Gas gauge voltage

[woody82]

My gas gauge is not reading properly.I hooked a hot wire from battery to the spot on the fuse box turned on the key nothing happened but maybe it went even more past full .I checked the voltage from the wire that goes to the gas tank It reads 5.3 volts. is this normal or should it be 12 volts?

[VetteVet]

Originally Posted by woody82

My gas gauge is not reading properly.I hooked a hot wire from battery to the spot on the fuse box turned on the key nothing happened but maybe it went even more past full .I checked the voltage from the wire that goes to the gas tank It reads 5.3 volts. is this normal or should it be 12 volts?

Don't put any voltage on that spot. That is the ground side of the fuel gauge. Connect a ground wire there and read the gauge. You should see the gauge go to empty with the key on. The wire from there to your sender is a ground wire, that goes to the tank sender, and runs through the resistor in the sender to the tank, and to the frame to complete the circuit.
If you put voltage on that wire, it will heat up the resistor in the tank and could act like an igniter on a gas stove. You know what that means!!!!!

If the sender is working properly, and is grounded to the tank, and the tank is grounded to the frame, the gauge should read correctly. If you get the empty reading on the gauge by grounding the FUEL terminal on the fuse panel, then ground the sending unit wire at the tank and check for the same reading.

If you get it, then the sender is bad, or the tank and sender are not grounded to the frame. If you don't get a reading then the wire may be open and not connecting to the sender. If so run a separate wire from the tank sender to the fuse panel FUEL terminal.

You can also test the sender by removing it from the tank and connecting the sending wire to it and grounding the flange to a good ground and then move the float arm up and down and watch for the fuel gauge needle to follow the float arm.

[woody82]

Okay so if I am getting a reading of 5 volts at that terminal then I better unhook it and leave it until I figure out where the voltage is coming from?
I must have a bare wire touching body somewhere?

[VetteVet]

Originally Posted by woody82

Okay so if I am getting a reading of 5 volts at that terminal then I better unhook it and leave it until I figure out where the voltage is coming from?
I must have a bare wire touching body somewhere?

You're getting a voltage reading because the 12 volts going to the fuel gauge is going through a resistor on the back of the gauge before it goes to the fuse panel. It's still positive voltage but the reading is reduced by the resistor on the gauge. When you ground the fuel terminal, the circuit from positive to negative is completed and the gauge will read empty.

Running the wire from the fuse panel to the sending unit adds in the variable resistance in the sending unit which varies from 0 to 90 ohms and the gauge should reflect this, depending on where the float arm is in the tank.

In your original post you said you put voltage on the terminal in the fuse panel.
That's the reason I said don't do that. You are bypassing the fuel gauge resistor and sending 12 volts into the fuel tank. This could overheat the variable resistor in the tank and ignite the fuel.

here is a diagram of the fuel gauge wiring circuit.



Notice how the current goes from the battery positive to the fuel gauge and through the resistor and then back to the fuse panel and then on to the sending unit. It doesn't show the fuse panel but after the wire leaves the fuel gauge it goes back to the fuse panel. Then from there it goes to the sending unit in the tank. If this wire does not connect to ground the gauge will show over full.