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

Power Distribution Center:

DESCRIPTION
The ASD relay and fuel pump relay are located in the Power Distribution Center (PDC) near the battery. A decal on the underside of the PDC cover shows the locations of each relay and fuse contained in the PDC.

OPERATION
The PCM operates the ASD relay and fuel pump relay through one ground path. The PCM operates them by switching on and off the ground path for the solenoid side of the relays. Both relays turn on and off at the same time.

The ASD relay connects battery voltage to the fuel injectors and ignition coils. The fuel pump relay connects battery voltage to the fuel pump.

The PCM turns the ground path off for both relays when the ignition switch is in the Off position. When the ignition switch is in the On or Crank position, the PCM monitors the crankshaft position sensor and camshaft position sensor signals to determine engine speed and ignition timing. If the PCM does not receive a crankshaft position sensor signal and camshaft position sensor signal when the ignition switch is in the Run position, it de-energizes both relays. For limp home capability the engine can run on crankshaft sensor input only. When the relays de-energize, battery voltage is not supplied to the fuel injectors, ignition coil and fuel pump.