Fuel Level Gauge Question

[Guest Classic Rog]

Hello, My 1955 Ford wagon has fuel level issues. Stays on full (actually past full) even after I put in new sender. Will return to empy level when I turn off key. Notice gas in tank (6" deep) was about 2" below full when I put sender in. Wondering what else I could check. Maybe I'm jumping the gun a bit. Maybe I'll drive some more gas out of it and see if level needle drops. Seems like the needle shouldn't be that far past full mark. Any ideas? Thanks:confused:

[Guest Commodore]

Well, on the Ford gas gauges, if you a full reading all the time that means you have short to ground somewhere. Note a short to ground may damage the gauge. To test remove the wire from the gauge and the sender and check the wire for a short to ground with an ohmmeter.

[Friartuck]

There may be a short somewhere in the circuit. This could be the wire from the sending unit is touching ground somewhere or the sending unit is defective like it's getting hung up at the full position. Remove the wire from the sending unit and observe the gauge. The gauge needle should not move (reads empty). If you have access to an electrical multimeter, measure the resistance between the sending unit connection and ground. Typical resistance values should be zero ohms for full tank (float at highest point) and somewhere between 25 and 100 ohms (or more) when the tank is empty (float in lowest position). Try shoving the car side-to-side to move the gas in the tank and and the float and observe the multimeter. Does the multimeter show varying results proving the sending unit is working properly. Was the replacement sending unit the correct one for that vehicle or a generic one? Did you connect and operate the sending unit before installing in the tank, going from float at highest to lowest? bending the arm maybe all that is needed. You may have to include a small resistor in the wire to the sending unit. This would calibrate the gauge to a full tank of gas and the gauge reading exactly at the full mark. This is a trial and error procedure. Start with a single 10 ohm resistor and observe the results. Add higher value resistors to adjust the needle to the full mark. When the sending unit was made, they should have taken this into account, but its not an exact science. An exact method is to use a 0-100 ohm variable resistor and adjust the knob for a full tank reading. Then remove the variable resistor and read that resistance value when disconnected from the tank using the multimeter. Get a single resistor closest to that value. If you use 1/2 watt resistors you should be fine. Please report back what you find. This is a good learning type question.

This thread reveals some useful info: http://www.fordbarn.com/forum/showthread.php?p=662674

Edited August 7, 2014 by Friartuck
added link (see edit history)