Case IH PB2 – Transmission Controller (TCU / PCU): Vehicle CAN Bus Off, Data Valid But Above Normal Range

The controller has stopped communicating on the network (Bus Off state), but the data or voltage signals it was processing just prior to the drop-off were recognized as valid, albeit exceeding the normal expected parameters. This is typically caused by a physical wiring issue, a failed CAN bus terminating resistor, or a voltage irregularity that forces the controller offline to protect itself.

• 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

   Check system voltage: Verify battery cables are clean and tight, and check voltage drop during cranking (a drop in the 5-volt sensor network will force the CAN bus offline).

2. 2

   Inspect fuses: Check all 5-amp fuses related to the transmission controller and the instrument cluster.

3. 3

   Test CAN bus voltage: Using a multimeter at the diagnostic connector, check CAN High and CAN Low voltages (should rest around 2.5V; erratic readings indicate a bad ground or shorted module).

4. 4

   Check CAN terminators: With the battery disconnected, measure resistance across the CAN network (should read approximately 60 ohms).

5. 5

   Isolate controllers: If the bus is completely down, disconnect controllers one by one (Transmission, Hitch, PTO, etc.) to see if the instrument cluster and CAN bus come back online, isolating a shorted ECU.

1. 1

   Power Down

   Disconnect the battery completely and let the machine sit to clear volatile memory and ensure safety while working on the wiring harness.

2. 2

   Wiring and Ground Repair

   Inspect the wiring harness for rub marks or pinched wires, particularly under the cab and around the transmission housing. Clean all chassis ground connections, as poor grounds cause the 'above normal range' voltage spikes.

3. 3

   Replace the CAN Terminator

   If the resistance test failed, locate and replace the faulty CAN bus terminating resistor.

4. 4

   Replace the Transmission Controller

   If the wiring, grounds, and terminators are perfectly intact but the controller remains offline or outputs out-of-range data, the Transmission Controller itself has suffered an internal hardware failure and must be replaced.

5. 5

   Recalibrate the Transmission

   If the transmission controller is replaced, or if non-volatile memory was reset, you must perform a Transmission/EDC Calibration. Start the engine, set engine speed to 1200 ERPM, and use the H-Menu (H1) to calibrate the clutches and synchronizers until 'END' is displayed.