Hi Stephen0523, here's a few things for you to check, not in any specific order.
Do you have a voltmeter or ohm meter? One would be highly desirable.
There's a small ground wire which runs between the fuel sending unit on the tank and the frame. The wire can corrode inside it's insulation, leaving you with what looks like a wire, but which has been reduced to a hollow vinyl tube! The ring terminal at the end of the wire is held to the frame by a screw which can corrode, blocking current flow. It might be worth unscrewing it, cleaning the terminal, then applying some antisieze or dielectric grease to the terminal and screw, then tightening it back up. Bad grounds are by and large the largest electrical issue vintage Dodge owners encounter...
Are your other gauges working? (Oil pressure, coolant temperature) If not, on the circuit board behind the instrument cluster there's a small mechanical voltage limiter/regulator. It's bolted right into the circuit board. You should measure battery voltage on one side, and a slowly pulsing voltage on it's output... The idea is to have around 5 volts output on average. This allows for the gauges to maintain some level of precision regardless of the state of charge of the battery. The other reason for this bizarre device is that the gauges aren't normal magnetic meter movements; they're thermal devices like thermometers, so the average amount of heat over time is what moves them. The mechanical regulators can last for decades...
The big round multipin connector which mates to the pins on the instrument cluster circuit board might be worth (carefully) reseating. Keep in mind that decades-old plastic becomes brittle, especially in an automotive environment. If it's never been removed, I'd consider using a screwdriver under opposite sides of it to push it up carefully, slightly, one side at a time, -after you disconnect the battery terminal!! And be careful with the plastic nose of the speedometer cable - you *don't* want to break it!
Inspect the base of the large pins on both sides of the circuit board. If the phenolic circuit board, Copper circuit board traces, or the silvery gray solder has developed noticeable cracks or crystallization, or milky white rings, they will require repair/resoldering.
There's not much else to the system other than the sending unit, the long run of wire up to the bulkhead connector, and the bulkhead connector. As with the sending unit ground, that long wire can corrode out inside.
You should be able to probe the terminal right at the bulkhead connector to see what sort of voltage or resistance is present. Disconnect your battery first if you're trying to measure resistance.
The pins inside the bulkhead connector are certainly subject to corrosion, since it's only marginally sealed. It's not the simplest thing to take apart, so I'd put an ohm meter on the sending unit to see if you get a reading before I'd consider wading into the bulkhead connector.
There were several different values of resistive element used in the sending units. You would need to have a decent idea which maximum value you've got to try to specify a replacement. The values are usually pretty low; you'd be using the lowest scale on an ohm meter to identify most sending units. The minimum resistance is at full tank, maximum at empty, so you'd either need to empty your tank or pull the sender.
Oh, the gauges themselves are nearly immortal. Not especially precise; the spec for an acceptable reading gauge is 3/32" of needle travel! Of course they claimed this was a 'feature', as it typically left you with a slight reserve of fuel when the gauge read empty...
A volt ohm meter is definitely your best friend for troubleshooting this system. At least it's an extremely simple circuit.
Last edited by Ram4ever; January 10th 2021 9:26 pm. Reason: spelling
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