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.

CIRCUIT OPERATION
Circuit A16 from fuse 4 in the Power Distribution Center (PDC) supplies battery voltage to the contact side of the Automatic Shut Down (ASD) relay. Circuit A16 is HOT at all times.

When the ignition switch is in the START or RUN position, it connects circuit Al from fuse 3 in the PDC to circuit A21. Circuit A21 powers circuit F18 through fuse 9 in the fuse block. Circuit F18 supplies battery voltage to the coil side of the ASD relay. The Powertrain Control Module (PCM) provides the ground path for the coil side of the relay on circuit K51. Circuit K51 connects to cavity 51 of the PCM connector.

When the PCM provides grounds the ASD relay, the relay contacts close and connect circuit A16 to circuit A1 42. Circuit A142 splices to supply voltage to the fuel injectors, ignition coil, and heated oxygen sensor. Circuit A142 also connects to cavity 57 of the PCM.