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

PURPOSE
The Powertrain Control Module (PCM) receives its power supply from the Variable Control Relay Module (VCRM). The VCRM is an integrated control module composed of multiple relays and controllers which also supplies constant and variable voltage power supplies to the A/C clutch, electric cooling fan, and fuel pump.

In addition to these functions, the VCRM performs self diagnostic checks. Information on any failure that is detected by the VCRM is sent via the Data Communications Link (DCL) to the PCM and stored as diagnostic trouble codes.

OPERATION
The VCRM receives battery voltage directly from the ignition switch when the switch is placed in RUN or START. The normally open PCM power relay (internal to VCRM) then energizes and supplies power to PCM and its outputs. A reverse polarity relay, and a diode in line with the PCM power relay prevents damage resulting from reverse polarity.

RELATED DIAGNOSTIC TROUBLE CODES

DTC 584 - Indicates the PWR GND circuit (pin 1) is open.

DTC 587 - Indicates a data communications link error between the PCM and the VCRM.