Case IH t39 – Transmission Control Module (TCM): Vehicle CAN Bus Transmit, Shorted To High Source

This code indicates a critical communication failure on the Controller Area Network (CAN) bus. Specifically, the "transmit" circuit of the CAN bus is shorted to a high voltage source (such as a 5V reference line or 12V battery voltage). This prevents the module from sending data to other controllers on the network, as the voltage on the transmit line is stuck high.

• MXM
• Case IH MXM 120
• Case IH MXM 130
• Case IH MXM 140
• Case IH MXM 155
• Case IH MXM 175
• Case IH MXM 190

1. 1

   Inspect the main transmission and cab wiring harnesses for chafing, melted insulation, or pinch points where the CAN transmit wire could be shorting to a power wire.

2. 2

   Check the connectors at the Transmission Control Module and the armrest controller for broken solder joints, corrosion, or pushed-out pins.

3. 3

   With the key ON, use a multimeter to probe the CAN transmit line. If it reads a constant high voltage (e.g., 12V) instead of the standard fluctuating CAN voltage (typically around 2.4V - 2.5V), the short is confirmed.

4. 4

   Disconnect the harness from the Transmission Controller to perform an isolation test. If the short to high voltage remains on the harness side, the wiring is at fault. If the short disappears from the harness but the controller pins output a constant high voltage, the controller itself has an internal short.

1. 1

   Disconnect Power

   Disconnect the tractor's battery to prevent further electrical damage during the repair.

2. 2

   Repair the Short

   If the short is in the wiring: Trace the CAN transmit wire from the TCM to the armrest/cab bulkhead. Cut out the damaged or shorted section of the wire and splice in a new section using weather-sealed heat-shrink butt connectors, or replace the affected harness section entirely. If the short is internal to the module: Remove the mounting hardware for the Transmission Control Module and replace it with a new or remanufactured unit.

3. 3

   Reconnect and Clear Codes

   Reconnect the battery and turn the key to the ON position. Clear the stored fault codes.

4. 4

   Perform Calibration

   Perform a transmission/EDC calibration setup procedure (such as the H1 Clutch and Synchroniser Calibration). Set engine speed to 1200 ERPM, enter calibration mode (display will show 'CAL'), and follow the H-Menu guide to recalibrate the clutches.