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GM/Detroit 6.5L – Diagnostics – OTHER DIAGNOSTIC AIDS

Several other on-vehicle items aid in diagnostics.

Data Link Connector (DLC)
The Data Link Connector (DLC) is located under the left side of the instrument panel (figure 6-11). It is a twelve terminal connector wired to the PCM. By connecting a jumper wire between terminals “B” and “A” or connecting the Tech 1 adapter to the DLC, you can gain access to Diagnostic Trouble Codes (DTC).

“Service Throttle Soon” (STS) Lamp
The “Service Throttle Soon” (STS) lamp alerts the driver to problems involving the Accelerator Pedal Position (APP) sensors. If any of the sensors are inoperative or operating out of normal range, the “Service Throttle Soon” lamp will illuminate. Diagnostic Trouble Codes (DTC) may also set, depending on the degree of sensor failure.

Malfunction Indicator Lamp (MIL)

The MIL in the instrument cluster alerts the vehicle operator to driveability problems requiring service. The lamp illuminates and stays illuminated under most DTC conditions. The MIL can also be used by technicians to displays codes in the event a Tech 1 “Scan” tool is unavailable.

The lamp initially comes “ON’ at ignition startup as a part of the vehicle bulb check. At engine “ON,” the lamp turns “OFF.”
If it remains “ON,” this indicates a problem with the PCM diagnostic system. If a problem goes away, the SES lamp will turn “OFF.” The DTC will be stored in the PCM memory as a history code and can be retrieved using the following procedure.

1. A jumper wire is applied across terminal “A” and “B” of the DLC.
2. The ignition key is turned to the “ON” position.
3. If no codes are stored in the PCM, the MIL will flash a DTC 12. This code indicates that the PCM
diagnostic system is operating correctly. The flash sequence for DTC 12 is “flash, pause, flash-flash” followed by a long pause. This sequence appears three consecutive times.
If DTC 12 is not indicated, there is a fault in the PCM diagnostic system, and the system should be checked using the appropriate diagnostic chart.
4. Each code is flashed three times. The first digit in the code number is indicated, followed by a
pause and the second digit. For instance, DTC 35 would be indicated “flash-flash-flash, pause,
flash-flash-flash-flash-flash.” A long pause occurs between each sequence display.

To manually clear diagnostic trouble codes from the PCM, do the following:
1. Turn the ignition “OFF.”
2. Ground DLC terminal “B” to “A.”
3. Turn the ignition “ON.”
4. Fully apply and hold the brake pedal.
5. Fully apply and hold the accelerator pedal.
6. Check the MIL for DTC 12.
7. Release the brake pedal.
8. Release the accelerator pedal.
9. Check the MIL for DTC 12.
10. Remove the jumper from the DLC.
11 . Turn the ignition “OFF.”

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