LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Ford >> 2008 >> Mustang GT, 2D Convertible, Automatic >> Repair and Diagnosis >> Heating, Ventilation & A/C (HVAC) >> HVAC Control Systems >> HVAC Control System - General Information And Diagnostics >> Diagnostic Tests >> Climate Control System >> Pinpoint Tests >> Pinpoint Test H: The Air Conditioning (A/C) is Always On - A/C Mode Always Commanded ON >> Pinpoint Test H: The Air Conditioning (A/C) Is Always On - A/C Mode Always Commanded On
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Pinpoint Test H: The Air Conditioning (A/C) Is Always On - A/C Mode Always Commanded On

  1. H1 CHECK THE AIR CONDITIONING REQUEST SIGNAL (AC_REQST) GEM PID 
    • Key in ON position.
    • With the engine running, select the OFF position on the HVAC module.
    • Enter the following diagnostic mode on the diagnostic tool: AC_REQST Generic Electronic Module (GEM) PID
    • Does the AC_REQST GEM PID read OFF? 
    1. YES  : Go to  H4.
    2. NO  : Go to  H2.
  2. H2 CHECK THE A/C SIGNAL WITH THE A/C OFF 
    • Key in OFF position.
    • Disconnect: SJB C2280a
    • Disconnect: SJB C2280b
    • Key in ON position.
    • With the engine running, select the OFF position on the HVAC module.
      • Fig 1: Checking A/C Signal With A/C OFF
      GF0001299Courtesy of FORD MOTOR CO.
    • Measure the voltage between SJB C2280b-29, circuit 1397 (GY/RD), harness side and SJB C2280a-36, circuit 1044 (WH/YE), harness side.
    • Is the voltage greater than 10 volts? 
    1. YES  : Go to  H3.
    2. NO  : Go to  H11.
  3. H3 CHECK CIRCUIT 1397 (GY/RD) FOR A SHORT TO GROUND 
    • Key in OFF position.
    • Disconnect: HVAC Module C294a
      • Fig 2: Checking Circuit 1397 (GY/RD) For A Short To Ground
      GF0016010Courtesy of FORD MOTOR CO.
    • Measure the resistance between SJB C2280b-29, circuit 1397 (GY/RD), harness side and ground.
    • Is the resistance greater than 10,000 ohms? 
    1. YES  : Go to  H12.
    2. NO  : REPAIR circuit 1397 (GY/RD) for a short to ground. TEST the system for normal operation.
  4. H4 CHECK THE AIR CONDITIONING COMPRESSOR CYCLING SWITCH (ACCS) PCM PID WITH THE A/C OFF 
    • With the engine running, select the OFF position on the HVAC module.
    • Enter the following diagnostic mode on the diagnostic tool: ACCS PCM PID
    • Does the ACCS PCM PID read OFF? 
    1. YES  : Go to  H5.
    2. NO  : Go to  H13.
  5. H5 CHECK THE AIR CONDITIONING CLUTCH (WAC/ACCR) PCM PID WITH THE A/C OFF 
    • Enter the following diagnostic mode on the diagnostic tool: WAC/ACCR PCM PID
    • With the engine running, select PANEL mode on the HVAC module and make sure the A/C button is OFF.
    • Does the WAC/ACCR PCM PID read OFF? 
    1. YES  : Go to  H6.
    2. NO  : Go to  H13.
  6. H6 CHECK THE A/C CLUTCH RELAY 
    • Key in OFF position.
    • Disconnect: A/C Clutch Relay
    • Key in ON position.
    • With the engine running, observe the A/C clutch operation.
    • Does the A/C clutch engage? 
    1. YES  : Go to  H7.
    2. NO  : CARRY OUT the A/C clutch relay component test. Refer to COMPONENT TESTING. If the relay tests OK, go to  H8.
  7. H7 CHECK CIRCUIT 883 (PK/LB) FOR A SHORT TO VOLTAGE 
    • Key in OFF position.
    • Disconnect: A/C Compressor Clutch Field Coil C100
    • Key in ON position.
      • Fig 3: Checking Circuit 883 (PK/LB) For A Short To Voltage
      G04002936Courtesy of FORD MOTOR CO.
    • Measure the voltage between A/C compressor clutch field coil C100-1, circuit 883 (PK/LB), harness side and ground.
    • Is any voltage present? 
    1. YES  : REPAIR circuit 883 (PK/LB) for a short to voltage. TEST the system for normal operation.
    2. NO  : ADJUST the A/C compressor clutch gap. REFER to Air Conditioning (A/C) Clutch Air Gap Adjustment . TEST the system for normal operation.
  8. H8 CHECK CIRCUIT 420 (DB/YE) FOR VOLTAGE 
    • Key in OFF position.
    • Disconnect: PCM C175b
    • Disconnect: A/C Cycling Switch C130
    • Key in ON position.
      • Fig 4: Checking Circuit 420 (DB/YE) For Voltage
      GF0016007Courtesy of FORD MOTOR CO.
    • Measure the voltage between PCM C175b-15, circuit 420 (DB/YE), harness side and ground.
    • Is any voltage present? 
    1. YES  : For 4.0L, REPAIR circuit 420 (DB/YE)/198 (DG/OG) for a short to voltage. TEST the system for normal operation.

      For 4.6L and 5.4L, go to  H10.

    2. NO  : Go to  H9.
  9. H9 CHECK THE A/C CYCLING SWITCH 
    • Key in OFF position.
    • Remove the A/C cycling switch from the vehicle.
    • Measure the resistance between the A/C cycling switch pins.
    • Is the resistance greater than 10,000 ohms? 
    1. YES  : Go to  H13.
    2. NO  : INSTALL a new A/C cycling switch. TEST the system for normal operation.
  10. H10 CHECK FOR A SHORT TO VOLTAGE 
    • Key in OFF position.
    • Disconnect: Dual-Function Pressure Switch C1078
    • Key in ON position.
      • Fig 5: Checking For A Short To Voltage
      GF0016007Courtesy of FORD MOTOR CO.
    • Measure the voltage between PCM C175b-15, circuit 420 (DB/YE), harness side and ground.
    • Is any voltage present? 
    1. YES  : REPAIR circuit 420 (DB/YE) for a short to voltage. TEST the system for normal operation.
    2. NO  : REPAIR circuit 198 (DG/OG) for a short to voltage. TEST the system for normal operation.
  11. H11 CHECK THE SJB MODULE CONNECTION 
    • Clear the DTCs.
    • Disconnect all the SJB connectors.
    • Check for:
      • corrosion.
      • pushed-out pins.
      • incorrectly seated connector.
    • Connect and correctly seat all the SJB connectors.
    • Operate the system.
    • Does the concern return? 
    1. YES  : INSTALL a new SJB. REFER to MULTIFUNCTION ELECTRONIC MODULES article. TEST the system for normal operation.
    2. NO  : The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.
  12. H12 CHECK THE MODULE CONNECTION 
    • Carry out the HVAC Module Cold Boot Process.
    • Operate the system.
    • Does the concern return? 
    1. YES  : INSTALL a new HVAC module. REFER to CLIMATE CONTROL article. TEST the system for normal operation.
    2. NO  : The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. TEST the system for normal operation.
  13. H13 CHECK THE PCM MODULE CONNECTION 
    • Clear the DTCs.
    • Disconnect all the PCM connectors.
    • Check for:
      • corrosion.
      • pushed-out pins.
      • incorrectly seated connector.
    • Connect and correctly seat all the PCM connectors.
    • Operate the system.
    • Does the concern return? 
    1. YES  : INSTALL a new PCM. TEST the system for normal operation.
    2. NO  : The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.
scientia non olet · About LEMON Manuals