---
title: A/C Signal Path Diagnostic
description: Step-by-step A/C electrical diagnostic guide for Battle Motors chassis. Trace circuits from cab controls to the compressor relay for efficient repair.
---
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# A/C Signal Path Diagnostic
### Objective
To provide a systematic diagnostic procedure for identifying electrical failures within the Air Conditioning system of Battle Motors chassis. This guide ensures technicians can accurately trace the circuit from the in-cab controls through to the compressor clutch engagement.
### 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.
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