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.