Fuel gauge not working

[machas74]

Hey guys and gals, hope someone can help me on this. I got a 69 c10 that the gauge does not work properly. I have not checked anything yet just wanted to hear from you guys where I should start checking. So when the truck is off the gauge reads 3/4 of tank. Then when I start it up the gauge goes to full and will not move from there regardless of how much fuel I have in the tank and the gauge will go back to 3/4 when the truck is off. Any help will be appreciated.

[demian5]

Pull gauge wire off at sender. Turn ignition on.
Ground gauge wire to the trucks body.
If gauge pegs when touching to ground and removing the wire from ground causes gauge to move the other way, bad sender.

If it reacts the same, senser may still be wonky, but then its the gauge itself possibly.

But I think its a bad sender OR the resistor on the back of the gauge is bad.

[machas74]

I probably have to drop my fuel tank to do that so I’ll wait until my fuel gets low. What color is the wire that goes to the sending unit? Asking because I see a wire behind my seat that I believe goes to the tank. Maybe I can cut it there and then ground it to the body to see if that plunges the fuel needle. Maybe?

[wilkin250r]

Testing isn't too terribly difficult, and it's even easier if your seat still moves forwards and backwards. If your seat doesn't move, you'll have to pull it out.

If you want to get fancy with your testing, you'll need to acquire a few resistors. They're not expensive, just difficult to find now that Radio Shack is essentially out of business. If you have a Frys Electronics in your area, you're good to go, or you can order online.

To test, you move your seat forward (or remove your seat), and your sending unit is at the top of your tank. It will have one or two fuel lines attached to it, and one electrical line. The electrical line should just pull off.

Take another piece of wire and connect that sending wire to ground (the easiest place is right in front of you, the tank body). Turn your key on, and the gauge SHOULD move all the way to empty. If it doesn't, first try a different ground spot.

If you wanted to get fancy with your testing, hook up a resistors between the sending wire and ground. The sending unit is just a resistance, with 0 to 90 representing empty to full. So a 30 ohm resistor should show 1/3 full, a 45 ohm resistor would show half tank, and so on. Doesn't have to be exact, if you just happen to have a 56 ohm resistor, hook it up. Gauge should read just under 2/3 full.

If the gauge is reading properly, then your problem is obviously in your sending unit. Easy fix. If the gauge is NOT reading properly, you have more testing to do, because your problem might be in the gauge itself, or might be a bad connection somewhere in the wiring.

[wilkin250r]

Your tank should look similar to this if you removed the seat. You can see the brown sending wire on the right. Like I said, if your seat still slides forward and backwards, it gets even easier because you can access it by just sliding the seat forward.

[machas74]

Sorry maybe I should of mentioned that my tank is relocated under the bed. But there is a single wire just hanging out behind my seat going to the back. Thanks wilkin250r.

[RustyPile]

Originally Posted by machas74

Sorry maybe I should of mentioned that my tank is relocated under the bed. But there is a single wire just hanging out behind my seat going to the back. Thanks wilkin250r.

"Just hanging out behind my seat...." doesn't tell us much.. What color is that wire?? Is it connected to anything?? Is any other wires connected to it??

The color of the wire at your "add on" tank is irrelevant, as it's not a factory install.. Don't cut any wire just for the purpose of testing.. Do whatever is needed to gain access to the sending unit in your tank..

Test as was stated by wilkin250r using resistors.. I use 33 ohm 2 watt resistors -- 3 of them soldered together in series.. Resistance adds in series.. Three resistors in series gives 96 ohms, close enough to the 90 ohms of the sending unit.. The test is looking for a "full to empty" response from the gauge.. If you get that using resistors, the sending unit is probably bad.. If you don't get the proper response from the gauge, then check the gauge circuit and the gauge itself.. Most of the time gauge failure is simply a failure of the "calibrating resistor" mounted to the back of the gauge.. Typically, it's an 85 - 87 ohm resistor.. If your testing leads you to a faulty gauge, remove that resistor for testing.. The value of the resistor cannot be tested while it's connected to a circuit.. Using a multi-meter, test the resistor.. You don't necessarily have to have a "genuine" resistor.. A common metal film 85 - 87 ohm resistor will work..

[wilkin250r]

If your tank was relocated, that makes it a little less likely that your sending unit is bad, as it was most likely replaced at that time.

However, the test is still essentially the same. Connecting the sending wire straight to ground (zero ohm resistance) should read as an empty tank. Leaving the wire disconnected should peg way past full.

Did the gauge ever work properly, that you know of? What is the possibility the last owner relocated the tank, but never bothered to connect the sending unit line?

[machas74]

Previous owner had the truck for 18 years and stated that the gauge always worked for him. He said he bought the truck with tank under bed. Not to believe him that it always worked.

[machas74]

Here is a pic of the sending unit with 2 wires hooked up to it. Is that normal?

[GypsyTwins]

The second wire on this set up is to ground. I would trace where the ground goes to and make sure it's not corroded.