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Pinpoint Test B: DTC B1251 Or DTC1253 - A/C In-Car Temperature Sensor Open Circuit Or Short To Ground

WARNING: This page is about a different car, the 2005 Ford Pickup, 2005 Ford F550 Super Duty, 2005 Ford F450 Super Duty, and 2005 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
  1. B1 CHECK THE SENSOR RESISTANCE 
    • Key in OFF position.
    • Disconnect: In-Car Temperature Sensor C233.
    • Measure the resistance between the in-car temperature sensor terminals.
      Fig 1: Checking Sensor Resistance
      G03910063Courtesy of FORD MOTOR CO.
    • Is the resistance within the specified values for these temperature ranges: 10-20°C (50-68°F), 37,000-58,000 ohms; 20-30°C (68-86°F), 24,000-37,000 ohms; 30-40°C (86-104°F), 16,000-24,000 ohms? 
    • Yes  : GO to B2.
    • No  : INSTALL a new in-car temperature sensor. TEST the system for normal operation.
  2. B2 CHECK THE EATC SENSOR OUTPUT VOLTAGE 
    • Key in ON position.
    • Press the AUTOMATIC button.
    • Measure the voltage between the in-car temperature sensor C233, circuit 790 (WH/OG) and circuit 470 (PK/BK).
      Fig 2: Checking EATC Sensor Output Voltage
      G03910064Courtesy of FORD MOTOR CO.
    • Is the voltage between 4.7 and 5.1 volts? 
    • Yes  : INSTALL a new EATC module. TEST the system for normal operation.
    • No  : If diagnosing DTC B1251, GO to B3.

      If diagnosing DTC B1253, GO to  B5.

  3. B3 CHECK CIRCUIT 790 (WH/OG) FOR AN OPEN 
    • Key in OFF position.
    • Disconnect: EATC Module C228a.
    • Measure the resistance between the EATC module C228a pin 19, circuit 790 (WH/OG) and the in-car temperature sensor C233, circuit 790 (WH/OG).
      Fig 3: Checking Circuit 790 (WH/OG) For Open
      G03910065Courtesy of FORD MOTOR CO.
    • Is the resistance less than 5 ohms? 
    • Yes  : GO to B4.
    • No  : REPAIR circuit 790 (WH/OG) for an open. TEST the system for normal operation.
  4. B4 CHECK CIRCUIT 470 (PK/BK) FOR AN OPEN 
    • Disconnect: EATC Module C228b.
    • Measure the resistance between the EATC module C228b pin 5, circuit 470 (PK/BK) and the in-car temperature sensor C233, circuit 470 (PK/BK).
      Fig 4: Checking Circuit 470 (PK/BK) For Open
      G03910066Courtesy of FORD MOTOR CO.
    • Is the resistance less than 5 ohms? 
    • Yes  : INSTALL a new EATC module. TEST the system for normal operation.
    • No  : REPAIR circuit 470 (PK/BK) for an open. TEST the system for normal operation.
  5. B5 CHECK CIRCUIT 790 (WH/OG) FOR A SHORT TO CIRCUIT 470 (PK/BK) 
    • Key in OFF position.
    • Disconnect: EATC Module C228a.
    • Disconnect: EATC Module C228b.
    • Measure the resistance between the in-car temperature sensor C233, circuit 790 (WH/OG) and circuit 470 (PK/BK).
      Fig 5: Checking Circuit 790 (WH/OG) For Short To Circuit 470 (PK/BK)
      G03910067Courtesy of FORD MOTOR CO.
    • Is the resistance greater than 10,000 ohms? 
    • Yes  : GO to B6.
    • No  : REPAIR circuit 790 (WH/OG) for a short to circuit 470 (PK/BK). TEST the system for normal operation.
  6. B6 CHECK CIRCUIT 790 (WH/OG) FOR A SHORT TO GROUND 
    • Measure the resistance between the EATC module C228a pin 19, circuit 790 (WH/OG) and ground.
      Fig 6: Checking Circuit 790 (WH/OG) For Short To Ground
      G03910068Courtesy of FORD MOTOR CO.
    • Is the resistance greater than 10,000 ohms? 
    • Yes  : INSTALL a new EATC module. TEST the system for normal operation.
    • No  : REPAIR circuit 790 (WH/OG) for a short to ground. TEST the system for normal operation.
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