Fuel gage : This is dumb

[Beer can]

In my '75 GMC the fuel gage does not work . With the key off it reads 3/4 full no matter how much fuel is in there , Key on the needle pegs way past full. I'm pretty sure this is a common problem because this is not the first square I've owned that had this problem. Usually I just fill up ever couple days ,but after running out of gas as I pulled into work Monday I thought this is dumb. Any ideas on we're I should start and what to look for ? Thanks

[rich weyand]

This is it:
http://www.chevyclassicsclub.com/the...ry-1965-newer/

[Nickelcharlie]

I just asked this question and below is everyone's response. Here is a very good resource I came across while having my problem!

http://www.chevytrucks.org/tech/gasgauge.htm

Here is how I fixed my gauge.

What worked on my truck was adding a bottle of Tehchron to a full tank. I have a 99 FRC Corvette and they are notorious for the gauge stop working. Techron cleans the contact points in the tank and in most cases the gauge will start working in the Corvette. So I thought what the heck, I added a can to the truck and after a couple of days of driving the gauge started working again. Maybe it cleaned up the arm inside of tank so it would swing easier. A large majority of electrical problems that we are having now on these old trucks can be cured with cleaning contacts and checking ground straps. One of the best electrical contact cleaners is a spray can of DeoxIT D5. Can be ordered of the net!

My question and everyones response

I have a 1986 GMC with dual tanks with a gas gage with the needle stuck on the far right side. It used to peg out when I switched to the left tank now pressing the switch up or down does not change the gage. When I press the switch either up or down I can hear the switching valve under the truck switch. Is this an indication that my cage is not working? Thanks in advance.

sounds like a loose ground on the tank

So your switch obviously activates a valve. That valve does two jobs, it changes the routing of your fuel flow, and also changes a set of electrical connections that determines which tank your fuel gauge is reading.

Pegged to the far right is an indication of a broken electrical path. Either a connection came undone, or a wire broke, or a loose ground on the tank as blazer suggested, or something broken in the sending unit itself. In electrical terms, it's called an Open Circuit. The electricity is not making a full path.

If it used to only happen on the left tank, but the right tank worked properly, then your gauge was obviously functional back then, so I suspect it's STILL functional now. I don't think the gauge is your problem.

These sending units are delicate, and now they're delicate and old. Most likely, your right side tank just gave up, and now you have the same problem with both tanks, which is why you're getting the same reading. Most likely.

Thank you blazer2007 and wilkin250r for taking the time to give an in-depth explanation.

Now for testing and repllacing, which way would be the easiest drop the tanks or take off the bed?

No need to do either, for testing. You just got to get a little creative. But when it comes to replacing (if that IS the problem), most people suggest removing the bed.

I don't have a wiring diagram, and I'm not 100% familiar with the 86 (it has a different valve than mine), but the theory is all the same. The gas gauge is just a resistance meter. Zero ohms is empty, 90 ohms is full. That is why your gas gauge is pegged to the right, with the open circuit it is seeing a zillion ohms.

SO, the basic theory for testing is simple. You pull the electrical connection, and send the fuel level wire to ground. This should read as an empty tank, and see if your gauge responds accordingly. If the gauge reads empty like it should, then the problem is in the tank or the valve. If the gauge does NOT respond properly, then the problem is in the wiring or gauge itself. If you wanted to get really creative, you can run to a Radio Shack or similar electronics supply, and pick up a couple resistors for about $1 and test the gauge at half-full, or 3/4, or 1/3, depending on resistor values.

No need to remove the bed if the problem is a faulty gauge.

So, simple in theory. Walking you through the process is slightly more difficult, because I don't have pictures to show you, I don't know the color codes, I can't SHOW you. But it's not all that difficult, the connection you need to test is the 6-pin connection at the fuel selection valve under the truck (NOT the selector switch on the dash).

Get some info on wiring. Somewhere on that 6-pin connector is a +12V feed, and if you short that one straight to ground, you're gonna fry your wires.

The terminals on the valve connector are A-F. F is a closed off cavity A is on the opposite end.
• A is the wire from the RH sender.
• B is the wire to the gauge.
• C is the wire from the LH sender.
• D & E are the valve motor power. Polarity depends on the dash switch position.
I would check that the tank is still getting a good ground. It's the most fragile part of the sender circuit. The ground strap from the frame to the tank is attached to a 1/4" male disconnect terminal on the sender locking ring to a machine screw on the frame. The disconnect tang typically rots off and then you loose your ground.

You can check the gauge by grounding terminal B. It'll drive the gauge needle to E.

[Beer can]

Ask and you shall receive! Thanks for the quick replies , I know what I'll be doing tomorrow. Can't believe I spelled gauge wrong

[Gregski]

I would ground the gauge behind the cluster with its own brand new black wire and if that doesn't fix it, I would do a home run from the sending unit ground to the negative on the battery, I hear guys saying, check your ground, check your ground often when this issue comes up, and that's what did it for me, though I did rewire the whole darn fuel gauge system eventually

[LT7A]

Gage is actually a usable variant of the word guage, just not as common.