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Pinpoint Test F: The Defrost System Is Inoperative

WARNING: This page is about a different car, the 2008 Mercury Sable, 2008 Ford Taurus X, and 2008 Ford Taurus. However, it is still accessible from the selected car via links, so may be relevant.
  1. F1 CHECK THE REAR WINDOW DEFROST GRID VOLTAGE 
    • Disconnect: Rear Window Defrost Grid Power C402A
    • Start the engine.
    • Activate the rear defrost switch and measure the voltage between rear window defrost grid power C402A, circuit CRD06 (BN/YE) and ground.
      • Fig 1: Checking Circuit For Voltage
      GF0019271Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
    1. YES  : Go to  F2.
    2. NO  : Go to  F3.
  2. F2 CHECK THE REAR WINDOW DEFROST GRID GROUND 
    • Key in OFF position.
    • Disconnect: Rear Window Defrost Grid Ground C402B
    • Measure the resistance between rear window defrost grid ground C402B, circuit GD147 (BK/VT) and ground.
      • Fig 2: Checking Circuit For Open
      GF0019274Courtesy of FORD MOTOR CO.
    • Is the resistance less than 5 ohms? 
    1. YES  : CARRY OUT the Grid Wire Test. REPAIR the rear window defrost grid or INSTALL a new rear window or liftgate glass. REFER to Window Grid Wire Repair . TEST the system for normal operation.
    2. NO  : REPAIR the circuit. TEST the system for normal operation.
  3. F3 CHECK THE POWER TO THE RELAY 
    • Key in OFF position.
    • Disconnect: Rear Window Defrost Relay
    • Key in ON position.
    • Measure the voltage between rear window defrost relay socket pin 85 and pin 30, harness side and ground.
      • Fig 3: Measuring Voltage
      GF0045456Courtesy of FORD MOTOR CO.
    • Is the voltage greater than 10 volts? 
    1. YES  : Go to  F4.
    2. NO  : VERIFY Battery Junction Box (BJB) fuse 20 (40A) and Smart Junction Box (SJB) fuse 45 (5A) are OK. If OK, REPAIR the circuit. TEST the system for normal operation.

      If not OK, REFER to the Wiring Diagrams Manual to identify the possible causes of the circuit short. TEST the system for normal operation.

  4. F4 CHECK THE HVAC MODULE 
    • Start the engine.
    • Activate the rear defrost switch and measure the resistance between rear window defrost relay socket pin 86, circuit CH122 (WH/OG), harness side and ground.
      • Fig 4: Measuring Resistance
      GF0045457Courtesy of FORD MOTOR CO.
    • Is the resistance less than 60 ohms? 
    1. YES  : INSTALL a new rear window defrost relay. TEST the system for normal operation.
    2. NO  : Go to  F5.
  5. F5 CHECK CIRCUIT CH122 (WH/OG) FOR AN OPEN 
    • Key in OFF position.
    • Disconnect: HVAC Module - Electronic Automatic Temperature Control (EATC) C228b or HVAC Module - Electronic Manual Temperature Control (EMTC) C294a
    • Measure the resistance between rear window defrost relay socket pin 86, circuit CH122 (WH/OG), harness side and C228b-23 (HVAC module - EATC) or C294a-3 (HVAC module - EMTC), circuit CH122 (WH/OG), harness side.
      • Fig 5: Measuring Resistance
      GF0045458Courtesy of FORD MOTOR CO.
    • Is the resistance less than 5 ohms? 
    1. YES  : Go to  F6.
    2. NO  : REPAIR the circuit. TEST the system for normal operation.
  6. F6 CHECK FOR CORRECT HVAC MODULE 
    • Disconnect all HVAC module connectors.
    • Check for:
      • corrosion.
      • pushed-out pins.
    • Connect all HVAC module connectors and make sure they seat correctly.
    • Operate the system and verify the concern is still present.
    • Is the concern still present? 
    1. YES  : INSTALL a new HVAC module. TEST the system for normal operation.
    2. NO  : The system is operating correctly at this time. Concern may have been caused by a loose or corroded connector. TEST the system for normal operation.
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