A/C Compressor Fails To Engage

Fault Symptom.
The A/C compressor clutch does not engage.

Functions/Definitions.
Information to the effect that the compressor is about to be engaged ("A/C IN") is received via Motronic control module pin 64.

The control module prepares for this load increase by opening the idle air control valve slightly.

Via pin 48 the Motronic control module then grounds the A/C relay which in its turn control tie A/C compressor ("A/C OUT" or "A/C RELAY").

Basic Requirement.
No diagnostic trouble codes.





Diagnostic Procedure.
1 Check the A/C request ("AC IN")

- Connect an ISAT scan tool.
- Start the engine.
- Contact the Motronic system.
- Select "READ ON/OFF".
- Select A/C IN".
- Activate the compressor from the dashboard:
^ Manual A/C: fan ON and A/C button depressed.
^ ACC: select the AUTO position Note that the antifrost thermostat will periodically interrupt communication between the Integrated Central Electronics (ICE) module and the Motronic system.
The ISAT scan tool should read "ON".

Does the ISAT scan tool read "ON"?

YES
^ Continue with point 2.
NO
^ The fault is located before the Motronic (between the A/C button or ACC control module and pin 12).

Continue fault diagnosis in the Climate Control System and the Electrical System, A/C-ACC.

NOTE: If the engine coolant temperature is too high or in the event of an incorrect engine coolant temperature sensor signal to the ICE, the ICE will break the A/C circuit.





2 Check the A/C relay ("AC OUT")

- Engine switched off.
- Turn the ignition switch to the ON position.
- Contact the Motronic system.
- Select "ACTIVATE".
- Select "AC RELAY".
Both the A/C relay and the compressor clutch should now be heard,

Conditions fulfilled?

YES
^ No electrical fault present.
NO
^ The fault is probably after the Motronic (i.e. between pin 48 and the compressor). Continue fault diagnosis in the Climate Control System.
If no fault can be found, continue at "Before Replacing a Control Module". Component Tests and General Diagnostics