Scan Tool Testing and Procedures

Circuit Description
The Class 2 Serial Data circuit to the DLC allows bi-directional communication between the VCM and the scan tool. This is accomplished through pin of the DLC. If the communication between the scan tool and the VCM cannot be established, use the procedure in the DLC diagnosis table in order to diagnose the condition.

Diagnostic Aids
Check for the following items:
^ For the VCM to establish communication with the scan tool, the system voltage must be between 9-16 volts. If the system voltage is not within this range, refer to Charging System Check in Starting and Charging.
^ Select the correct application (model year, truckline, VIN code) on the scan tool.

An intermittent may be caused by any of the following conditions:
^ A poor connection
^ Rubbed through wire insulation
^ A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. Symptom Related Diagnostic Procedures

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 2 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. Refer to [1][2]Data Link Communications System Check in Data Link Communications.
7. This step isolates the VCM by disconnecting all the other components on the Class 2 Serial Data circuit. If VCM Class 2 Serial Data exists after disconnecting all other components on the Class 2 Serial Data circuit, then refer to [1][2]Data Link Communications System Check in Data Link Communications.
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.