No Serial Data

This article has been updated with bulletin No. 81-65-40

NO SERIAL DATA












CIRCUIT DESCRIPTION
The Class II serial data circuit to the Data Link Connector (DLC) allows bi-directional communication between the VCM and the scan tool. This is accomplished through terminal 2 of the DLC. If communication between the scan tool and the VCM cannot be established, this diagnostic procedure 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
The numbers below refer to the step numbers on the diagnostic tables.

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. An inactive module is a module which had previously established communications 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.

7. This step isolates the VCM by disconnecting all the other components on the Class II Serial Data circuit, refer to Electrical Diagnosis for the shared components on the Class II Serial Data circuit. After making repairs, it is necessary to clear DTCs from the modules that can communicate. Clearing DTCs will avoid future misdiagnosis.

12. This step determines if the 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.