heating controls do not work properly on a 928 S4

hello,
both controls of the heater are always giving hot and do not operate
somebody knows about this problem
thank you and regards

The heater control valve is a primary culprit when the output of the HVAC is always hot.   There are vacuum operated solenoids in the console just to the right of the top of the throttle pedal that may also need addressing.   Then, 2 or more vacuum pods under the dash that can fail.   Not a simple system.   I suggest a search of the Rennlist 928 Forum for lots of details.

Gary

The 928 heater hot water flow valve has a default "FULL ON" position.  So if there is a vacuum leak, or if the valve itself has failed, the heat will be full on.

I suggest first checking the heater valve.  It is located at the rear of the engine.

 

It looks like this.  Use a vacuum tester (such as a MightyVac) to see if it actuates and holds vacuum.  It it has failed, replacement may fix your problem.

The Porsche 928 also has several vacuum motors that move the diversion doors for airflow and temperature.  These motors fail and work to replace can be easy to having to remove the entire dash to replace.