Main Relay

Main Relay

the ECM controls its own power supply, via the main relay in the engine compartment fusebox. When the ignition is turned to position II the ECM provides a ground to the main relay coil. the main relay then energizes and connects the main power feed to the ECM. the ECM controls the main relay and therefore its own power supply, so that when the ignition is turned off it can follow the power-down sequence, during which it records values from various sensors and writes adaptions into its memory etc. the last action the ECM carries out before completing its power-down sequence is to turn off the main relay. This will occur approximately 7 seconds after the ignition has been switched off ,as long as the coolant temperature is not rising. For NAS vehicle with tank leakage detection and under some vehicle system fault conditions this period could be extended up to 20 minutes.

Failure of the main relay will result in the engine failing to start. the engine will stop immediately if the main relay fails while the engine is running.