Most fuel gauge circuits of that vintage can be diagnosed by disconnecting the sender wire, with it disconnected the gauge should go to one extreme, with it grounded it should go to the other. If there is no change then the next thing to check is the wire from the sending unit to the gauge. If the wire is ok then the circuits to the gauge is the next step. There will probably be 2 more terminals in the gauge one will be a ground the other is a power supply. The power wire is usually a lower voltage some are around 8 volts this comes from a small voltage regulator for the gauge. A voltage regulator is used instead of battery voltage so that the gauge does not fluctuate as the battery voltage does when driving. Some gauge circuits share that voltage regulator with other gauge so other gauge would not be working if it was a problem.
Another problem that can cause gauge so to be inoperative or read incorrectly is if diesel fuel has leaked through the terminal or wires on the sending unit, diesel fuel becomes conductive as it absorbs the moisture in the air, cleaning with brake clean would only be a temporary fix.
Checking the sending unit, they should have a resistance of 20 to 120 ohms depending on the position of the float. Note: depending on the type of float used they can saturate with fuel and cause incorrect readings or read empty if they are sunk.
the sending unit may have 1,2,or 3 wires on it. A common ground, the fuel level wire and sometimes a power wire for an internal lift pump.
If the resistance spec is known the accuracy of the gauge circuit can be checked by putting a resistor of the approximate values between the gauge wire and ground wire.