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

Fig. 25 Power Distribution Center (PDC):

PURPOSE
The Automatic Shutdown (ASD) relay supplies battery voltage to the fuel injectors, generator field, electronic ignition coil and the heating elements in the oxygen sensors. Refer to Diagrams/Electrical for circuit information.

The Automatic Shutdown (ASD) sense circuit informs the PCM when the ASD relay energizes. When energized, the ASD relay supplies battery voltage to the fuel injectors, ignition coils, and the heating element in each oxygen sensor. If the PCM does not receive 12 volts from this input after grounding the ASD relay, it sets a diagnostic trouble code.

OPERATION
The PCM controls the ASD relay by switching the ground path for the solenoid side of the relay on and off. The PCM turns the ground path off when the ignition switch is in the Off position unless the O2 Heater Monitor test is being run. When the ignition switch is in On or Start, the PCM momentarily turns on the ASD relay.

The PCM also monitors the ASD relay output voltage. If the PCM does not receive 12 volts from this input after grounding the ASD relay, it sets a diagnostic trouble code.

While the relay is on the PCM monitors the crankshaft and camshaft position sensor signals to determine engine speed and ignition timing (coil dwell).
- If the PCM does not receive crankshaft and camshaft position sensor signals when the ignition switch is in the Run position, it will de-energize the ASD relay.

The ASD relay is located in the PDC Fig. 25. The inside top of the PDC cover has label showing relay and fuse identification.

Fig. 1 ASD And Fuel Pump Relay Terminals: