The air-conditioning compressor is switched on and off by DDE control unit via the A/C-compressor relay. The DDE control unit receives a message from the heating and A/C control unit as to whether the air conditioning system button is pressed and whether the heating and A/C control unit is requesting that the compressor be switched on.
If a request is received to switch on the A/C compressor, the DDE control unit checks whether conditions that do not permit the compressor to be switched on apply and sends a corresponding message back to the heating and air conditioning control unit.
With the air-conditioning compressor switched on, the idling speed is increased.
The line between the DDE control unit and the relay is monitored for open circuit and short circuit. Fault code ”A/C power output” is stored in the case of defect.