Fuel gauge wiring

[Tricky's Zzzz]

When i got my 73, the dash wiring was messed up. I put new guages, but the fuel guage is not working. Reads past full. Is there seperate wiring that goes to it, or does it work off the printed circuit board? Thanks

 

[Twisted_Metal]

"Past Full" means you have an open connection.

A fuel gauge is just an Ohm Meter which measures the resistance at the sending unit.
If any connection is broken, the resistance will be infinite and the gauge will read off the full side of the gauge.

Start troubleshooting at the middle of the circuit.
The wire in question is a tan wire and it should be connected at the rear harness above the driver's side fender well, in the trunk.

If you ground that wire (with the key "ON") and the gauge goes to "Empty"... The problem is somewhere to the rear of the car.
If grounding the tan wire at the rear of the car makes no difference... The problem is with the cluster connection or the tan wire someplace between the cluster and rear harness connection.

The second place to look is under the car, by the fuel tank.
There should be a black wire (ground) coming from the sending unit and screwed to someplace near the tank. (Frame, trunk floor etc.)
A bad ground connection will exhibit the same "Past Full" symptom.

Good Luck!

 

[Tricky's Zzzz]

Thanks for the reply. I am also wondering on the gauge itself, is there any external wire going to it or does the tan wire go into the harness, and run by the printed circuit board?

 

[Twisted_Metal]

It's through the printed circuit board.

You are apparently getting power and a ground to the gauge itself.
Which tells me the tan wire circuit is the culprit, whether your car has factory gauges or "Idiot Lights".

That circuit includes the sending unit and its ground.

I would start in the middle of the circuit... It's easier to access than the gauge cluster or the fuel tank.

 

[Tricky's Zzzz]

Gotcha, appreciate the help, i see the wire coming through the trunk, i will ground it there. Thanks.

 

[Tricky's Zzzz]

Ok, i grounded the tan wire in the trunk going up to the dash with a test light and the guage still read past full and the test light lights up dull. Where should i go from here? I disconncet the tan wire t the connector before grounding it.

 

[Twisted_Metal]

Ground the tan wire leading to the cluster plug.

If there is still no change... It's probably the connection at the gauge cluster.

 

[Tricky's Zzzz]

I did ground it it going up to the guage at in the trunk, should i go under the dash and try it there. Is the test light supposed to light up when grounding the tan wire? It lights up dull.

 

[Twisted_Metal]

Yes, ground it under the dash. (That will eliminate any problem with the wire from the dash to the trunk if the gauge goes to empty.)
There is a small amount of current through that circuit so the light will glow.
You are adding the resistance of the bulb but that shouldn't be much.

I would try to ground it directly, without the test light/probe.

 

[Tricky's Zzzz]

Ok thanks.