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Home >> Chevrolet >> 2020 >> Express 2500 LS, 6.0L Eng VIN G >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 2 (Automatic Transmission - 8L90 (M5U MQD) - Diagnostic Information And Procedures) >> Diagnostic Information and Procedures >> DTC P1840, P1841, P18B9, P18BE, or P18C3 >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different variant/trim than selected.
  1. Ignition/Vehicle - Off
    2. NOTE:

    It may take up to 2 min for all vehicle systems to power down before an accurate ground or low reference circuit continuity test can be performed.

  3. Disconnect the electrical connector: X175 @ T12 Automatic Transmission Assembly
  4. Test for 8.3 to 9.3 V between the test points: 9 V Reference terminal 31 @ X175
    • If less than 8.3 V 
    1. Ignition/Vehicle - Off
    2. Disconnect the appropriate electrical connector: K71 Transmission Control Module
    3. Test for infinite resistance between the test points: 9 V Reference terminal 31 @ X175 & 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 1 ohms between the test points: 9 V Reference terminal 31 @ X175 & The other end of the circuit
      • If 1 ohms or greater - Repair the open/high resistance in the circuit.
      • If less than 1 ohms - Replace the component: K71 Transmission Control Module
    • If greater than 9.3 V 
    1. Ignition/Vehicle - Off
    2. Disconnect the appropriate electrical connector: K71 Transmission Control Module
    3. Ignition/Vehicle - On
    4. Test for less than 1 V between the test points: 9 V Reference terminal 31 @ X175 & 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 between 8.3 and 9.3 V 
  5. Ignition/Vehicle - Off
  6. Connect a 3 A fused jumper wire between the test points: 9 V Reference terminal 31 & Signal circuit terminal 34 @ X175
  7. Ignition/Vehicle - On
  8. Verify the scan tool parameter: Internal Mode Switch S = 4.8-5.2
    • If greater than 5.2 V 
    1. Ignition/Vehicle - Off
    2. Disconnect the appropriate electrical connector: K71 Transmission Control Module
    3. Ignition/Vehicle - On
    4. Test for less than 1 V between the test points: Signal circuit terminal 34 @ X175 & 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
    • If less than 4.8 V 
    1. Ignition/Vehicle - Off
    2. Disconnect the appropriate electrical connector: K71 Transmission Control Module
    3. Test for infinite resistance between the test points: Signal circuit terminal 34 @ X175 & 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 1 ohms between the test points: Signal circuit terminal 34 @ X175 & The other end of the circuit
      • If 1 ohms or greater - Repair the open/high resistance in the circuit.
      • If less than 1 ohms - Replace the component: K71 Transmission Control Module
    5. Test for infinite resistance between the test points: Signal circuit terminal 34 &
      • Signal circuit terminal 21 @ X175
      • Signal circuit terminal 22 @ X175
      • Signal circuit terminal 23 @ X175
      • Signal circuit terminal 24 @ X175
      • Signal circuit terminal 28 @ X175
      • If less than infinite resistance - Repair the short between the circuits.
      • If infinite resistance - Replace the component: K71 Transmission Control Module
    • Go to next step: If between 4.8 and 5.2 V 
  9. Test or replace the component: B15 Transmission Internal Mode Switch
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