I've been attempting to run down a problem with my external air temperature sensor and its effect on the AC System output. My 1990 928 digital display shows the exterior temp as -40. Which cause the heater valve to be wide open while the AC output is trying to reach 65 degrees. The exterior sensor has been replaced, resulting in no change to the -40 display. System ac lines, compressor, dryer and freon all working fine. Electrical continuity to the external air sensor has been checked along with the heater valve and vacuum lines.
Any suggestion on what to checkout to fix the problem?
The dash display temperature has no effect on your automatic A/C - Heat control. The sensors involved are the inside sensor which is to the right of the center AC vent (behind the grill) and the external sensor which is actually located on the intake of the alternator vent tube (see the illustration below, item #23). Check the connection to the sensor.
There is a vacuum operated heater water flow valve located to the rear of the engine under the air filter box (illustration item #3). If the valve fails (doesn't hold vacuum) the fail safe is full heat. If the vacuum line comes disconnected, the valve opens to full heat. So this could be the cause of your problem. I suggest replacing this valve every 10 or 15 years along with the hose (illustration item #4) as the valve and hose do fail.