1968 GTX Temperature gauge indicating hot

[rikhemi]

I just replaced the temperature gauge with an NOS one that I had and when I turn the car on the gauge goes to Hot ???????

 

[super-bee_ski]

With engine off, send the gage sensor wire to GND
Verify is deflects to hot...(Pegs to HOT)
Try your old gage...
Do you have access to a thermal reader?

"Just because we buy new gages, doesnt mean it works"
Also, are you getting correct water flow?

How is your thermostat?

Do more research...then get back to us..

 

[Meep-Meep]

If the gauge goes all the way to hot range, either the voltage limiter has an issue or likely the sending wire (or sender itself) is going to ground. There is a silver box on the back of the PC board and that takes the 12V and crudely switches it down to 5V. The sender completes the circuit to ground through a resistor that adjusts based on temperature. Fuel gauge works the same way, with a resistor based on the position of a float in the tank.

 

[mopar 3 B]

And if your gas gauge is working properly the (limiter-resistor) is not the problem.

 

[Meep-Meep]

Just to clarify, the limiter is not a resistor. It controls the voltage to the gauges in a duty cycle method using a bi-metal contact arm. There are a set of points in that box with the movable arm made out of a bi-metal strip. As current passes through that circuit the contact arm gets hot and the bi-metal deforms causing the contacts to open. Once open the bi-metal cools and the contacts close completing the circuit. The result of the make and break connection averages out to about 5V. It is possible for the contacts to stick closed and send 12V to the gauges - not good for the gauges! By the way this is how the old turn signal flasher works.