John Deere 639.08 – Steering System Control Unit (XMC): Vehicle CAN Bus Fault

The control unit is unable to process all the CAN Bus messages it is receiving. This is typically caused by a CAN Bus overload (too much data traffic) or a physical communication problem (such as a wiring issue or voltage drop) between the control unit and the vehicle's CAN bus network.

• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 6145J
• 6165J
• 6180J
• 6205J
• 8235R
• 8245R
• 8260R
• 8285R
• 8310R
• 8335R
• 8360R
• 8310RT
• 8335RT
• 8360RT
• 8400R
• 9470RT
• 9520RT
• 9570RT
• 4940

1. 1

   Cycle the key switch or power down the tractor completely, then restart the engine to see if the code is a temporary glitch and clears on its own (attempt vehicle recovery)

2. 2

   Check the unswitched supply voltage to the Steering System Control Unit. Inspect battery terminals, alternator output, and relevant fuses (e.g., F23)

3. 3

   Inspect the CAN Bus wiring harness connecting to the XMC module for visible damage, chafing, or loose pins. Look specifically for incompatible or damaged harnesses on the steering system

4. 4

   Use a diagnostic tool to check if other modules are also reporting CAN bus faults. If multiple modules report CAN errors, the issue is likely a network-wide wiring short or hub failure rather than an isolated XMC module failure

1. 1

   Attempt Recovery

   Safely park the machine, turn off the engine, and cycle the tractor power. Restart the engine.

2. 2

   Address Voltage Issues

   If the code persists and is accompanied by low voltage codes, reduce the electrical load or increase engine RPM to ensure adequate voltage is being supplied to the modules.

3. 3

   Inspect Fuses

   Disconnect the battery and inspect the fuses related to the Steering System Control Unit. Replace any blown fuses.

4. 4

   Check Connections

   Disconnect the harness at the XMC module. Clean the contacts with electrical contact cleaner and check for pushed-back pins, moisture, or corrosion. Reconnect securely.

5. 5

   Test CAN Lines

   Perform a continuity and short-to-ground test on the CAN High and CAN Low wires between the XMC and the main vehicle CAN bus hub. Repair or replace the wiring harness if faults are found.

6. 6

   Software Update

   If the wiring and voltage are good, the XMC module may require a software reflash (reprogramming) by a John Deere dealer using Service ADVISOR.

7. 7

   Module Replacement

   If reprogramming fails or the hardware self-test fails, replace the Steering System Control Unit (XMC).