Case IH PA – Transmission / EDC Controller or Engine Control Unit (ECU): Ambient Pressure Sensor, Shorted To Low Source

The control module is receiving a signal voltage from the ambient (atmospheric) pressure sensor that has fallen below the minimum acceptable threshold. This is typically caused by a short circuit to ground (low source) in the sensor's wiring harness, a complete failure of the sensor itself, or a defect within the internal circuitry of the control module. Note: The fault code 'PA' is a visual misreading of the code 'tA' (or 'TA').

• MXM
• MXM 120
• MXM 130
• MXM 140
• MXM 155
• MXM 175
• MXM 190
• AFX Series
• AFX8010
• Magnum Series
• TM115-TM165
• T-Series

1. 1

   Determine sensor location: Check the service manual to see if the ambient pressure sensor is an external component on the engine harness or built internally into the Engine Control Unit (ECU).

2. 2

   For external sensors (Wiring & Connector Inspection): Inspect the wiring harness between the sensor and the ECU. Look for chafed wires shorting to the chassis (ground) or moisture/corrosion inside the connector.

3. 3

   For external sensors (Disconnect Test): Disconnect the sensor. If the diagnostic code changes from 'Shorted to Low Source' to 'Shorted to High Source', the wiring harness is likely intact, indicating the sensor itself is internally shorted.

4. 4

   For internal sensors (ECU Test): If the sensor is internal to the ECU (e.g., EEM3 controllers), test the system by swapping in a known-good replacement ECU. If the fault clears, the original ECU's internal sensor has failed.

1. 1

   Disconnect battery

   Turn the key switch to the OFF position and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Repair external wiring or replace sensor

   For an external wiring/sensor fault: Repair any damaged sections of the wiring harness. If the sensor is faulty, unbolt it, plug the connector into the new OEM replacement sensor, and mount it securely.

3. 3

   Replace ECU (if internal sensor fault)

   For an internal ECU sensor fault: Remove the ECU mounting hardware, carefully disconnect the main harness blocks, and install a replacement ECU.

4. 4

   Reconnect battery

   Reconnect the battery and turn the key switch to the 'ON' position.

5. 5

   Perform calibration

   If the transmission controller, ECU, or a related potentiometer was replaced or its non-volatile memory was reset (Mode H8), you must perform the Transmission/EDC Calibration Set Up Procedure. For MXM tractors, this involves blocking the wheels, entering the H-Menu calibration mode, setting engine speed to 1200 RPM, and holding the High/Low range buttons until 'END' appears on the display.

6. 6

   Clear fault codes

   Clear the inactive fault codes from the system memory.