Fendt 05.1.51 – MFA (Multifunction Armrest) / A039: A039 - MFA, multifunction armrest. Diff. lock 100% button.

The system has detected a faulty signal from the 100% differential lock button located on the multifunction armrest (MFA). This is typically caused by a failed button sensor (Signal fault, button/A004) or a communication error on the internal armrest bus (Faulty bus A004/A002).

• 309 Vario COM III
• 310 Vario COM III
• 311 Vario COM III
• 312 Vario COM III
• 922 Vario COM III
• 924 Vario COM III
• 927 Vario COM III
• 930 Vario COM III
• 933 Vario COM III
• 936 Vario COM III
• Fendt 300 Vario COM III
• Fendt 400 Vario COM III
• Fendt 700 Vario COM III
• Fendt 700 SCR
• Fendt 800 Vario COM III
• Fendt 820 Vario greentec
• Fendt 800 SCR
• Fendt 900 Vario COM III (916, 920, 924, 926, 930, 933, 936, 939)
• Fendt 900 SCR
• Fendt 900 S4

1. 1

   Check the physical condition of the 100% differential lock button on the armrest keypad for sticking, collapse, or liquid damage.

2. 2

   Attempt to clear the fault code via the Vario terminal to see if it returns.

3. 3

   Connect the FENDIAS diagnostic tool and monitor live data for the MFA (Module A039) to observe voltage changes when the button is pressed.

4. 4

   Check the internal wiring harness of the armrest for pinched wires or a faulty bus connection (A004/A002) if there is no communication.

1. 1

   Power Down

   Turn off the tractor ignition and disconnect the battery to prevent electrical shorts while working on the armrest.

2. 2

   Disassemble the Armrest

   Carefully remove the top cover/keypad of the multifunction armrest to access the internal comfort module and switch panel.

3. 3

   Inspect Internal Connections

   Check the ribbon cables and connectors linking the differential lock switch to the main MFA circuit board. Clean any corrosion or debris.

4. 4

   Replace the Component

   If the button itself has failed internally (common with microswitches), the specific keypad module or the entire armrest comfort module must be replaced.

5. 5

   Reassembly

   Reconnect the internal harnesses and secure the armrest cover.

6. 6

   Calibration/Clearing

   Reconnect the battery, turn on the ignition, and use the Vario terminal or FENDIAS to clear the stored 05.1.51 fault code.

7. 7

   Testing

   Test the 100% differential lock button to ensure the fault does not return and that the differential lock engages properly.