I have a 88 cheyenne mud truck and I seem to have ground problems all the time with the old truck. odd things start happening when corrosion get into the gorunds...

98 Silverado k1500 4x4 350


 
Chevytech 
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01-06-09 01:48 PM - Post#1600743    
    In response to jason8902

I have never used WINALDL, but I know it shows sensor readings, much like scan tools do.

If you have WINALDL, use it, instead of trying to monitor the wave signal to the IAC.

You should be able to see the IAC “count” or position.

You should also be able to see what the ECM reads for a coolant temp which will affect the command it sends to the IAC.

I am curious what IAC count it has and at what rpm it has it. Check this after the temp reaches 195 degrees F.

Also use the WINALDL to see if the park neutral switch is functioning.


For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
jason8902 
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jason8902
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01-06-09 02:50 PM - Post#1600777    
    In response to Chevytech

I have already monitored everything with WinALDL to verify that everything is functioning correctly and the computer is giving the IAC the correct commands. The computer shows that the IAC starts off with a high number (pintle off the seat) and starts getting down to a smaller number and eventually to zero (pintle on the seat, no IAC air). The idle doesn't change through any of the IAC commands. Correct IAC position at idle should be around 25.

I want to scope it because I want to ensure that the computers commands are actually making it correctly to the IAC. But all the sensors are working correctly, the motor warms up to operating temp just fine (about 190-200 on WinALDL) and you can watch the IAC number slowly decrease as it warms up. I haven't seen the Park/Neutral switch command verification on there yet. I did look for that yesterday. But also regardless of that, THE COMPUTER IS COMMANDING IT TO STEP ZERO. So regardless of Park/Neutral, temp, etc., the computer tells it to close and it does not.

It is idling around 1700 rpm with the IAC step at zero. but plugging the hole in the TBI will bring the idle down so the IAC is obviously not doing anything. Is the IAC bad? Is the command getting scrambled before it makes it to the IAC? I have replace the IAC and installed a new connector. I took the second Niehoff IAC back and got a Borg Warner but I don't expect that it's possible that I received 2 bad IACs in a row.

I was hoping to see a high IAC count in WinALDL which would tell me that it's just obeying the computer and the computer wants it to idle high for some reason. Unfortunately that's not the case.

I've also checked the grounds... I'll be installing the new IAC tomorrow. Does anybody know what the scope should read for a zero step? maybe there's a computer ground that is acting funny? I'd expect to see more that just the IAC problem if this was the case though. Any other diagnosis steps that anybody can recommend? Thanks for all the help.. I know this isn't the most exciting subject.



 
someotherguy 
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01-06-09 03:33 PM - Post#1600809    
    In response to jason8902

Is it even remotely possible you mixed any of the wires up when installing the new connector?

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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01-06-09 04:49 PM - Post#1600901    
    In response to someotherguy

definitely a good thing to check but I did check and re-check that. I do have the factory wiring manual too. blue and white goes to terminal A, blue and black on B, and the green and white/black are opposite (so the whites are diagonal from one another). The blues are on the clip side and the greens on the bottom.

One thing to mention-before I replaced the terminal the blue-white wire was nasty. I lubricated very well but eventually chopped it and reterminated everything. Nothing was melted so it wasn't necessarily excessive current draw but it is worth noting. Hopefully all the connecting and reconnecting screwed something up and plugging in the new sensor with the new harness will be good. Although the sensor I returned did show 54 Ohms across the two inductors.

I would like to check the resistance of each wire and look for one with a higher value than normal. I suppose I could do this directly from the computer. Is there an easier place to access the main harness?

One thing I've learned is that usually problems that you start overthinking like this turn out to be something so stupid... either a $5 part or free. Please continue with the ideas!! I don't want to get so focused on one aspect that I don't check something easy.



 
someotherguy 
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01-06-09 05:00 PM - Post#1600923    
    In response to jason8902

You can unplug the connectors from the ECM and remove the screw holding the harness to the firewall, and feed the connectors back into the engine compartment so both ends of the wiring are easier to access with your meter. Hope that helps.

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
Chevytech 
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01-06-09 06:38 PM - Post#1601017    
    In response to jason8902

  • jason8902 Said:
… The computer shows that the IAC starts off with a high number (pintle off the seat) and starts getting down to a smaller number and eventually to zero…

It should NEVER get to zero. If the IAC is working there is a vacuum leak somewhere.

  • jason8902 Said:
…, the computer tells it to close and it does not. …

IAC, wiring, or ECM problem.

  • jason8902 Said:
… It is idling around 1700 rpm with the IAC step at zero. but plugging the hole in the TBI will bring the idle down so the IAC is obviously not doing anything. Is the IAC bad? Is the command getting scrambled before it makes it to the IAC? I have replace the IAC and installed a new connector. I took the second Niehoff IAC back and got a Borg Warner but I don't expect that it's possible that I received 2 bad IACs in a row. …


Does plugging the IAC hole bring it down to a VERY low idle?

Did you invoke an IAC relearn AFTER you worked on the IAC wiring? This is a must after you experience erratic movement when moving the connector.

If the IAC does not move as it should for each pulse, the IAC count gets off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumed it has moved every time it has been sent a pulse.

When wiggling the IAC wires the ECM could have lost track of where the IAC is and the ECM may think it has it at the fully extended position when the IAC is far from fully extended.

At a zero IAC count don't expect to see the ECM sending any pulses (electricity)to the IAC.

If the store will take back an IAC you had installed, I would not buy any parts that are not in a seal box from that store.



For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.


 
jason8902 
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jason8902
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01-06-09 08:31 PM - Post#1601085    
    In response to Chevytech

A relearn has been attempted on several occasions with several different IAC motors.

I believe the computer is sending it to zero because that's the closed position and it's still idling too high. Computer sees high idle, it decreases IAC count. Regular operation should be 25-35.

Anyway I pulled the harness out of the computer and checked for resistances. circuits are perfect for all IAC wires (0.1 Ohms). On the larger connector there are two grounds, tan and tan w/ white stripe. Tan grounds behind coolant temp sensor, tan/white grounds on rear of passenger side cylinder head. When checking resistance between these two wires on the connector I got around 9.4 Ohms. I took off all the grounds and cleaned them with scothbrite and now it's about 0.8 Ohms. I did check resistance between Battery/CoreSupport/Frame /Motor/Firewall (every possible combination of the two) and ensured that everything was a good circuit. Hopefully that helps. I'll find out tomorrow. Thanks for all the continued comments and suggestions.



 
jason8902 
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jason8902
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01-07-09 10:01 PM - Post#1601886    
    In response to jason8902

Okay- still not working. Lets recap:

New Borg Warner IAC Motor

New connector on harness (Harness probed for continuity from ECU. Negligible resistance. NO Continuity with ground (short) on any wire).

All Grounds cleaned. Ground verified at computer. Tan has 0.2 Ohms between tan/white. Both have 0.2 Ohms between ECU harness and chassis ground. All parts of vehicle (chassis / motor / frame / computer ) haves 0.2 Ohms or less between them.

When I install jumpers between the harness and the IAC (so electrically it's the same to the computer but now I can probe it) I get about 1.0 mV on the green wire side and 30-70 mV on the blue wire side. This is with the engine warmed up. I tried this with and w/out AC and in Park or drive. I need to do this again cold though. I know this doesn't receive a DC voltage but this is just the best I can do to see what the computer is saying to the IAC at the moment. Seems low but at this point the IAC count is down to zero per the computer scan.

IAC reset has been attempted by shorting A-B in the data terminal,
Running for 5 with throttle slightly open, killing for a while, then running again.
Driving at 40 mph for a while, then highway speed then back down.

Through all of this the ALDL scan shows the IAC count changing but the IAC is not moving ever. I made note of where the pintle was when installed and it hasn't moved.

At this point the only thing that I can think of is that the computer thinks it's controlling it but due to something blown on the computer it's unable to get a full power pulse to it? I don't know what to expect but ranges in the mV seem low. I need to do this again when cold so I can see what the values are when the computer is trying to retract the IAC motor. If it couldn't pulse the IAC then it probably couldn't pulse the injectors and the motor wouldn't run at all. But I don't know anything about power distribution inside the ECU.

How do you verify a bad ECU? I hate throwing parts at a problem. Please help before I burn this car down. My ECU PN is 16084551. Where do you shop for one of these? I'll call the dealer tomorrow but if somebody knows a good spot let me know.

THANKS!!!




 
someotherguy 
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someotherguy
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01-07-09 10:19 PM - Post#1601892    
    In response to jason8902

  • jason8902 Said:
How do you verify a bad ECU? I hate throwing parts at a problem. Please help before I burn this car down. My ECU PN is 16084551. Where do you shop for one of these? I'll call the dealer tomorrow but if somebody knows a good spot let me know.


Hard to say; the only bad ECM I've ever dealt with was one on an '88 T/A that was giving a false MAF error code. That one really stank because it resulted in buying an expensive MAF that I didn't need.

Should be plenty of these ECM's out there in boneyards for you to try. I didn't see if you'd mentioned it, but does your 2500 'Burb have a 5.7 or 7.4? What's the service # on the ECM? (I'll go out on a limb and guess it's a 1227747?)

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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01-08-09 01:21 AM - Post#1601913    
    In response to someotherguy

Funny you mentioned a T/A... the last computer I threw at a problem I couldn't figure out was my 87 T/A. It didn't fix it though.

I'll go back out and look at it tomorrow. Is the 16084551 not the service number? Can I pull this out of any tbi Van/Truck/whatever and just swap the chips? It is a 5.7 liter. There were a lot of 3/4 TBI vans and the parts yard last time I went. Sort of weird to sit in the passenger seat while I took off a cruise control diaphragm.



 
someotherguy 
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someotherguy
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01-08-09 07:52 AM - Post#1602056    
    In response to jason8902

16084551 isn't a service number I recognize, so I think it's the number under it that you read. Service number is similar and same # of digits, but it's generally on the top line of the sticker and says "SERV. NO." just before it.

You might be able to use just any computer from a TBI 5.7, but it'd be best if you get one with the same service number, and put your chip into it. Observe static precautions and all that stuff.

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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01-08-09 08:18 AM - Post#1602067    
    In response to someotherguy

16084551 is the manufacturers PN but it may have been succeeded by another number. When I communicate with it with the computer I use a 12** number I think.

I am leaning towards replacing it. There is another ground and two power supplies to the ECU that I want to check first. I realized my error in comparing the IAC to the injectors last night since the injectors are wired straight to switched battery and don't receive power from the ECU, only a ground, and the IAC has to receive everything from the ECU. So I would expect low power since they aren't going to run a lot of amps through the ECU. One wire was somewhat nasty but it didn't look like it was fried from high current, it just looked very dirty. Maybe it was current in which case the ECU could have been damaged. I'll check the last bit of wiring today and start shopping!



 
someotherguy 
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01-08-09 08:27 AM - Post#1602070    
    In response to jason8902

Okay, the confusion here is that I'm asking what the service number is, not the part #. The service number is clearly identified on a sticker on top of the ECM.



Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
jason8902 
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01-10-09 04:19 PM - Post#1603939    
    In response to someotherguy

Okay, I haven't pulled it out yet but I found out that the new P/N is 88999159. Before I order it does anyone want to do a science project for me?

I verified my harness was good by unplugging from the computer and probing:

Grn/Blk comp to Grn/Blk at IAC -> 0.2 Ohms
Grn/Blk comp to Tan comp -> inf Ohms

I did this for Grn/Wht, Blue/Blk, Blue/Wht and got the same results, perfect continuity on the circuit and no short to ground.

Today just out of curiosity I did it again but this time from the engine side (so the computer is plugged in). 33 Ohms from Grn/Blk to ground strap. then I unplugged to battery to halt any current and got 16 Ohms. Since I knew the harness was okay the only place the short could be was in the computer. I unplugged the computer and resistance went to inf (on all 4 connectors). Blue circuit was good with the computer plugged in or not (4.5 Mega-Ohms plugged in.. normal). So this makes me 99% sure that the computer has an internal short on this particular circuit, but just to be sure I want to verify that this is indeed abnormal.

Does anyone want to volunteer to probe resistance between IAC wires and ground?



 
jason8902 
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Age: 45
Loc: San Diego CA
Reg: 12-18-08
01-20-09 10:39 AM - Post#1611639    
    In response to jason8902

FYI

Computer fixed it. Glad I did the troubleshooting first though just the same. I failed smog about a month ago with high NOx, I figured the vacuum leak created by the IAC being open might be enough to push it lean but I guess the motor compensated. It actually failed worse this morning. To be continued in a new thread......

Thanks to everyone who contributed to this post!



 
someotherguy 
Senior Moderator
Posts: 29526
someotherguy
Loc: Texas
Reg: 08-01-03
01-20-09 11:12 AM - Post#1611661    
    In response to jason8902

Crazy! But not totally unheard of. Thanks for posting back that you solved the problem. It's unlikely for the ECM to be the issue but as you have proved, it's certainly not impossible.

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
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