Data Link Connector Diagnosis

CIRCUIT DESCRIPTION
The Class II Serial Data circuit to the DLC allows bi-directional communication between the VCM and the scan tool. This is accomplished through pin 2 of the DLC. If communication between the scan tool and the VCM cannot be established, the procedure in the DLC Diagnosis table should be used to diagnose the condition.

DIAGNOSTIC AIDS
Check for the following items:
^ For the VCM to establish communication with the scan tool, system voltage must be between 9.0 and 16.0 volts. If the system voltage is not within this range, refer to Charging System for diagnosis.
^ Ensure that the correct application (model year, truckline, VIN code) has been selected on the scan tool.
^ An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside the insulation. Check for a poor connections or a damaged harness. Inspect the VCM harness and the connectors for improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

TEST DESCRIPTION
2. This step determines if the scan tool is operating correctly.
6. This step monitors the actively communicating modules with the scan tool's Diagnostic Circuit Check function. An active module is a module that is successfully communicating on the Class II Serial Data line with the scan tool, but currently is not communicating. If a module is not listed at all, then the module never successfully established communications with the scan tool. Refer to Data Link Communications System Check in Wiring Systems for any unlisted modules.
7. This step isolates the VCM by disconnecting all the other components on the Class II Serial Data circuit. If VCM Class II Serial Data exists, then refer to Data Link Communications System Check in Wiring System.
12. This step determines if voltage is not available at the DLC due to an open battery feed fuse. If the fuse is open, determine if the open was due to a short in the battery feed circuit before replacing the fuse.