ECM Main Relay Control

When the ECM receives the KL 15 ignition switch input, it activates the ECM Main Relay providing operating power for the ECM and the following control modules if equipped:
- EGS/AGS
- ABS/ASC/DSC
- ADS

The Main Relay also provides operating power for most of the engine control system components if so equipped such as:
- Fuel injectors
- Idle Control Valve
- Purge Valve
- Fuel Pump Relay
- Air Mass Sensor
- VANOS Camshaft Actuator
- Camshaft position sensor
- O2 Sensor Heaters (directly or via the Heater Relay)

The ECM maintains the ground for the ECM Main Relay for a short duration of time after the ignition has been switched off. On M3.X systems and earlier this was required to keep the purge valve closed just after the engine was turned off. On M5.X systems and newer; this function provided operating power directly to the control module for the purpose of storing adaptation values and fault codes ("memory house keeping").