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Home >> Chevrolet >> 2024 >> Tahoe Commercial, 4WD >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Automatic Transmission - 10L80 (MHS MHU MQC) - Diagnostic Information And Procedures (2 Of 2) >> Diagnostic Information and Procedures >> DTC P18A2-P18A4 >> Circuit/System Testing

Circuit/System Testing

  1. Ignition/Vehicle - Off
    2. NOTE:

    When the following connector is disconnected, the ignition cannot be turned off due to lack of a valid park input to the TCM. If the ignition needs to be turned off, the connector will need to be reconnected.

  2. Disconnect the electrical connector: X1@T12 Automatic Transmission
  3. Test for 15 to 25 ohms between the test points: Control circuit terminal 11 (component side) & Control circuit terminal 12 (component side)
    • If not between 15 and 25 ohms 
    1. Remove the component: Automatic Transmission Fluid Pan
    2. Disconnect the electrical connector: M123 Transmission Park Valve Lock Solenoid Actuator
    3. Test for 15 to 25 ohms between the test points: Control terminal 1 & Control terminal 2
      • If not between 15 and 25 ohms - Replace the component: M123 Transmission Park Valve Lock Solenoid Actuator
      • If between 15 and 25 ohms - Replace the component: Automatic Transmission Wiring Harness
    • Go to next step: If between 15 and 25 ohms 
  4. Test for greater than 500 kohms between the test points: Control circuit terminal 11 (component side) & Transmission Case
    • If 500 kohms or less 
    1. Remove the component: Automatic Transmission Fluid Pan
    2. Disconnect the electrical connector: M123 Transmission Park Valve Lock Solenoid Actuator
    3. Test for infinite resistance between the test points:
      • Control terminal 1 & The component's housing
      • Control terminal 2 & The component's housing
      • If less than infinite resistance - Replace the component: M123 Transmission Park Valve Lock Solenoid Actuator
      • If infinite resistance - Replace the component: Automatic Transmission Wiring Harness
    • Go to next step: If greater than 500 kohms 
  5. Ignition - On/Vehicle - In Service Mode
  6. Connect a test lamp between the test points: Control circuit terminal 11@Component harness (harness side) & Ground
  7. Perform the scan tool control function: High Side Driver - On and Off

    Verify the test lamp turns On and Off.

    • If the test lamp does not turn On and Off 
    1. Ignition/Vehicle - Off & Remove - Test Lamp
    2. Disconnect the electrical connector: K71 Transmission Control Module
    3. Test for infinite resistance between the test points: Control circuit terminal 11 & Ground
      • If less than infinite resistance - Repair the short to ground on the circuit.
      • Go to next step: If infinite resistance
    4. Test for less than 2 ohms between the test points: Control circuit terminal 11 & The other end of the circuit
      • If 2 ohms or greater - Repair the open/high resistance in the circuit.
      • If less than 2 ohms - Replace the component: K71 Transmission Control Module
    • If the test lamp is always On 
    1. Ignition/Vehicle - Off & Remove - Test Lamp
    2. Disconnect the electrical connector: K71 Transmission Control Module
    3. Ignition - On/Vehicle - In Service Mode
    4. Test for less than 1 V between the test points: Control circuit terminal 11 & Ground
      • If 1 V or greater - Repair the short to voltage on the circuit.
      • If less than 1 V - Replace the component: K71 Transmission Control Module
    • Go to next step: If the test lamp turns On and Off 
  8. Disconnect the electrical connector: K71 Transmission Control Module
  9. Ignition - On/Vehicle - In Service Mode
  10. Test for less than 1 V between the test points: Control circuit terminal 12@X1 (harness side) & Ground
    • If greater than 1 V 

      Repair the short to voltage on the circuit.

    • Go to next step: If less than 1 V 
  11. Ignition/Vehicle - Off
  12. Test for infinite resistance between the test points: Control circuit terminal 12@X1 (harness side) & Ground
    • If less than infinite resistance 

      Repair the short to ground on the circuit.

    • Go to next step: If infinite resistance 
  13. Test for less than 5 ohms between the test points: Control circuit terminal 12@X1 (harness side) & The other end of the circuit
    • If greater than 5 ohms 

      Repair the open/high resistance in the circuit.

    • Go to next step: If less than 5 ohms 
  14. Test or replace the component: K71 Transmission Control Module
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