Pinpoint Test C: DTC B1255 Or DTC B1257 - A/C Ambient 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.

C1 CHECK THE AMBIENT TEMPERATURE SENSOR RESISTANCE
- Key in OFF position.
- Disconnect: Ambient Temperature Sensor C132.
- Measure the resistance between the ambient temperature sensor terminals.
  
  Fig 1: Checking Ambient Temperature Sensor Resistance
  Courtesy 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 C2.
- No : INSTALL a new ambient air temperature sensor and bracket. TEST the system for normal operation.
C2 CHECK THE EATC MODULE OUTPUT VOLTAGE
- Key in ON position.
- Press the AUTOMATIC button.
- Measure the voltage between the ambient temperature sensor C132, circuit 3061 (LB/OG) and circuit 470 (PK/BK).
  
  Fig 2: Checking EATC Module Output Voltage
  Courtesy 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 1255, GO to C3.
  If diagnosing DTC 1257, GO to C5.
C3 CHECK CIRCUIT 3061 (LB/OG) FOR AN OPEN
- Key in OFF position.
- Disconnect: EATC Module C228a.
- Measure the resistance between the EATC module C228a pin 7, circuit 3061 (LB/OG) and the ambient temperature sensor C132, circuit 3061 (LB/OG).
  
  Fig 3: Checking Circuit 3061 (LB/OG) For Open
  Courtesy of FORD MOTOR CO.
- Is the resistance less than 5 ohms?
- Yes : GO to C4.
- No : REPAIR circuit 3061 (LB/OG) for an open. TEST the system for normal operation.
C4 CHECK CIRCUIT 470 (PK/BK) FOR AN OPEN
- Measure the resistance between the EATC module C228b pin 5, circuit 470 (PK/BK) and the ambient temperature sensor C132, circuit 470 (PK/BK).
  
  Fig 4: Checking Circuit 470 (PK/BK) For Open
  Courtesy 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.
C5 CHECK CIRCUIT 3061 (LB/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 ambient temperature sensor C132, circuit 3061 (LB/OG) and circuit 470 (PK/BK).
  
  Fig 5: Checking Circuit 3061 (LB/OG) For A Short To Circuit 470 (PK/BK)
  Courtesy of FORD MOTOR CO.
- Is the resistance greater than 10,000 ohms?
- Yes : GO to C6.
- No : REPAIR circuit 3061 (LB/OG) for a short to circuit 470 (PK/BK). TEST the system for normal operation.
C6 CHECK CIRCUIT 3061 (LB/OG) FOR A SHORT TO GROUND
- Key in OFF position.
- Measure the resistance between the EATC module C228a pin 7, circuit 3061 (LB/OG) and ground.
  
  Fig 6: Checking Circuit 3061 (LB/OG) For Short To Ground
  Courtesy 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 3061 (LB/OG) for a short to ground. TEST the system for normal operation.