Chart 2 Scan Tool Cannot Communicate With PCM

Diagnostic Chart (2 Of 2):

Wiring Diagram:

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

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

The class II data bus is used by the PCM as well as ABS (if equipped). The Scan tool communicates to both controllers through pin 2 of the DLC connector. The Scan tool also uses pin 4 (circuit 350) of the 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.