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

PURPOSE
The Automatic Shutdown (ASD) relay supplies battery voltage to the fuel pump relay. It also supplies voltage to the fuel injectors, ignition coil, and O2 sensor heating element. The ASD is located in the engine compartment.


OPERATION
The ground circuit for the ASD relay is controlled by the Powertrain Control Module (PCM). The PCM operates the relay by switching the ground circuit on and off.

The PCM monitors distributor signal to determine engine speed and compute injector synchronization. If the PCM does not receive a distributor signal when ignition switch is in RUN position, it will de-energize (remove ground) from ASD relay. Battery voltage will be cut to the fuel pump relay. It will also be cut to the fuel injectors, ignition coil, and O2 sensor heater element.

CIRCUIT DESCRIPTION
When the ignition switch is in either the START or RUN positions, it connects circuit A1 from fuse 12 in the Power Distribution Center (PDC) to circuit A21. Circuit A21 supplies battery voltage to fuse 6 in the PDC.

From fuse 6, circuit F86 supplies power to the coil side of the fuel pump relay At the fuel pump relay, circuit F86 is double pinned and supplies power to the coil side of the Automatic Shut Down (ASD) relay. The Powertrain Control Module (PCM) provides ground for the coil sides of the ASD relay and fuel pump relay on circuit K81. Circuit K81 connects to cavity 51 of the PCM.

When the PCM grounds the ASD relay contacts inside the relay close and connect circuit A5 from fuse 2 in the PDC to circuit A61. Circuit A61 splices to the generator field terminal, fuel injectors, and ignition coil. Circuit A61 also connects to cavity 57 of the PCM.