If you have owned an early A-body car long enough you probably have experienced the failure of the gas and temp gauges. The cause of the failure of both gauges is the instrument cluster regulator. This regulator maintains a constant voltage to the gas and temp gauges so that they won't fluctuate when the battery voltage changes during charge and discharge cycles.
In these early cars, the regulator was built into the gas gauge, which made repair difficult. The regulator is actually a bimetal spring with a coil wound around it. As the current going through the coil heats up the spring, it breaks a set of points that disconnects the voltage from the gauges. The output of the regulator is actually a pulse of about 4 volts (2 per second). In 1967, Chrysler made the instrument regulator a unit that plugs into the back of the instrument panel. This article shows how to use the 1967 regulator to repair your gauges.
NOTE: If your gauges work but read low, this repair will probably not restore full functionality to the gauges. This is because some of the current from the replacement regulator will feed back into the old internal regulator. On one car I tried with partially functioning gauges, it brought the readings up closer to normal, but not all the way. If your gauges are completely dead, this should restore full functionality.
My test setup was the instrument panel from a 1963 Dart. You should be able to duplicate this setup on a '63-'66 Dart, Valiant or 'Cuda. It may also work on the '60-62 Valiant and Lancer, but I do not have any other instrument panels to play with. You will need to find the appropriate places to connect the new regulator. You can also do this while the panel is installed in the car if you can get your hands back there to attach the wires. (Disconnect the battery first!)
You will need 3 "spade" connectors, 3 "ring lugs" (#8), 3 pieces of wire about 6" long and the new regulator. Use a crimping tool to assemble the connecting wires. The killer here is the regulator. At one store I paid $27!
The bottom of the regulator has one terminal marked "I". This connects to the ignition circuit. The terminal spot-welded to the cover is the ground, and the remaining terminal is the regulator output.
Shown below is the unit attached to the instrument panel.
It would also be a good idea to tape up the regulator to prevent shorting to other terminals on the panel.