Fuel gauge troubleshooting

1973 C20 with factory dual tanks. The truck has been in my family since ‘84 and most everything is factory or stock. The wiring is not modified or hacked up. The fuel gauge worked fine for both tanks until one day the needle started pointing to the 3 o’ clock position for both tanks. I am aware that going past full means high resistance or open circuit.

In the trouble shooting process I’ve replaced the gauge with one from LMC and the selector switch with one from Napa. The switch over valve is getting power and works fine.

I’ve been trouble shooting it on and off for a couple of months now and can’t figure it out. I pulled the cluster out yesterday and checked the supply voltage going to the cluster and it’s 12V and the gauge is receiving 12V at the gauge clamp via the printed circuit. However, on the gauge output that goes to the switch / tank I’m only getting 8.5V. I’ve tested both the factory gauge and LMC replacement gauge with a known good working sender and both work correctly on the test bench. Is 8.5V correct for the output voltage of the gauge?

I don't really know of any "output" test there. It is the load side, and, when hooked up and properly working, (the senders) may read as low as a volt or 2, as the sender "grounds" it.
I suspect a wire/connection problem in the wire from the dash to the switching valve, or the switch itself, as there were 2 different setups.

Robert Bare said:

I don't really know of any "output" test there. It is the load side, and, when hooked up and properly working, (the senders) may read as low as a volt or 2, as the sender "grounds" it.
I suspect a wire/connection problem in the wire from the dash to the switching valve, or the switch itself, as there were 2 different setups.

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I’ve never heard of an output test either, but in the spirit of checking everything I tested it and would’ve expected 12V in, 12V out. I checked the terminal on the plug that plugs into the switch and it’s getting the same 8.5V I measured at the prong on the gauge going to the sending unit. I’m starting to the think there’s a broken wire somewhere.

Ya, I think the 6 wire solenoids switched tanks senders, where the other way the dash switch had lots of wires. But I may be wrong, been a long time,lol. I do remember the 6 wire connector being troublesome, when older.

dsteelejr said:

I’ve never heard of an output test either, but in the spirit of checking everything I tested it and would’ve expected 12V in, 12V out. I checked the terminal on the plug that plugs into the switch and it’s getting the same 8.5V I measured at the prong on the gauge going to the sending unit. I’m

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I've been chasing the same issue. Mine ended up being the wire out of the main bulkhead fitting (18-TAN30) that splices into the sending unit supply wires further back. Or, so I believe. I yanked it when I did my resto for lack of knowledge. Use to run down the passenger side frame rail. Did a test connection today with some test leads and boom gauge works on both tanks.

Dumb me thought it was part of the old valve selector solenoid when I was removing all the carb stuff. Cost me a lot of troubleshooting, and some new parts for no reason....Either that or I just made a time bomb. I assume the senders have some voltage across them to ground to get the resistance measurement. I'm no electrician.

8.5v is what I get off my circuit as well. I believe thats to do with the resistor in the gauge so I haven't worried about it. Someone can chime in if I'm mistaken.

Seems like your problem could be similar to mine. That circuit (TAN30) looks like it goes all over the place in a giant loop on the schematics. Could make sense if both sides quit. Either that or somehow both sending unit wires broke at the same time.

Just my $.02