Circuit Description
The Class II Serial Data circuit to the data link connector (DLC) allows bi-directional communication between the vehicle control module (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 Check in Engine Electrical for diagnosis. |
| • | Ensure that the correct application, such as model year, truckline, VIN code, has been selected 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 Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step determines if the scan tool is operating correctly.
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This step monitors the actively communicating modules with the scan tool 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.
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This step isolates the VCM by disconnecting all the other components on the Class 2 Serial Data circuit. After making repairs, it is necessary to clear the DTCs from the modules that can communicate. Clearing the DTCs will avoid future misdiagnoses.
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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.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | |
Install the scan tool on a known good vehicle. Does the scan tool display the VCM data? | -- | Go to Step 4 | Go to Step 3 | |
3 | Repair the scan tool or cables. Are the repairs complete? | -- | -- | |
4 |
Does the scan tool power up? | -- | Go to Step 5 | Go to Step 9 |
5 | Verify that the correct vehicle application was selected on the scan tool. Is the vehicle information correct? | -- | Go to Step 6 | |
Are any of the modules on the serial data circuit communicating? | -- | Go to Step 13 | Go to Step 7 | |
Does the scan tool display the VCM data? | -- | Go to Scan Tool Does Not Communicate with Class 2 Data Line in Data Link Communications | Go to Step 8 | |
8 |
Is the test lamp ON? | -- | Go to Step 20 | Go to Step 13 |
9 | Probe the battery positive voltage circuit (DLC pin 16) with a test lamp connected to a ground. Is the test lamp ON? | -- | Go to Step 10 | Go to Step 12 |
10 | Probe the ground circuit (DLC pin 4) with a test lamp connected to B+. Is the test lamp ON? | -- | Go to Step 11 | Go to Step 16 |
11 | Probe the ground circuit (DCL pin 5) with a test lamp connected to B+. Is the test lamp ON? | -- | Go to Step 18 | Go to Step 16 |
Inspect the fuse in the fuse block. Is the fuse OPEN? | -- | Go to Step 14 | Go to Step 15 | |
13 | Check for an open or poor connection in the Class 2 Serial Data circuit. Did you find a problem? | -- | Go to Step 17 | Go to Step 19 |
14 |
Is the action complete? | -- | Go to Step 23 | -- |
15 | Repair the open in the battery feed circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 23 | -- |
16 | Repair the open in the DLC ground circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 23 | -- |
17 | Repair the open or the poor connection in the Class 2 Serial Data circuit. Refer to Wiring Repairs or Connector Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 23 | -- |
18 | Check for poor connections or improper mating at the DLC connector. Did you find a problem? | -- | Go to Step 21 | Go to Diagnostic Aids |
19 | Check for a poor connection of the Class 2 Serial Data circuit at the VCM and DLC connectors. Did you find a problem? | -- | Go to Step 21 | Go to Step 22 |
20 | Repair the short to ground in the Class 2 Serial Data circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 23 | -- |
21 | Repair the circuit as necessary. Refer to Wiring Repairs or Connector Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 23 | -- |
22 |
Important: The replacement VCM must be programmed. Replace the VCM. Refer to VCM Replacement/Programming . Is the action complete? | -- | Go to Step 23 | -- |
23 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
