I have a 64 with the temp gauge pegging full hot with the key off. With the key on or engine running it reads normal. Shouldn't it go to full cold with the key off? Is it a bad gauge?

 

Welcome to the forums! Not getting a flood of comments I see. I can tell you what I think, I have no Idea. I never had a 64 to compare with, but most gauges do 3 things, stay where the power was discontinued, or move to the left or right. My opinion is that what you have is a gauge trait. You could replace it if you want a different result or the new one might do the same thing. It`s up to you. You didn`t say when it pegged, was it a slow drift to hot or a quick slam to hot? (maybe a short if it slams to hot ) You can pull the gauge and see if it acts the same way. If it reads correctly while running I would say leave it alone. Good luck let us know what you find out or decide.

 

Welcome to the forum. You could check the voltage at the temperature sensor with the key off.

 

While I haven't diagnosed a '64, virtually all fuel gauges from this era work the same way regardless of manufacturer and model. A fuel gauge is simply a meter that measure current (ammeter). One side of the gauge has power to it (12V positive), the other side of the gauge has a wire that goes to the sending unit in the tank. One side of the sending unit goes to ground. Remember the old formula V=IR. where v is voltage and I is current, R is resistance? Reordering the formula gives I=V/R. The sending unit is nothing more than a variable resistor. When the tank is full, the resistance of the sender is the lowest, thus more current "I" through the fuel gauge, i.e. "Full". As the fuel is consumed and the level in the tank goes down, the sender resistance increases, thus lowering the current. The highest resistance reading is when the tank is "Emply" (thus the smallest amount of current "I" through the gauge).

You either have a gauge that isn't correct for the car (reverse reading) or you have a heck of a wiring issue.

1. Per the other responses, with the ignition off, put the negative voltmeter probe to a solid ground. Put the positive probe to one of the terminals on either side of fuel gauge If you have a positive voltage on the gauge, it is time to take out your schematic from the shop manual and figure out what is happening and why you have voltage at the gauge with everything ostensibly off. This will eventually discharge your battery. But even with voltage on the gauge, for it to peg to full, it needs to see "zero" resistance, i.e. a short. Why would there be a short with the ignition off but not otherwise (the fuel gauge works!). If the gauge is right for the car, I really suspect funky wiring.
2. I don't think you have a bad fuel gauge, because if it were bad it would either behave as a dead short (zero resistance) or a complete open (infinite resistance). Even if your sender had a dead short, the fuel gauge wouldn't register full (high current) with the key off unless you ALSO had voltage at the fuel gauge because you need voltage on that one side of the gauge to make a current go through it. No voltage, no current.

Good luck with your hunt!