Fuel System: how does the sending unit work for the gas gauge?

[GasWorksGarage]

Is it a ground wire that runs of sending unit and creates resitance as it goes up or is it a hot wire? How do i test the unit?

Thank in advance.

[345 DeSoto]

Hot wire...the tank is the ground.

[swvalcon]

It basically and adjust able rheostat. When empty it shows zero ohms and as the gas in the tank moves the float up towards full the sending unit increases the ohms and this is what moves the gas gauge.

[thunderations]

The wire going to the sending unit in the tank sends an ohm reading to the gauge. The sending unit is grounded by being bolted to the tank which is bolted to the frame. Sometimes this ground is not good and running a jumper wire from a screw holding the sending unit into the tank to a good ground helps. There is NO power wire going to the tank, which could create a bomb.
Test the sender by removing it from the tank. Ground the sender with a jumper wire. Raise and lower the float to change the ohm reading. By leaving the wire on to the gauge, you can see if the gauge is working by raising and lowering the float. It takes a little time for the change to take place, so be patient. If the gauge moves, both the sender and the gauge are working. Another problem is the cork float over years may become fuel logged and sink instead of float. Plastic and copper replacement floats are available to correct this.

[bensherb]

Hot wire...the tank is the ground.

It depends on whether you car is positive or negative ground, and how you define "hot wire".

[GasWorksGarage]

Well I found a red wire coming out of the harness that has been cut. There isnt a wire in the unit. I took a test lead and with it hooked the wire to the unit, turned on the key no movement on gauge. I took a test light hooked it to the red wire turn on the key and the test light come on. So its a hot " live " wire.

[kxet]

Hot wire will spark when touched to ground.

[Dwain G.]

Studebaker gas gauges don't respond instantly. Give it a full minute to see if there is any needle movement.

[thunderations]

Don't play with that "Hot" wire around the tank. There are no positive charged wires going to or from the fuel tank sending unit. Any positive wire would be behind the dash where an oops won't cause a big boom.

Well I found a red wire coming out of the harness that has been cut. There isnt a wire in the unit. I took a test lead and with it hooked the wire to the unit, turned on the key no movement on gauge. I took a test light hooked it to the red wire turn on the key and the test light come on. So its a hot " live " wire.

[bensherb]

What year is your car? Positive or negative ground? In my experiance with Studebakers, at least with Hawks before '63 many of the red wires are ground. It makes it hard when you're use to other makes, or when previous owners used red wire for everything and anything they did, as was the case with my '62. The fuel sender wire of my '62 is red with a black tracer at the gauge, but by time it gets to the sender it's black. I think it changes at the connector at the rear bumper. It's the ground side of the circuit, the power side, on the other gauge terminal, is white with a black tracer.

[gordr]

If you unplug the wire from the sending unit, and meter it with a voltmeter, you will see 12 volts. But because there is a fairly high resistance in the dash unit, it can't deliver much current; maybe 1/10 of an amp. Not enough to make a spark.

[GasWorksGarage]

well, I cleaned the terminal and reattached the test lead turned on the key and the gauge moved. its a 58 Packard hawk. according to the wiring diagram the wire running from the gauge to the unit is red.