Main Relay (Computer/Fuel System): Description and Operation

The main relay is located in the relay board under the facia at the steering wheel.

The relay supplies the injectors, [1][2]ignition discharge module, idle air control valve and evaporative emission canister purge valve. The main relay is controlled from pin 31 of the ECM which is grounded for the relay to operate.

If the engine has been started and the ignition has then been switched off for more than 45 seconds, the main relay will operate for approx. 6 seconds when the ignition is next switched on to allow for pre-injection. On the other hand, if the ignition has been switched on but the engine has not been started, the ignition must be switched off for 15 minutes before further pre-injection takes place.

The main relay then operates as soon as the ECM receives pulses from the crankshaft position sensor. When the engine has been stopped, the main relay continues to operate for approx. 6 seconds, in order to supply the [1][2]ignition discharge module and allow burn-off of the spark plugs.