Instrument cluster

[ray_mcavoy]

Originally Posted by tdangle

I belive ray_macvoy has it backwards for the fuel guage. The guage reads E with no ground to the guage. if you short it to ground it will read full. I know this because when my sending unit went out the guage read E with the wire disconnected and full when shorted to ground.

Originally Posted by VetteVet

Ray and I have both been helping trouble shoot these gauges for several years and yours is the first that I have ever heard of that reacts this way.

Yes, I've never come across a GM fuel gauge that has reacted like that either.

Here is the basic circuit for the 0 - 90Ω GM fuel gauges (the earlier 0 - 30Ω gauges are wired the same, only the resistance values are different).


As you can see, shorting the sending unit connection to ground bypasses the "full" coil and causes all the current to go through the resistor on the back of the gauge and the "empty" coil. That pulls the needle all the way to E.

Now, there are some non-GM fuel gauges that work with sending units that have a higher resistance when empty than when full. Those would read backward compared to a GM gauge when the sending unit lead is disconnected or when it's grounded.