Fuel System: Gas Gauge or Sender Faulty

[my2R5]

I have a single wire sending unit in my '51 pickup. The gas gauge does not show any signs of life. I get power at the gauge when I turn the ignition key, as well as power at the sender (based on a test light). Grounding the fuel tank did not change anything but the gauge went to full when I grounded the sending unit. Not sure if this means my gauge or the sending unit is at fault. I've read a number of threads but most deal with 2 wire sending units and hoping someone can assist. Thanks.

[thom]

Exactly what do you mean by "power at the sender" ? I don't believe there should be "power" at the sender. The sender should offer varying amounts of "ground" depending on the position of the float. A test light touching a source of power and grounded to the sender should vary in brightness as the sender moves as long as the sender/tank is grounded to complete the circuit.

[gordr]

Sounds like you have a bad sending unit. You are seeing voltage at the sending unit, and the gauge goes to full when you you ground the sending unit wire, so the gauge is presumably good.Yes, you do see voltage at the "sender" stud on the gauge, and at the other end of the wire. The gauge is "looking for" a path to ground through the sending unit, and the only tool it has to find that path is the voltage from the "ign" stud. There is resistance within the gauge, so open-circuit voltage will be equal to supply voltage, but with any normal amount of resistance in the sending unit, the voltage will drop to a fraction of that. If you ground the sending unit wire, the voltage on the "sender" stud will drop to zero, and all the system voltage will appear across the resistor (in fact a tiny heating element) in the gauge, and make it peg out past "full".

[my2R5]

Thanks Gord. Great explanation. So I need a new sending unit. I don't suppose these are fixable are they?

[2R2]

First step would be to remove the sending unit from the tank, ground it, hook up the wires, and manually move the float rod to see if the gauge registers. If so, then I would imagine your old cork float is bad. These can be replaced with a modern plastic one.

[chet445]

Interesting thread. My gauge and sending unit work great on the bench but in the car the gauge reads full. Can't figure why. I replaced the sending wire thinking it had a quirk...no luck. Both the gauge case and sending unit have additional ground wires making sure the units are properly grounded. '62 Hawk. Any thoughts?

[gordr]

Sometimes they are. I did a thread about it, with pics, IIRC, many moons ago. Depends if the wire-wound resistance element in the sender is intact. The "wiper" contact, which is attached to the float arm, and sweeps the resistance element (a tapered strip of fiberboard wound with fine Nichrome wire, about an inch long) had a "button" stamped in one end, so as to make point contact with the wire. Over time, that button wears through, leaving a hole, and eventually, it can wear enough that contact is lost altogether.I discovered that I could take the unit apart, ream the hole in the wiper a little bigger, and then solder a tiny round-head copper or brass rivet into the end to make a new contact button. I think I also used the melted-round end of a piece of brazing rod in lieu of a rivet. You need something round and smooth as a button, with a radius of maybe about .025", so the contact button will glide smoothly over the wires, and not snag.

[gordr]

Are you sure that you have not got the "ign" and "sender" studs on the dash unit interchanged? Just try swapping the two wires briefly, to see what happens.

[chet445]

Well I can say both the gauge and sender are new and work on the bench however the gauge tops out when installed on the car. Gremlins??

[StudeRich]

Try removing the un-needed Case ground on the Dash Unit, and you may be good to go.