Fuel Gauge Constantly Reads Full

[Gregski]

I'm sorry for asking I should know better, but should my brand new sending unit inside the fuel tank in the cab behind the seats have a ground wire or is it grounded simply by touching the tank itself which in turn is grounded by touching the cab, I believe my brand new Intelitronix digital gauge is reading 99 at all times do to a bad ground issue, but I may be wrong. 1971 GMC 1/2 ton short box with cigarette lighter, ha ha

[Mbeef61]

In cab tanks are grounded by the tank.

Could there be a mismatch in ohms between your sending unit and gauge? Both need to be the same ohm rating. 0-30 or 0-90 etc

[Gregski]

Originally Posted by Mbeef61

In cab tanks are grounded by the tank.

Could there be a mismatch in ohms between your sending unit and gauge? Both need to be the same ohm rating. 0-30 or 0-90 etc

thank you so much for the quick reply, I am using the same gauge in my '74 and it works, could GM have switched ohm ratings in their sending units from second to third gens? the '74 sending unit has a separate external ground wire that bolts to the frame

[RustyPile]

What does Intelitronix say about the resistance "range" their gauge needs. As previously stated, the gauge and sending unit resistance reading range have to match. GM uses 90 ohms, while Ford and others use 30 ohms.. The stock sending unit in your truck is a 0 - 90 ohm range.. The gauge has to have a 0 - 90 ohm range also.

As for a ground circuit, the sending unit normally doesn't need a ground wire.. The unit touches the tank and the tank is grounded through the mounting bolts to the cab. You can test for a proper ground by using a jumper wire of suitable length with alligator clips on each end.. Clip one end directly to one of the tabs on the sending unit and clip the other end to a bare metal area of the cab.. If the gauge then reads properly, you'll have to make a permanent ground wire for the sending unit.

[Mbeef61]

I do not know about that gauge. The manufacturer should be able to tell you the ohm rating. Did you install the gauge in your other truck? If not the other owner may have changed the sending unit to match. Also some gauges come in other ohm ratings. Maybe you got the incorrect one.

You have to make sure the ohm rating are the same for each. The manufacturer of the gauges should be able to tell you what yours is. You can test your sending unit when it’s out.