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Diagnostic Procedure

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the diagnostic system check? If so, go to next step. If not, see DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.
  2. Check the PNP switch and shift linkage for proper adjustment. See SERVICING - "C" & "K" SERIES (ALLISON) article. Is the PNP switch and shift linkage properly adjusted? If so, go to next step. If not, adjust as necessary and retest.
  3. Install the scan tool. Turn ignition on, engine off. Record the DTC failure records. Using the scan tool, monitor PRNDL "A", "B", "C", "P" status while moving the gearshift lever through each position. Does each switch toggle on and off in the correct sequence and does the gear range displayed on the scan tool match the actual gear range for the selector position chosen? If so, see DIAGNOSTIC AIDS . If not, go to next step.
  4. Turn ignition off. Disconnect the TCM connector C2. Install Breakout Box (J-39700) and Breakout Box Adapter (J-43799). Turn ignition on. In sequence, connect TCM connector C2 terminal No. 20 to terminals No. 5, 6, 7, and 8. Using the scan tool , monitor PRNDL "A", "B", "C", "P" status. Does each switch toggle on and off in the correct sequence as each terminal is grounded? If so, go to next step. If not, go to step  8.
  5. Turn ignition off. Reconnect the C2 connector to the TCM. Disconnect the 4-way connector at the PNP switch. Turn ignition on. Using the scan tool, monitor PRNDL "A", "B", "C", "P" status. In sequence, connect 4-way PNP connector terminals 4A, 4B, 4C, and 4D to 7-way PNP connector terminal 7D. Does the scan tool indicate the status as ON? If so, go to next step. If not, go to step  7.
  6. Replace the PNP switch. See PARK/NEUTRAL POSITION SWITCH  under REMOVAL & INSTALLATION. Is the replacement complete? If so, go to step  9.
  7. Inspect and repair the transmission wiring harness assembly. Replace the wiring harness if necessary. Is the repair complete? If so, go to step  9.
  8. Replace the TCM. See TRANSMISSION CONTROL MODULE  under REMOVAL & INSTALLATION. Is the replacement complete? If so, go to next step.
    CAUTION: In most cases, the TCM is not at fault. Investigate thoroughly before replacing the TCM.
  9. Perform the following procedure to verify the repair:
    • Clear the DTC.
    • Using the scan tool, monitor PRNDL "A", "B", "C", "P" status while moving the gearshift lever through each position.

    Does each switch toggle on and off in the correct sequence as the gearshift lever is moved? If so, system is okay. If not, go to step  1.

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