85 GMC fuel gauge non-op

[85c10owner]

It hasent really worked since I bought the truck. dude said it works sometimes but it has always read empty. I have switched back and forth on the tanks and both tanks read empty so not sure if it`s a gauge issue or sending units or something else. This is my daily driver so Im not going to be able to tear into it really heavy but if there a a couple things that are easy enough to check to varify what the issue is that would be great.

 

[foamypirate]

It hasent really worked since I bought the truck. dude said it works sometimes but it has always read empty. I have switched back and forth on the tanks and both tanks read empty so not sure if it`s a gauge issue or sending units or something else. This is my daily driver so Im not going to be able to tear into it really heavy but if there a a couple things that are easy enough to check to varify what the issue is that would be great.

First thing to check would be the sending unit grounds. They run from the sending unit to the frame rails. Make sure they are clean and tight.

 

[85c10owner]

First thing to check would be the sending unit grounds. They run from the sending unit to the frame rails. Make sure they are clean and tight.

i can handle that. Thanks

 

[chengny]

A poor ground connection in the sensing leg circuit will result in the gas gauge reading higher than actual level - or in the case of a broken ground wire, the gauge will peg high.

I would first check for proper operation of the dash gauge:

1. Pull the harness connector off the tank select valve body. This will create an open circuit in the gauge's sensing leg (pink lead). Turn the ignition switch to the RUN position and observe the gas gauge - it should peg high.

2. With the harness plug still off the Pollak valve, locate the pin (or socket) on the plug that is connected to the pink wire. Attach one end of a suitable jumper connection to the pink wire's pin and connect the other end to a good clean/rust-free spot on the frame. Now with the ignition switch in the RUN position, the gas gauge should read right down to the "E".

If the gauge reacts as above, you have confirmed proper operation of the gauge, the cluster PC board, the common section of the sensing leg and it's power supply.

So basically, you will have eliminated everything except a switching issue within the tank transfer valve.

The other possibilities are:

1. Two grounded sensing legs - between the transfer valve and tank sender - or

2. You have two failed sending units


Two have both of either those fail is not likely. Here is another more realistic possibility;

GM used the same tank sensing wire harness on dual tank trucks as with single tanks - up to a point.

The common pink wire has to be connected to the tank transfer valve - which is mounted over by the auxiliary tank. The pink wire in the standard harness (which only needed to go to the production tank on the LH side) was not long enough. To gain the additional length - that the pink wire needed in a dual tank setup - the simply tacked on another 4-5 feet.

That splice was made just a little bit forward of the rear wall of the cab - up on the top flange of the LH frame rail. It is not to hard to locate and check for a good connection. It is certainly easier to access than the sender connections.

Here is the wiring diagram for an 86 with dual tanks and a image of that hidden common splice connector:

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[85c10owner]

A poor ground connection in the sensing leg circuit will result in the gas gauge reading higher than actual level - or in the case of a broken ground wire, the gauge will peg high.

I would first check for proper operation of the dash gauge:

1. Pull the harness connector off the tank select valve body. This will create an open circuit in the gauge's sensing leg (pink lead). Turn the ignition switch to the RUN position and observe the gas gauge - it should peg high.

2. With the harness plug still off the Pollak valve, locate the pin (or socket) on the plug that is connected to the pink wire. Attach one end of a suitable jumper connection to the pink wire's pin and connect the other end to a good clean/rust-free spot on the frame. Now with the ignition switch in the RUN position, the gas gauge should read right down to the "E".

If the gauge reacts as above, you have confirmed proper operation of the gauge, the cluster PC board, the common section of the sensing leg and it's power supply.

So basically, you will have eliminated everything except a switching issue within the tank transfer valve.

The other possibilities are:

1. Two grounded sensing legs - between the transfer valve and tank sender - or

2. You have two failed sending units


Two have both of either those fail is not likely. Here is another more realistic possibility;

GM used the same tank sensing wire harness on dual tank trucks as with single tanks - up to a point.

The common pink wire has to be connected to the tank transfer valve - which is mounted over by the auxiliary tank. The pink wire in the standard harness (which only needed to go to the production tank on the LH side) was not long enough. To gain the additional length - that the pink wire needed in a dual tank setup - the simply tacked on another 4-5 feet.

That splice was made just a little bit forward of the rear wall of the cab - up on the top flange of the LH frame rail. It is not to hard to locate and check for a good connection. It is certainly easier to access than the sender connections.

Here is the wiring diagram for an 86 with dual tanks and a image of that hidden common splice connector:

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

Great info. Thank you. BTW, I cant see the images. When I click on them it takes me to the turbo image site but its all about some dating site.

 

[chengny]

Sorry, about the dating site pop-up. Maybe I forgot to select "family safe" in the content field. Try again - I just re-upped them as family safe.

I just tried them and they all came up clean.

 

[85c10owner]

Still cant see them. still has the dating site stuff. See if this link shows you what I`m seeing.

http://www.turboimagehost.com/p/24290597/IMG_20131231_123520.jpg.html

 

[chengny]

No, nothing about dating. All I see is my photos. Check your pop-up settings?

Can someone else try and see what comes up?

 

[85c10owner]

Ok, was at work earlier and could not see anything. At home I can see the images but when I go to the link I posted there is still a link to a dating site. Doesnt matter now that I can see the images.