1972 gmc fuel gauge

[Drewbee]

Hey I'm hoping someone can help. First, I know there are a few threads of this topic but they seem scattered in opinions because some people say if it's this it's this, and others say if it's this it's that.

Second, my truck has had some bad wiring from previous owner that I'm trying to correct.

I have replaced everything between the fuel gauge to the sender...even the resistor except the wires. I have gotten the gauge to read at 3 o'clock when the wires is unhooked, and past empty when hooked up.

I checked the new fuel sending unit and was getting 0-90ohms. However because of the previous symptoms described I decided to put the old sending unit in....it now makes a slight clicking sound and bounces around empty. So, I think the new sending unit maybe bad, and the old one is going out that's the reason for the bouncing.

If someone with more knowledge on this please speak in on this. Also, if you say to test the volts or ohms somewhere, pictures would be great and more detail on which lead you put where - such as positive goes here negative goes there.

[glider_6969]

Originally Posted by Drewbee

Hey I'm hoping someone can help. First, I know there are a few threads of this topic but they seem scattered in opinions because some people say if it's this it's this, and others say if it's this it's that.

Second, my truck has had some bad wiring from previous owner that I'm trying to correct.

I have replaced everything between the fuel gauge to the sender...even the resistor except the wires. I have gotten the gauge to read at 3 o'clock when the wires is unhooked, and past empty when hooked up.

I checked the new fuel sending unit and was getting 0-90ohms. However because of the previous symptoms described I decided to put the old sending unit in....it now makes a slight clicking sound and bounces around empty. So, I think the new sending unit maybe bad, and the old one is going out that's the reason for the bouncing.

If someone with more knowledge on this please speak in on this. Also, if you say to test the volts or ohms somewhere, pictures would be great and more detail on which lead you put where - such as positive goes here negative goes there.

Hello Drewb. this is one of those issues that can drive you crazy as I am in the same boat. I finally ordered a universal replacement kit, has new gauge float , sending unit and wiring. Mind you I have a custom tank in the bed that I had fabricated. If you are looking for more info on trouble shooting the issue I suggest as I did do a quick search (google) on electric fuel gauge. There is a lot of info out there on how and what to test.

[VetteVet]

This is one of the simpler circuits in our trucks but it seems to be the hardest to understand.

Here is the circuit.
Gauge power goes to the gauge via a pink wire on terminal 3 of the cluster plug.


The sender wire runs from the fuel tank sender to the fuse panel on the terminal marked "FUEL". Then from there to the cluster plug on terminal 4 and to the fuel gauge. It is a tan or light brown wire.




Right edge in center.



The sender wire runs under the left sill plate with the dome light wires and a lot of times it will get bare and rub on the metal creating a ground for the gauge which will read below empty. If you disconnect this wire from the fuse panel you should get an over full reading on the gauge.

If you remove it from the sending unit on the tank you should also get a full reading on the gauge. This is because you are only reading the power side of the gauge. If you continue to get an empty reading with the sending wire off the sending unit then the wire is grounded at some point.

The first thing I do is to run a temporary wire to the sending unit from the fuse panel and eliminate the sending wire.

The readings you have gotten so far are correct from the fuse panel to the gauge. If you did not get a past empty reading with the sending unit wire off and then you did when you grounded the wire then your wiring is OK and the problem is in the tank or in the sending unit.

I would pull the sending unit out and connect the sending wire to it, then ground the sending unit flange and move the float arm. You should see a corresponding movement in the gauge needle. You can use the old gauge or the new one.

You can also connect an ohmmeter to the sending unit terminal and the flange and move the arm looking for the 0 to 90 ohm reading.

Here's a complete diagram of the circuit.

[Cali72K20]

Check for corrosion in all the connectors, both female and male, also if the circuit wiring has any crimp style butt connectors from the PO. Those are notorious for corroding and causing a current resistance. Not to sound mean, VetteVet's fuse block looks a bit rusty, but everything he said is sound advice. If you have this factory set up, being 40 plus years old, corrosion most likely is your culprit. You might have to check the back of the fuse block and inspect the factory wiring. Any "LIGHT BLUE" fuzzies on any wiring is bad, clean it and check with a multi-meter, test lights lie. Best of luck.

[VetteVet]

No offense taken. it's not mine. I posted it for reference only.

Here's a better diagram.

[Cali72K20]

You might also try a quick wiggle test. Have someone watch the gauge with it being powered up, run along the circuit wiring wiggling the wires every foot or so, any change in the gauge, that's were your problem is.

[Cali72K20]

Cool, that is a perfect diagram of where to check!
Thank You!

[Cherry 383 stroker]

Got the same where Duse the weir go to from the tank my truck has no wires thanks

[VetteVet]

Originally Posted by Cherry 383 stroker

Got the same where Duse the weir go to from the tank my truck has no wires thanks

Check my post 3 and post 5. The wire from the tank runs to the fuse panel under the drivers sill plate up to the fuse panel to the terminal marked FUEL.
The fuel tank and the sending unit must be grounded to the cab and frame or the gauge won't read correctly.