Case IH 12505 – Vehicle Control Unit (VCU) or Pneumatic Trailer Brake Controller: Air pressure sensor (circuit 2) (rear axle) - short circuit to power supply.

The control module is receiving a voltage signal from the rear axle's pneumatic air brake pressure sensor (circuit 2) that exceeds the maximum expected threshold. This indicates that the sensor's signal wire is shorted directly to a power source (such as a 5V or 12V reference line), or the sensor has failed internally.

• Magnum 250
• Magnum 280
• Magnum 310
• Magnum 340
• Maxxum
• Puma
• Farmall 105U
• Farmall 110U
• Farmall 115U

1. 1

   Inspect the wiring harness running to the rear axle air pressure sensor for chafed, pinched, or melted wire insulation that could cause the signal wire to touch a power wire.

2. 2

   Disconnect the sensor and inspect the electrical connector for water ingress, corrosion, or bent pins, which can bridge the power and signal terminals.

3. 3

   With the sensor unplugged and the key ON, use a multimeter to check the voltage at the signal pin on the harness side. If it reads the full reference voltage (e.g., 5V or 12V), the short to power is located within the wiring harness. If the voltage is normal (0V on the signal return), the short is internal to the sensor.

1. 1

   Depressurize the System

   Turn off the tractor and safely bleed all compressed air from the pneumatic brake system to prevent injury when removing the sensor.

2. 2

   Disconnect Power

   Disconnect the tractor's battery.

3. 3

   Locate the Sensor

   Locate the circuit 2 air pressure sensor on the rear axle pneumatic valve block/lines.

4. 4

   Harness Repair (If Applicable)

   If the diagnostic test revealed a harness short, splice and repair the damaged wires using weather-sealed connectors, or replace the affected harness section.

5. 5

   Sensor Replacement (If Applicable)

   Unplug the electrical connector. Unscrew the faulty air pressure sensor from the pneumatic port. Thread the new OEM sensor into the port and tighten to the manufacturer's torque specification. Reconnect the electrical harness.

6. 6

   System Test

   Reconnect the battery, start the tractor, and allow the air compressor to build system pressure. Check the new sensor fitting for air leaks.

7. 7

   Clear Codes

   Use the Case IH Electronic Service Tool (EST) or the in-cab diagnostic display to clear the 12505 fault code and verify it does not return.