Fuel gauge failure diagnosis

[BDR]

1973 Series III 88

My fuel gauge suddenly stopped working. I've been through all the fuses and they seem to be okay. I uncapped the cover plate in the passenger seat and am now starting at the (nounted) sending unit and the connecting wire.

Before I bust into the fuel tank to replace the sending unit, I thought I'd best test the wiring to the panel, and perhaps test the sending unit itself. Problem is ... I don't quite know how to go about this, as I'm unclear exactly how a sending unit works. Should I simply be receiving varying voltage levels based on the fuel state? Since there's a single lead, how could I send (or receive) a power signal from the sending unit to confirm that it is or is not working?

Thanks,
Don

[siiirhd88]

Power to the fuel gauge circuit will be from a solid green wire off of the fuse box, going to the voltage stabilizer behind the instrument cluster. A light green with ? color tracer wire will go from the stabilizer to one side of the fuel guage. The other side of the fuel gauge will have a dark green and black tracer wire that will go into the harness. The dark green and black tracer wire should end up at your sending unit on top of the fuel tank. If the circuit is powered up and intact, the fuel gauge will go to full if you unplug the wire from your sending unit and ground the end of the wire. (The sending unit varies the resistance to ground as the tank empties.) If the fuel gauge doesn't go to full, the problem is likely an open in the circuit, and not a failed sending unit. IIRC, the dark green and black tracer wire is one of the wire connections in the multi wire plug connector going from the front harness to the rear harness. It could be cruddy.

HTH,
Bob

[a109]

Its well worth grounding the tank. Its a favourite series trick for the Tank to lose its ground due to rusty bolts etc. When this happens the gauge won't work.
John

[BDR]

Thank you both. I will investigate both suggestions.
Don

[BDR]

I have performed the recommendations of you both and it seems that the sending unit / transmitter is the culprit.

- The gauge goes to Full when the gauge wire is grounded.

- The tank appears to be grounded, as I can ground the gauge wire directly to the tank.

So, popping the portal to R&R the sending unit seems a little dangerous, what with metal-on-metal screwing action while fumes are percolating up. Painful as it may be, seems to me prudent to drain the fuel and fill the tank with water to evacuate the fumes. But then, will the water damage a new sending unit?

Or am I being paranoid? My last gas tank experience, attempting to re-solder a connector pipe on a Jeep Wagoneer gas tank nearly full of water after a couple complete rinse outs still did not end up well (boom!) But admittedly that was an open flame versus a metal-induced spark. Still, more painful to blow myself up than to tank up with water.

Thanks again,
Don

[siiirhd88]

"So, popping the portal to R&R the sending unit seems a little dangerous, what with metal-on-metal screwing action while fumes are percolating up. Painful as it may be, seems to me prudent to drain the fuel and fill the tank with water to evacuate the fumes. But then, will the water damage a new sending unit?

Or am I being paranoid?".......


I've never worried about the fuel in the tank when swapping sending units, just make sure the tank is not full... Don't do anything crazy, like smoking... Of course, your results may vary.

Check the float on your old sending unit. If brass, it might have a pinhole and could be repaired by soldering.

If they are really after you, are you still considered paranoid?

Bob