Fuel gauge wiring

[jperdue]

hey guys, I have an 82 GMC Sierra I just replaced the gauge cluster. Went from idiot lights to gauges. And just repinned the pig tail for the new cluster. The only gauge that I'm having trouble with is the fuel gauge. Both clusters have the big gauge. any suggestions?

[jperdue]

The needle doesn't move at all.

[ray_mcavoy]

Welcome!

I assume the gauge worked prior to the cluster swap, correct? If so, you can likely rule out problems with the sending unit and it's ground and concentrate instead on the gauge & cluster wiring that was changed.

You can leave the cluster in place and just take off the lens & front "tin". That will allow you to take out the fuel gauge and have easy access to it's terminal clips. As viewed from sitting in the truck, use a voltmeter to measure between the lower/center (ground) clip and the upper/left (power) clip with the key on. You should see battery voltage there if the wiring and connections are okay. Next, use an ohmmeter to measure between the lower/center (ground) clip and the upper/right (sending unit) clip with the key off. You should get a reading somewhere in the 0 to 90Ω range depending on how much fuel is in the tank. A considerably higher reading would indicate a bad connection.

If the above meter readings indicate a problem, remove the cluster plug and take a close look at the terminals. One of them might have gotten bent/distorted during the re-pinning process. Or possibly pulled back out if the locking tangs weren't bent back far enough.

But if everything is okay there, it could be a defective fuel gauge in the replacement cluster. I believe the large fuel gauges to the right of the speedometer are the same for idiot light & gauge clusters. So you could try swapping in your old fuel gauge as a quick test.

[jperdue]

I tried swapping the old gauge out this evening. I'm starting to wonder if I pinned the pig tail correctly or not for the gauge. After trying the other gauge I'm not sure have power to the gauge.

[ray_mcavoy]

Originally Posted by jperdue

I tried swapping the old gauge out this evening. I'm starting to wonder if I pinned the pig tail correctly or not for the gauge. After trying the other gauge I'm not sure have power to the gauge.

You can measure for power at the fuel gauge clips in the cluster using a voltmeter as I described earlier. Or a 12V test light will work if you don't have a multimeter.

As for the pin-out, you should have a pink wire with a black stripe going to cavity #16. That feeds power to the fuel gauge (and also to the brake warning light). Ground for the fuel gauge should be a black wire going to cavity #10. That's shared with several other gauges/lights so a problem there would affect more than just the fuel gauge. The pink fuel gauge sending unit wire stays located in cavity #18 for the large fuel gauge to the right of the speedometer.

[motornut]

I did my 78 idiot to tach and gauges,i had to add a ground for the gauge,because it moved.
in my case neg and pos switched.....your gauge dosn't move but the wires do,have a look at the 78 below,
*note*I think some wire colours do change later years

[jperdue]

I checked and I have everything correct. The part that confuses me is I have two wires cavity #18 and another fuel wire for cavity #7.. My old cluster only has one pinned out in cavity #18?

[hatzie]

Originally Posted by jperdue

I checked and I have everything correct. The part that confuses me is I have two wires cavity #18 and another fuel wire for cavity #7..
My old cluster only has one pinned out in cavity #18?

According to the GM wiring diagrams...
#18 is the fuel sender wire on idiot light & Tachometer clusters.
#7 is the fuel sender wire for gauge clusters.
Tach clusters show two wires in position 18. One of them is a jumper wire from position 18 to position 7 and the other is from the sender. Since position 18 feeds the small fuel gauge I doubt GM actually put the shown jumper wire in the harness. But I've never looked closely at a factory tach truck.

[jperdue]

Okay so I pinned my new cluster to #7 for the fuel sender that was originally pinned to #18 in my old cluster. And pinned a switch ignition feed to #18 which obviously gave the gauge power..in which case the needle went clear over to the right and doesn't move now?

[motornut]

my 79 K non tach/big fuel has the 7-18 jumper.
If the gauge is working and just the sender is off/broken wire it will read over full.
grounding the sender will read E

[hatzie]

John
Apparently they put the 7-18 jumper wire on the gauge equipped trucks at least up to 1979... The full pinout you show is only partially correct. You have the TEMP sender wire marked as "Fuel Grnd"

Jperdue
The sender (circuit 30) should be in 7 or 18 or both. Neither of those cluster plug positions should have anything else. Recheck your work.

This is the 1982 GMC CK wiring manual section that includes the gauge panels for all three variations. BASE=Idiot lights.

[jperdue]

I did some testing with a blot meter and with Oms on my gauge and the volts are overpowering the gauge and making it peg out to the right?

[ray_mcavoy]

Originally Posted by jperdue

I did some testing with a blot meter and with Oms on my gauge and the volts are overpowering the gauge and making it peg out to the right?

What are the volt and ohms readings you obtained? And from which terminals/locations were they measured.

Here is a picture of an empty gauge cluster housing with the fuel gauge terminal locations labeled.



With the fuel gauge temporarily removed for testing, you should have:

Full battery voltage (or close to it) between the terminals I've labeled +IGN and GROUND when the key is on.

And with your meter set to resistance (and the key off to be safe), you should get a reading somewhere between zero and 90Ω if you measure between the terminals I've labeled SENDING UNIT and GROUND.

The exact reading you get here will depend on how much gas is in the tank. But if it is significantly higher than 90Ω it indicates a bad connection somewhere between the gauge and sending unit (or a bad sending unit or poor sending unit ground). Any of which will cause the gauge to peg to the right.

[motornut]

Originally Posted by hatzie

John
Apparently they put the 7-18 jumper wire on the gauge equipped trucks at least up to 1979... The full pinout you show is only partially correct. You have the TEMP sender wire marked as "Fuel Grnd"

Ooops K I'll fix that

[jperdue]

I can't seem to get a very constant reading on the resistance. I've read any where from 4 to 150 ohms.