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Username Post: 1980 K10 fuel Gauge\Tank Switching Valve Problems        (Topic#215685)
seawolf18 
Forum Newbie
Posts: 30

Loc: South Dakota
Reg: 04-27-09
04-27-09 08:08 PM - Post#1686499    

I just bought a 1980 chevy 1/2 ton for cheep. I can not figure out the gas gauge. The gauge stays on over full. The truck has dual tanks, and selecting the tanks does nothing. I checked the grounds on both sending units and they are good. On the passenger side I noticed theree are three wires comming of the wiring harress : green one goes to the tank switcher valve, white one goes to the tank sending unit, and a red one with a male connector on it, that is not connected to anything. On the driver's side there is the standard ground and gauge wire on the unit, and both are connected.

I took a resistance reading of both sending units and came up with 53.5Ohms passenger tank and 23.5Ohms driver's tank. I believe the readings are within specs for the sending units.

I am also not sure If the tank switching valve is hooked up right. Right now there is one green wire going to the valve. Does the valve have more than one wire going to it?

1980 Chevy K-10 4X4

2001 Monte Carlo SS Daily Driver


 
52chevybob 
Dedicated Enthusiast
Posts: 5628

Reg: 05-27-08
04-27-09 08:53 PM - Post#1686535    
    In response to seawolf18

There should be a voltage on the tank you are drawing from. It sounds like the relay for switching the gauge line isn't installed or isn't working and there is no voltage to the tanks. The overfill indication is that there is no gauge connected to the gauge - resistance is more than the 90 ohms of a full tank.

 
seawolf18 
Forum Newbie
Posts: 30

Loc: South Dakota
Reg: 04-27-09
04-27-09 09:12 PM - Post#1686551    
    In response to 52chevybob

There isn't any voltage to either tank sending units. Looks like all the wiring for the switching unit is messed up. Afeter looking at the selector valve, it looks like a wire broke off at the valve, and someone electrical taped the end of it.
1980 Chevy K-10 4X4

2001 Monte Carlo SS Daily Driver


 
seawolf18 
Forum Newbie
Posts: 30

Loc: South Dakota
Reg: 04-27-09
04-28-09 08:11 AM - Post#1686810    
    In response to seawolf18

anyone?
1980 Chevy K-10 4X4

2001 Monte Carlo SS Daily Driver


 
75gmck25 
Contributor
Posts: 345

Loc: Alexandria, VA
Reg: 12-25-07
04-28-09 06:36 PM - Post#1687259    
    In response to seawolf18

I am working from memory on wiring colors, but this is how my tank switch works. Also - the main tank in '80 should be on the passenger/right side, in later years they switched the "main" to the driver's side.

Switch in the dash is dual pole, dual throw.
-- One side has the gauge wire (tan) on the center and the two tank sending unit wires on each pole (one tan, one tan/white stripe).
-- Other side has a red wire on the center pole, and this runs to a switched ignition source. One pole has a wire that runs power to the switching valve (maybe the green wire), and the other pole is open (no wire).

- Main position - no power feed from red wire to switching valve, gauge now connected to tan wire; gauge shows main tank and valve feeds from main tank
- Aux position - power feeds from red wire to switching valve, gauge connected to tan/white wire.

The wiring has changed from year to year, and some of the schemes are very complicated because it made it easier to add dual tanks at the factory. For example, the tan wire out of my gauge cluster runs through the firewall, down the right frame rail, then hits a junction right next to the tank, and runs back up the frame rail to the engine compartment, and back through the firewall to the dash switch. If I only had one tank (no switch) they just leave off the wiring running back to the switch and connect the wire at the junction directly to the sending unit lead for the right side tank.

Bruce

 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15453

Loc: Colorado Springs, CO
Reg: 08-08-04
04-30-09 12:57 PM - Post#1688589    
    In response to 75gmck25

When gas gauge reads way past full, it means that there is an open circuit between the gas gauge and the sender. When the gas gauge reads empty, it means there is a direct short to ground.

Where did you measure your resistances? At the tank, tank selector valve, or the gas gauge?

If you want to go to the 60-66 forum, there is a sticky at the top I wrote on diagnosing fuel gauge problems. IT won't include the switching valve, but the rest of the info is pertinent.

 
seawolf18 
Forum Newbie
Posts: 30

Loc: South Dakota
Reg: 04-27-09
05-01-09 06:29 PM - Post#1689548    
    In response to Vaughn

I measured the resistance at the fuel tank sending units.
1980 Chevy K-10 4X4

2001 Monte Carlo SS Daily Driver


 
Vaughn 
"13th Year" Gold Supporting Member
Posts: 15453

Loc: Colorado Springs, CO
Reg: 08-08-04
05-02-09 01:00 PM - Post#1690013    
    In response to seawolf18

That is your problem. You should have measured it at the gauge also. Measuring at the tank is fine - to verify that the senders are working.

Your problem is upstream of the sending units - the wiring and the switching valve between the tanks and the gauge (the switch inside the cab only switches the valve under the truck, it doesn't do any switching of the feed to the gauge, this happens down at the switching valve). The wiring may be corroded or e3ven broken, increasing the resistance along the way, or the contacts in the switching valve may be corroded, adding resistance.

 
seawolf18 
Forum Newbie
Posts: 30

Loc: South Dakota
Reg: 04-27-09
05-07-09 09:54 AM - Post#1693137    
    In response to Vaughn

I found the problem. There was a broken wire upstream from the fuel tank sending units. repaired the wire, and now have the gauge working for both fuel tanks. Thanks.
1980 Chevy K-10 4X4

2001 Monte Carlo SS Daily Driver


 
jhudson78 
Forum Newbie
Posts: 28

Reg: 08-20-08
06-13-09 02:45 PM - Post#1716435    
    In response to seawolf18

I have the exact same problem on my '80 K10. I found a broken wire on the pass. side, but I can't find the other half so I don't know where exactly it goes!
06 Impala SS
85 Caprice Classic
80 K10
55 Bel Air Wagon


 
75gmck25 
Contributor
Posts: 345

Loc: Alexandria, VA
Reg: 12-25-07
06-14-09 05:59 PM - Post#1717163    
    In response to jhudson78

The tank wiring on my '75 truck is from about an '81 vintage truck, so I'll explain how it was wired. Its been a while, but I think I remember the routing.

The tan wire from the dash cluster runs from the cab through the firewall, then across the back of the engine/firewall (very hard to see/reach), down near the passenger side exhaust header, and then down the frame rail to the area near the passenger side fuel tank. If there is only one tank the tan wire would just connect there to the sending unit wire.

However, if there are two tanks, the tan wire has a very strange route. Instead of connecting directly to the sending unit, it runs back across under the truck (along the same bracket that holds the fuel lines) over to the driver's side frame rail, and back up through the firewall to the fuel tank dash selector switch.

From the tank selector switch (for the sending unit) there is a tan wire and a tan wire with white stripe. Both wires run back through the firewall and down under the truck, with one connecting to each tank sending unit. The tan wire goes to passenger side and tan/white to driver's side.

Bruce

 
Anonymous 

06-15-09 06:08 PM - Post#1717723    
    In response to 75gmck25

I missed this post, or would have posted sooner.

I can't speak about the color coding of wires, but here is a schematic from my 85 Shop Manual.



If the original Fuel Tank Selector valve is kaput, a replacement for an 85 C-10 Truck is $153.00 plus tax.

A substitute (which require doing some wiring changes) is available from NAPA. It is part number 2-2057 ($63.00). But it requires a new switch, 2-2058 ($19.00).

You do have to do some rewiring, but the Valve comes with the instructions. here is the schematic you receive with the new valve:



AND, you have to remove the original switch, fabricate a small plate to cover the stock, square hole, so you can install the new toggle switch.

OH, something I forgot to add...both the original Chevy schematic, and the aftermarket schematic, consider the left tank (driver's side) to be the primary tank, and the right side, the "auxiliary" tank...why???...a single tank installation from the factory is on the left side!!!!

 
Colby 
Forum Newbie
Posts: 1

Reg: 07-28-11
07-28-11 11:02 PM - Post#2119193    
    In response to

I'm working on rebuilding a 1980 chevy c20 3/4 ton and can't find the stock fuel switch to replace it (with the return lines)know anywhere where I can purchase the part (I want the single wire) to help cut down on extra wiring?
Thanks

 
sartobuilt 
Forum Newbie
Posts: 64
sartobuilt
Age: 41
Loc: Camdenton, MO
Reg: 06-05-09
08-30-11 11:36 AM - Post#2131894    
    In response to Colby

Napa sells the single wire one, with 3 ports. That is the only choice I've found so far. I'll post if I find some. the Napa one can be used if you are using a fuel pump with no return line. if your running a return line you can tee the return to bolth tanks, but then the return fuel will go to bolth tanks. Also the picture I saw of the switch is backwards from the origional. It wont connect to the lines directly.

 
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