Yesterday when I went to take my '69 out for a little Sat afternoon crusin', as soon as I turned the key to the "on" position (engine hasn't started yet), the temp gage reading went immediately from C to beyond H to the fully pegged position to the right. Car was obviously not "overheating" as I hadn't even started it yet. I disconnected the wire from the temp sending unit, turned the key back to the "on" position, and the temp gage continued to read fully pegged to the right. With the wire disconnected, it should have come back to read C.
The temp gage circuit functions based on the variable resistance of the temp sending unit as a function of engine temp. Cold engine, higher internal resistance in the temp sending unit, and the gage should read C. As the engine heats up, the internal resistance in the temp sending unit decreases and the needle on the temp gage moves to the right.
I have verified the circuit operation to myself in the past by warming the engine up, noting the temp gage reading, and then disconnecting the wire at the temp sending unit. As soon as you disconnect the wire (open circuit - high resistance), as you would expect, the temp gage reading would instantly go to C; reconnect the wire to the temp sending unit and the temp gage goes back to reading normal engine temps.
I've checked the resistance of the temp sending unit in the car (135 ohms with engine temp of 190F; 530 ohms with cold engine) and compared the "cold" reading to a spare sending unit that I have. The spare unit also inidcates approx 530 ohms ambient conditions.
Bottom line, I do not believe the problem is in the sending unit but rather the problem is the gage itself. I'm basing this on the similar resistance values between the sending unit in the car and the spare one that I have, and more importantly, the fact that when I disconnected the wire from the temp sending unit (open circuit - high resistance), the temp gage does not come back and read "C", but stays pegged to the right which indicates a "low" resistance. I have also checked the resistance through the "green" wire that connects to the temp sending unit to the ground at the battery and get about 135-140 ohms - basically measuring the resistance of the wire and the internal resistance of the gage. Does anyone know if this reading is "normal"? I'm guessing that it is lower than normal and is telling me something about why the gage is staying pegged to the right.
However, before taking the console apart to get the gage cluster out and change the gage, I wanted your thoughts in case I'm missing something.
Thanks in advance.