A/C Compressor Fails To Engage

Fault Symptom.
The A/C compressor's clutch fails to engage.

Functions/Definitions.
Information to the effect that the compressor is to be engaged ("A/C IN") is received via pin 12 of the Motronic control module.

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

Via pin 36 the Motronic control module then grounds the A/C relay which in its turn controls the A/C compressor ("A/C OUT" or "A/C RELAY").





Basic Requirement.
No diagnostic trouble codes.

Diagnostic Procedure.
1. Check the A/C request ("A/C IN")
- Connect the 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) 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 A/C-ACC system.

NOTE: In the event of high engine coolant temperature or a fault in the coolant temperature sensor signal sent to the ICE, the ICE will break the A/C circuit.





2. Check the A/C relay ("A/C OUT")
- Engine switched off.
- Ignition switch in ON position.
- Select "ACTIVATE"
- Select "A/C RELAY'.

Both the A/C relay and the compressor, clutch should now be heard.

Can the A/C relay and compressor clutch be heard?

YES: No electrical fault present.
NO: The fault is probably after the Motronic (i.e.between pin 36 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