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Home >> Chevrolet >> 2020 >> Express 2500 LS, 6.0L Eng VIN G >> Repair and Diagnosis >> External Pages >> Different car >> Section 57 (Automatic Transmission - 8L90 (M5U) - Diagnostic Information And Procedures) >> Diagnostic Information and Procedures >> Tow/Haul Switch/Indicator Malfunction >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2019 GMC Savana 3500, 2019 GMC Savana 2500, 2019 Chevrolet Express 3500, and 2019 Chevrolet Express 2500. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ignition/Vehicle - Off
  2. Disconnect the electrical connector: S18 Exhaust Brake Switch
  3. Test for infinite resistance between the test points: Signal circuit terminal D (component side) & Ground circuit terminal A (component side)
    • If less than infinite resistance 

      Replace the component: S18 Exhaust Brake Switch

    • Go to next step: If infinite resistance 
  4. Press and hold the switch: S18 Exhaust Brake Switch

    Test for less than 5 ohms between the test points: Signal circuit terminal D (component side) & Ground circuit terminal A (component side)

    • If 5 ohms or greater 

      Replace the component: S18 Exhaust Brake Switch

    • Go to next step: If less than 5 ohms 
  5. Test for less than 10 ohms between the test points: Ground circuit terminal A (harness side) & Ground
    • If 10 ohms or greater 
    1. Disconnect the ground connection: G302
    2. Test for less than 2 ohms between the test points: Ground circuit terminal A (harness side) & Terminal G220
      • If 2 ohms or greater - Repair the open/high resistance in the circuit.
      • If less than 2 ohms - Repair the open/high resistance in the ground connection.
    • Go to next step: If less than 10 ohms 
  6. Disconnect the electrical connector: X1 @ K9 Body Control Module
  7. Ignition - On / Vehicle - In Service Mode
  8. Test for less than 1 V between the test points: Signal circuit terminal D (harness side) @ S18 Exhaust Brake Switch & Ground
    • If 1 V or greater 

      Repair the short to voltage on the circuit.

    • Go to next step: If less than 1 V 
  9. Ignition/Vehicle - Off
  10. Test for infinite resistance between the test points: Signal circuit terminal D (harness side) @ S18 Exhaust Brake Switch & Ground
    • If less than infinite resistance 

      Repair the short to ground on the circuit.

    • Go to next step: If infinite resistance 
  11. Test for less than 5 ohms between the test points: Signal circuit terminal D (harness side) @ S18 Exhaust Brake Switch & The other end of the circuit @ Control module harness
    • If 5 ohms or greater 

      Repair the open/high resistance in the circuit.

    • Go to next step: If less than 5 ohms 
  12. Replace the component: K9 Body Control Module
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