Fuel gauge sending unit

[spyder_xlch]

The sending unit is your ground. Ground goes through a resistor controlled by the float. When the tank is empty you should have zero resistance and when it's full you should have 90 ohms resistance. To test it you'd have to tap into the wire for the sender (tan as Sunbirdman said) and put your other lead to ground. It's easier to test if the sender is out of the tank, you can move the float and see it the resistance changes. You could also start with no fuel in tank and add fuel. Or if you have a full tank measure the resistance and siphon fuel out to see if the resistance changes. You could tell if the guage and all the wiring are good by unhooking the wires at the tank. The guage should go to full, or past full. Ground the tan wire and the guage should go to empty.

[SunbirdMan]

The sending unit is your ground. Ground goes through a resistor controlled by the float. When the tank is empty you should have zero resistance and when it's full you should have 90 ohms resistance.

clintjohns had another thread on this topic and said the symptom he is having is the needle is pegged out past full.
So if the sending unit was not grounded, or had an extremely poor ground, the resistance would be higher than 90 ohms. Or if the current seeking ground was not going to the tan wire in the first place. That's why I suggested to see if there is current . If zero resistance is empty, he could ground the tan wire with the car running and see if the needle drops to empty. That would not require measuring anything and would determine if the gauge is working. If the guage drops then the problem is probably at the tank. In fact I recently had the same problem. I replace the elctric pump and the float assembly. The assembly I had was sitting out for several years and the terminals had a slight rust to them. I guess the fuel pump was grounding thru the tank to the chassis but until I cleaned the ground terminal where the connecter plugs on, the gauge was pegged out.

[clintjohns]

SO I UNPLUGGED THE WIRE IN THE TRUNK GOING TO THE SENDING UNIT. I TOOK A JUMPER WIRE AND GROUNDED THAT WIRE OUT AND THE GAS GAUGE SLOWLY WENT FROM WIDE OPEN (f) TO EMPTY SO I KNOW THE WIRE TO THEGAUGE IS GOOD AND THE GAUGE FUNCTIONS SO IT MUST BE A BAD SENDING UNIT. I HAVE A GUY SENDING ME ONE, BUT IT IS OUT OF A CAR THAT DIDN'T HAVE A ELECTRIC FUEL PUMP/SENDING UNIT ALL TOGETHER. SO MAYBE I CAN RIG IT ALL BACK UP AND MAYBE SWAP SOME PARTS OVER.

[spyder_xlch]

Yes, if your guage reads full now and you ground the wire it should go to empty. If so your guage and wires are fine. If this doesn't make the guage move then it could be a bad guage or wires. You'd have to take the guage out to check it and eliminate the possibility of bad wiring.

[HI WINDING MONZA]

I dont know if this will help you, but my guage read over full after it had been working fine and sat for a while and the wires to the sending looked ok.

It was driving me nuts because I had a newer Autometer fuel guage installed for years and it was allways working correctly.

I finally found that the ground wire to the sending unit tang had rusted so bad even though it still was attached to the sending unit , the ground on it was NOT GOOD enough.

I cleaned up one of the metal outlet tubes on the sending unit with sandpaper, wound the ground around a few times and reattched the wire with a small mini hose clamp so it was on a CLEAN ground on the sending unit itself.

Doug in AZ