I recently purchased 77 300d. It hasn't seen a lot of use, just 51000mls but I have some climate control problems. I know the heater is designed not to work until the engine is heated up with the exception that the defrost overrides the temperature sensor.
The heater will not blow under any condition, even with the car warmed up unless the defrost button is first activated. The heater will then operate. It will continue to be operational even if I turn the car off until the point that the engine cools all the way down again at which point the defrost will have to be pushed first again. The heater will also not turn off once it is activated until the engine cools down again. The off button just seems to have no effect.
I'm assuming this is some problem with the temperature sensing device but I am not sure. Where is the sensing device and how difficult is it to replace? Does anyone have any other ideas about what may be malfunctioning?