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Circuit/System Testing

WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.

ECM Park Switch Circuit Malfunction (4T65-E)

  1. Ignition OFF, disconnect the harness connector at the AT inline 20-Way connector.
  2. Ignition ON, install a 3A fused jumper wire between the signal circuit terminal W and ground. Verify the scan tool ECM PNP switch parameter displays Park/Neutral.
    • If not the specified value, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the ECM.
  3. If all circuits test normal, test or replace the internal mode switch (IMS).

Crank Relay Circuit Malfunction

  1. Ignition OFF, disconnect the crank relay.
  2. Ignition ON, verify a test lamp illuminates between the relay coil B+ circuit terminal 85 and ground.
    • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
  3. Ignition ON, verify a test lamp illuminates between the relay switch B+ circuit terminal 30 and ground.
    • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
  4. Connect a test lamp between the relay coil control circuit terminal 86 and the relay coil B+ circuit terminal 85.
  5. With the transmission range selector in park or neutral, cycle the ignition key between the OFF and CRANK position. The test lamp should turn ON and OFF when changing between the commanded states.
    • If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM.
    • If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  6. Ensure the parking brake is applied and the transmission is in PARK. Momentarily install a 40A fused jumper wire between the B+ circuit terminal 30 and the control output circuit terminal 87. The starter solenoid should engage and the engine should begin cranking.
    • If the solenoid does not engage, test the control output circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the starter motor.
  7. If all circuits test normal, test or replace the crank relay.
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