A/C Signal Path Diagnostic

Tools Required

• Digital Multimeter (DMM)

• Automotive-safe Test Light

• Back-probe pins (recommended to avoid connector damage)

• Wheel Chocks

Vehicle Preparation

1. Safety First: Ensure the vehicle is parked on a level surface.

2. Secure the Unit: Place the transmission in Park (or Neutral for manuals), engage the parking brake, and chock the wheels.

3. Ventilation: Ensure the workspace is well-ventilated, particularly when testing in the engine bay.

4. Hazards: Be mindful of moving parts (fans/belts) and hot surfaces. Electric cooling fans may cycle on even with the engine off.

5. Schematic Procurement: Obtain both the Vehicle Chassis Schematic and the HVAC System Schematic (Document 156A-0087). Note that the hand-off between these documents occurs at the "A/C Heater Unit" section, typically found on page 2 of the chassis schematic.

Procedure

Part 1: In-Cab Circuit Tests

Verify the signal path from the fuse panel to the HVAC box output.

Test 1: Main Fuse and Power to A/C Switch

1. Inspect the 2 Amp fuse at location F13 in the in-cab fuse panel.

2. Turn the ignition Key to the ON position.

3. Locate the 18-pin X20 connector.

4. Measure for 12V+ at Pin 1/18 of connector X20.

   • If NO power: Check fuse F13 and the wiring between the fuse panel and X20.

   • If power is present: Proceed to Test 2.

Test 2: A/C Switch Output

1. Ensure X20 remains connected.

2. Toggle the A/C Switch to the ON position.

3. Probe Pin 9/18 of connector X20. Use caution to avoid shorting adjacent pins.

   • If NO power: The A/C switch or control panel is faulty or bad internal connection.

   • If power is present: Proceed to Test 3.

Test 3: Anti-Icing Switch Output

1. Locate the 4-pin X9 connector for the Anti-Icing Switch.

2. With the A/C switch ON, check for 12V+ at Pin 2 of connector X9 (Wire 056).

   • If NO power: The Anti-Icing Switch is failed open or there is a harness break between X20 and X9.

   • If power is present: The in-cab circuit is functional. Proceed to Part 2. Fault is downstream in the dash harness or the components that receive the 056 signal.

Part 2: Engine Bay Circuit Path Analysis

Trace the signal from the cab pass-through to the compressor.

1. A/C Request Signal (Wire 056): The signal exits the cab via the floor pass-through (Pin 21) and a 35-pin connector (Pin 19).

2. Trinary Switch: Wire 056 enters Pin A of the Trinary Switch.

   • The switch must be closed (proper refrigerant levels).

   • Confirm 12V+ exits on Pin B.

3. Relay Trigger (Wire 057): Signal travels from Trinary Pin B to the external relay module.

4. A/C Compressor Relay (Relay 3): Located in the front left fender fuse panel.

   • Pin 86: Receives trigger from Wire 057.

   • Pin 85: Ground.

   • Pin 30: Receives 12V+ from Wire 604 (Protected by 10A Fuse F4) in same panel.

   • Pin 87: Output to compressor.

5. Compressor Clutch: 12V+ flows from Relay Pin 87 to the clutch coil.

Part 3: Diagnosis and Repair Summary

• Failure at Test 1: Replace Fuse F13. If it blows again, trace for a short to ground between the fuse and X20. If the fuse is good but there is no power, repair the open circuit in the same wire.

• Failure at Test 2: The A/C switch is not sending the signal. Most likely need to replace the A/C control panel assembly.

• Failure at Test 3: Inspect wiring between X20 and X9. If wiring is intact, replace the Anti-Icing Switch.

• Failure in Engine Bay:

  • Inspect floor pass-through and 35-pin connectors for corrosion or pushed-out pins.

  • Check Trinary Switch continuity. If open, verify refrigerant pressures before replacing the switch.

  • Swap Relay 3 with a known good relay and check Fuse F4.

  • Inspect the final lead from the relay to the compressor clutch for damage or an open circuit.

• Verify that the A/C system operates correctly after repair has been completed.