Chart 2

Diagnostic Chart:

Schematic:

If a Scan tool cannot communicate with PCM chart 2 of 2 will isolate cause. If engine starts and runs ok it indicates the PCM is functional and problem is in serial data link between BCM and DLC.

A problem with loss of power to PCM or faulty ground circuits will be indicated by loss of serial data as well as no PCM controlled lamps being illuminated when ignition is turned On.

The class II data bus is used by PCM, BCM, IPC, SDM. The Scan tool communicates to both controllers through pin 2 of DLC connector. The Scan tool also uses pin 4 (circuit 350) of DLC as a ground reference when reading serial data. The class II data bus is normally around 0.5 volts and is switched high (seven volts) to low when communications are taking place.

IMPORTANT: Class II communications are terminated by PCM when ignition voltage is too high (approximately 16 volts) or to low (9.0 volts). Verify a battery charger or system voltage is not causing lack of communications.