Fuel gauge issues, puzzled

[trailturd]

I recently acquired a 87 comanche that I swapped to 4wd. I noticed that when the motor is running the fuel gauge is pegged past full, but if I turn the key to the 2nd position, just before starting, the gauge seems to read correct. I know that the always full reading points to grounding issues, but what about what I have going on? Planning on doing this procedure outlined below.

 

The official test procedure from the 1988 FSM is:

 

1. Disconnect C139 -- Needle should go to Full. If the ground to G197 is okay. replace sender. If not ==>

 

2. Connect C139. Test C203 terminal 15. Reading should be between 0 and 88 ohms. If okay, replace gauge. If not okay, repair open to sender.

 

 

The simplified version of that is, unplug the 3-wire connector outside the fuel tank. That gives you an open circuit on the sender (infinite resistance). The gauge should read Full with the connector unplugged. If so, jumper the small wire (violet?) to black on the chassis side of the connector (zero resistance). The gauge should swing to empty. If that doesn't work, try jumping the violet directly to the chassis (making certain you get a good ground).

 

On the tank side of that same connector, put an ohmmeter on the violet and black wires. That's the circuit through the sending unit. The reading should be somewhere between zero and 88 ohms. (I don't think the scale is linear, though, so don't assume that a half tank will read exactly 44 ohms.)

 

[Eagle]

That scale should be linear -- unlike the temperature gauge.

 

[Pete M]

you can try cleaning the fuel pump ground which is behind the drives side taillight.