Compressor Control

Using the E38 (99 MY) for this test, activation of the compressor clutch is an output control function of the IHKA module. Activation is carried out directly through a final stage (no relay required).

If the evaporator temperature is above 37°F (3°C), and the refrigerant pressure sensor indicates acceptable pressure, the control panel/module then signals the ECM that it is about to activate the compressor. The IHKA module will then activate the compressor if a cancellation signal is not received from the ECM.

Additional criteria:

• If the "snowflake" button is on at engine start-up, the control module delays compressor clutch activation until the engine speed exceeds 600 RPM for at least 5 seconds.
• If the "snowflake" is switched off while the engine is running, the control module continues to cycle the compressor (with increasingly longer off times) for up to 15 minutes.

Compressor deactivation:

• ECM detects full load - signalling IHKA
• Evaporator temperature < 2°C
• Coolant temperature > 120°C
• Refrigerant pressure sensor indicates pressure is too low/high
  Fig 1: Compressor Control Circuit Diagram

  Courtesy of BMW OF NORTH AMERICA, INC.