'86 duel tank GMC Gas Gauge issue

[Ken Hepting]

When either tank is full fuel gauge reads 2/3 to 3/4 empty. Hence, I never know when reading "empty" really means, "EMPTY".

I have replaced (1) tank sending unit, dash switch and fuel distributor-switching unit with no success.

I suspect the gas gauge needs replacing since it indicates both tanks being faulty readings????

Thanks for any input.

 

[nerfherder78]

Most likely the resistor on the back of the gauge:
The GM Fuel Gauge Mystery 1965 & Newer |
Pull it off and see if the gauge moves.

There are 5 different color resistors, you'll want to replace it with the same color:
Lt blue-123 ohms
green -89
yellow-88
orange-86
blue black-81

I had one fail on the back of my voltmeter last year, threw me off when I looked down and saw 0 volts. Managed to find a replacement after scouring the local wrecking yard.