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

Holding Relay:

A holding relay is integrated into the Motronic control module. The holding relay is used for starting the engine, power supply for the "Burn-off" of the hot wire in the mass air flow sensor and output check diagnosis.

When the ignition is switched ON and before the engine is started, the holding relay supplies power to the:

^ Mass air flow sensor
^ Fuel injectors
^ Idle air control valve
^ EGR frequency valve
^ Evaporative emissions frequency valve

When the ignition is switched OFF, the holding relay supplies power to pin 37 for five (5) seconds. This ensures that the mass air flow sensor has voltage to burn off (clean) the hot wire.

At the same time, power is supplied to the evaporative emission frequency valve. The valve is closed by the Motronic control module. This helps prevent engine "after-run".

After engine start up the power for these components is supplied by the Fuel Pump Relay