Gas Gauge problem

[mrwill17]

Hi Everyone, I have an 85 GMC 3500 with dual tanks and cannot get the gas gauge to work. I replaced the dual tank switch on the dash, the tank selector valve and checked the fuel gauge and nothing. The only thing that will make the gauge move is when I discount the tank selctor valve connector. Then the gauge goes to Full. I have been reading blogs and some say that it is a ground when the gauge is stuck on E. Any help would be GREat!!!!!!!!!!!!! Thanks to all.

[joe231]

That is correct. Open circuit will read past full and a grounded circuit will read empty.

How many wires go to your switching valve?
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[tucsonjwt]

How did you check the fuel gauge? Are the sending units good?

[75ChevyGuy]

Check the ground on the frame for the sending unit. I've seen them get all corroded and screw up all the readings.

[mrwill17]

The sending units are fine. I put a new tank in last year. Is the ground accessible without dropping the tanks?

[baclay9]

In my experience when it is the ground on the sending units the guage will go way past full. Like "Extremely Full"

[joe231]

Exactly what Brandon^ said.

How many wires at the switching valve?
If the sender wires run through the switching valve, and unplugging it the gauge reads past full, you have something grounded that shouldnt be in the switching valve. A pic of the switching valve and connector would be helpful. Do you have access to a multi-meter?
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[tucsonjwt]

Just curious, how did you test the fuel gauge? I "tested" mine by installing a new gauge after trial and error on everything else.

[bucannon]

Originally Posted by baclay9

In my experience when it is the ground on the sending units the guage will go way past full. Like "Extremely Full"

this is true, i had the same problem

[mrwill17]

The plug has 6 prongs with 5 wires and one prong that is empty. If it is grounding out before it reaches the gauge I can assume that I will have to check the wires from the plug to the gauge. Any tips on making it easier would help. Yes I do have a multi meter but I do not know enough to check wires if they are grounding out. Any help or tips to check the wires would be would great Thanks!!!!!!!

[joe231]

If you unplug the connector at the switching valve and the gauge reads past full then the wiring is good from the valve to the gauge. Your problem is somewhere from the switching valve to the tanks. If both tanks show the same on the gauge with the connector plugged in then I suspect the switching valve.

I'll see if I can find a diagram so I can give you specific things to check.
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[hatzie]

Originally Posted by mrwill17

The plug has 6 prongs with 5 wires and one prong that is empty. If it is grounding out before it reaches the gauge I can assume that I will have to check the wires from the plug to the gauge. Any tips on making it easier would help. Yes I do have a multi meter but I do not know enough to check wires if they are grounding out. Any help or tips to check the wires would be would great Thanks!!!!!!!

The sender is a variable resistor in the ground side of the fuel gauge circuit. I think it's 90 Ohms max sweep.

Max resistance (90 Ohms) is Full and Minimum resistance (Shorted to ground or 0 ohms) is Empty.

An open circuit, bad ground or unplugged sender, will show way past Full @ 3 O'clock on the gauge. Your ground is good. You unplugged the sender at the tank valve and it behaved as expected so the pink wire up to the gauge is ok.

If the gauge always shows Empty there is a short to ground in the wiring, valve, or senders. I'd test everything starting at the valve.

I usually pull the bed back onto planks or timbers slipped over the bumper and frame to access the senders. Much easier than pulling a tank.

With a partial fill the sender stud to a good ground should read more than 0 and less than max ohms.

Here's the 87-91 R/V dual tank schematic complete with the electrical internals of the switches, valve, and senders. Ignore the fuel pump motors and their power feeds everything else should be the same on the 85.

[tucsonjwt]

The ground is a black wire from the right side to the top of the frame rail - a special large star washer/bolt thingy. (See picture, next to rubber bumper for my dump hoist conversion). You could snake your hand up there to wiggle, and maybe remove/clean/reinstall it with an inspection mirror and a lot of dexterity. I think I did this once. In the 3 squares I have owned, none of them had a bad ground.
You can check from the sending units to the fuel gauge, or from the fuel gauge to the sending units. I would have been better off checking from the fuel gauge to the sending units on my 3 trucks. If you have a known good fuel gauge, you can remove the instrument bezel and temporarily install the good gauge in 20 minutes tops. That is a lot easier then removing the bed or crawling under your truck and poking around. If the gauge is good, then you have to go the other way.
The only way to test a fuel gauge I have read about is to get a special test set with a variable resistor to simulate the resistor(s) in the sending unit(s), or make up a test set of your own - too much science for me, so I just got a good gauge to be my test set.
If the vent lines on your sending units just have stub hoses and caps, I can post a rerouting of the lines to the rear of the truck to eliminate gas smell in the cab if you want to see that - assuming you don't have a vapor recovery canister.

[tucsonjwt]

To clarify, each sending unit has its own ground wire to the nearby frame rail.

[Yupiamlo]

So if are gauge is reading EXTREMLEY full, its probably the ground from the sending unit right? If it helps when I turn off my headlights it reads empty, when I turn them on I have EXTREMLEY full

[hatzie]

Originally Posted by Yupiamlo

So if are gauge is reading EXTREMLEY full, its probably the ground from the sending unit right?

Yes. The gauge pinned past full at @ 3 O'clock is usually a bad tank ground.

[joe231]

Originally Posted by Yupiamlo

So if are gauge is reading EXTREMLEY full, its probably the ground from the sending unit right? If it helps when I turn off my headlights it reads empty, when I turn them on I have EXTREMLEY full

your headlights should have no effect on your gas gauge, have you looked at the wiring behind the gauge cluster? I fear that someone may have done some custom wiring

[Yupiamlo]

Ill check in a little bit, well everything was all good, and than I rode around for a week almost on empty, and finnally filled it up, and now its being a doucher