67-72 Chevy Truck fuel gauge diagnostics 101

[IIGW]

Great read!!

[bamabronc]

Hey guys, I’m stumped with my fuel gauge. Here’s my story. The fuel gauge never really worked since I got my 72 so I decided to change the sender. After install it stayed at 1/4 tank. After searching here I determined it had to be the resistor, so I checked the ohms on the resistor and nothing. Ok, so now I order a resistor and new printed circuit from truck shop. I then installed the new printed circuit and the resistor. It read 3/4 tank so I drove to fill up and it stayed at 3/4. So I take the cluster out again and thought well maybe I put the resistor on wrong because I had the circuit under it. I changed it so the circuit is on top of the resistor. Now it reads 1/4 again, but I get 86 ohms on the resistor, so it should be good. The gauge ohms are very similar to the ones in this thread and I get 90 ohms on the #4 wire and 14v on the #3 wire (on the tender). I have already verified that the sender ohms moves from 0 empty to 90 full. Also, the gauge test with the wire off the sender goes to around 2 o’clock and grounded goes to empty. Is the copper on the resistor supposed to touch the copper on the circuit? Any ideas as to what may be my problem? Your help would be greatly appreciated. Thanks.

[bamabronc]

More info. With the gauge outside of the cluster and hooked up, it works. I’m thinking it must be something with how I have the resistor on or the wiring coming into the cluster. Does anyone know what the readings should be on each wire at the plug?

[bamabronc]

Originally Posted by bamabronc

More info. With the gauge outside of the cluster and hooked up, it works. I’m thinking it must be something with how I have the resistor on or the wiring coming into the cluster. Does anyone know what the readings should be on each wire at the plug?

Also, feed wire at plug is 90 ohm, but once plugged in it goes to 36 ohms.

[67 Burb]

Originally Posted by bamabronc

Also, feed wire at plug is 90 ohm, but once plugged in it goes to 36 ohms.

Yes, once you plug it in, you are introducing more circuitry into the ohm measurement.

Run a wire from the ground stud on the back of the gauge to a good ground.

[67 Burb]

Seeing how you have already verified that the sender ohms moves from 0 empty to 90 full, test the gauge on the bench like I have done. I use three 33 Ohm resistors soldered together as in photo. Apply 12V + to the IGN post on the gauge and 12V - to the GROUND post of the gauge.

Attach one end of a wire to the SENDER post on the gauge and attach the other end to the resistors at point "A" on the resistors. Attach a wire to the gauge ground post and touch the other end to points A, 1, 2 and 3 on the resistor string and if the gauge is working properly, it should read A - empty, 1 - 1/3 tank, 2 - 2/3 tank, 3- full tank.

[67 Burb]

OK. I missed your last post as I was posting mine. Now you know that the sender is OK and the gauge works out of the dash unit, make sure the gauge cluster is well grounded.

Where did you test the sender from? At the sender or from under the dash?

[bamabronc]

Originally Posted by 67 Burb

OK. I missed your last post as I was posting mine. Now you know that the sender is OK and the gauge works out of the dash unit, make sure the gauge cluster is well grounded.

Where did you test the sender from? At the sender or from under the dash?

I have tested the sender both in the tank and out. Ohms moved as it should while moving the float up and down. The tank is full and reading 90 ohms. At the dash plug and tank both read 90 before plugging up to the cluster

It’s weird. Seems to work until I connect the printed circuit

[67 Burb]

The one I have the black jumper on.

[bamabronc]

Tried grounding stud with a wire and no change while harness plugged in

[67 Burb]

I'm stumped. All seems to be OK, but still it doesn't work.

[bamabronc]

Here is a great YouTube explanation on fuel gauges I thought I would share. It covers the resistors and such. https://youtu.be/bVT2Jh_p02A

[57taskforce]

I just wanted to thank Vettevet and Tbone for this great thread. I had to replace my gauge resistor but thanks to this thread I was able to get my gauge working again. You guys are awesome.

[TBONE1964]

Originally Posted by 57taskforce

I just wanted to thank Vettevet and Tbone for this great thread. I had to replace my gauge resistor but thanks to this thread I was able to get my gauge working again. You guys are awesome.

Glad the information provided has helped you. It's always a pleasure to help .