Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
A/C Compressor Control
The ECM controls the A/C Compressor Relay based on signals from the IHKA/IHKR Control Module. The IHKR system in an E46 is shown.
The IHKR control module sends the following signals to the ECM over the K-bus-Kombi-CAN -bus connection:
- IHKR on stand-by (signal AC)
- Request for A/C activation (signal KO)
- Calculated compressor load
- Request for auxiliary fan
The IHKR determines the load torque for compressor activation and required auxiliary fan speed from the refrigerant pressure sensor mounted on the high side line next to the receiver/dryer.
The refrigerant pressure sensor provides a voltage input signal (0-5 volts) to the IHKR. The voltage value increases as pressure in the high side refrigerant circuit increases. The IHKR processes this signal to determine the calculated load that will be placed on the engine when the compressor is switched on. Pressure values that are too high or too low will cause the compressor to be switched off.
The ECM prepares for the additional load of the compressor by modifying the ignition timing and stabilizing idle speed.
Once all of the criteria for compressor operation have been met, the ECM will activate a gound circuit to the compressor relay to energize the compressor magnetic clutch.
The A/C Compressor Relay is deactivated during wide open throttle acceleration at low speeds to allow the engine to quickly achieve maximum power.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.