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

PURPOSE
The Automatic Shutdown (ASD) Relay supplies battery voltage to fuel pump relay, fuel injectors, ignition coil, and O2 sensor heating element, when energized by Powertrain Control Module (PCM).

OPERATION
The PCM energizes the ASD relay by supplying relay ground, when ignition switch is in START or RUN position and PCM is receiving a reference signal from the camshaft and crankshaft position sensors.

When the ground is applied, the relay is energized, and voltage is allowed to flow to fuel pump, fuel injectors, ignition coil, generator field winding, and O2 sensor heating element.

When PCM does not receive reference signal from ignition system, (indicating engine is not running), then the PCM interrupts relay ground circuit and no voltage is supplied to fuel pump, fuel injectors, ignition coil, generator field winding, and O2 sensor heating element.

The PCM controls fuel pump relay and ASD relays simultaneously, through same ground circuit.