Scan Tool Does Not Communicate With VCM/PCM

CIRCUIT DESCRIPTION
The provision for communicating with the Powertrain Control Module (PCM) is the Data Link Connector (DLC). The DLC is located under the instrument panel. The DLC is used to link a scan tool to the PCM serial data stream. Some common uses of the scan tool are listed below:
^ Identifying the stored Diagnostic Trouble Codes (DTCs)
^ Reading the serial data stream
^ Clearing the DTCs
^ Storing the Freeze Frame data

DIAGNOSTIC AIDS
Some scan tools may require an external power supply. Make sure that your scan tool is using the correct power feed.

Check that the correct application (model year, carline, VIN code) has been selected on the scan tool. If communications are still not established, try the scan tool on another vehicle in order to ensure that the scan tool or the connecting cables are not the cause of the malfunction.

An intermittent malfunction may be caused by a fault in the serial data circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions in Computers and Control Systems.

TEST DESCRIPTION

Steps 1-5:

Steps 6-15:

Steps 16-20:

The numbers below refer to the step numbers in the diagnostic table.
3. This step checks whether the scan tool can communicate with other control modules on the vehicle. If the scan tool can communicate with one of the other modules then the DLC connector, the shared circuits, and the scan tool are OK.
5. The modules supply a low current (about 1.85 mA) voltage on the serial data circuit. The normal circuit low current will NOT illuminate a test lamp. If the circuit is shorted to battery positive the higher current will illuminate the test lamp.
7. If the engine starts and runs, the main relay is OK.
8. This step checks whether the PCM is receiving ignition positive voltage from the IG fuse located in the Junction Block. The main relay will not energize and the PCM can not provide serial data without ignition positive voltage from the IG fuse circuit.
9. This step checks whether the PCM is receiving ignition positive voltage from the main relay when the ignition is ON. The PCM can not provide serial data without ignition positive voltage from the main relay.
10. The PCM can not provide serial data without the correct power and grounds.
14. The scan tool may be damaged by high current flow on the serial data circuit at DLC terminal 7. Check the scan tool for proper operation on a known good vehicle that uses terminal 7 of the DLC for serial data communication.