Case IH PB3 – Transmission Control Module (TCM): Vehicle CAN Bus Off, Shorted To High Source

The Controller Area Network (CAN) bus is the communication network that allows the tractor's computers to talk to each other. A "Shorted to High Source" means that one of the CAN communication wires (CAN High or CAN Low) is physically shorted to a high voltage source (typically the 12V battery power). To protect the network, the system shuts the bus down ("Bus Off"), resulting in a loss of communication between the tractor's modules.

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

1. 1

   Check CAN Bus Voltage: With the key ON, measure the voltage on the CAN pins (typically pins 3 and 4) to a good ground. A healthy CAN bus should read approximately 2.5V to 2.7V on CAN High and 2.3V to 2.5V on CAN Low. If you see 12V on either line, the short is currently active.

2. 2

   Check Terminating Resistors: With the battery completely disconnected, measure the resistance across the CAN High and CAN Low pins. It should read exactly 60 ohms. If it reads 120 ohms, one resistor is dead or disconnected. If it reads 0 ohms, the wires are shorted together.

3. 3

   Isolate the Modules: Disconnect the electronic controllers (Armrest controller, Instrument Cluster, TCM) one by one. If the CAN voltage drops back to normal 2.5V after unplugging a specific module, that module has an internal short.

4. 4

   Inspect the Wiring Harness: Check for chafing, pinched wires, or melted insulation where the main harness routes under the cab, near the transmission controller, and at the armrest pivot point. Pull gently on the wires near the connectors; if the wire stretches like a rubber band, the copper is broken inside the insulation.

5. 5

   Check Clean Grounds: Verify the 'clean ground' for the ICU (usually a ground strap above the batteries bolted to the cab mount). A bad ground can cause voltage to backfeed through the CAN network.

1. 1

   Power Down

   Turn off the tractor, remove the key, and disconnect the negative battery cable.

2. 2

   Locate the Short

   Based on your diagnostic isolation (unplugging modules), determine if the short is in the wiring harness or inside a module.

3. 3

   Repair Wiring (If Applicable)

   If the harness is chafed and touching a 12V power wire, cut out the damaged section of the CAN wire. Solder in a new section of twisted-pair wire (CAN wires must remain twisted to reject interference) and seal it with adhesive-lined heat shrink.

4. 4

   Replace Terminators (If Applicable)

   If the terminating resistors are burnt out or have broken solder joints (common on the active terminators located behind the rear cover and in the engine bay), unplug the old terminator and plug in the updated OEM replacement.

5. 5

   Replace the TCM (If Applicable)

   If the Transmission Control Module is internally shorted to 12V, unbolt it from its mounting bracket, disconnect the multi-pin connectors, and install the new module.

6. 6

   Recalibrate

   Reconnect the battery. If the TCM was replaced or its memory was wiped, you must perform a Transmission/EDC Calibration. Start the engine, hold the High and Low range buttons until "CAL" appears on the display, set engine speed to 1200 RPM, and follow the on-screen prompts to calibrate the clutches.