Fendt 05.1.52 – MFA (Multifunction Armrest) control module: A039 - MFA, multifunction armrest: Differential lock auto system button - Sensor faulty / Signal fault

The system has detected a signal failure, short, or open circuit originating from the button used to engage the automatic differential lock. As a direct consequence of this fault, the tractor disables the automatic differential lock mode; only the manual "100% differential lock" function remains available to the operator.

• 922 Vario COM III
• 924 Vario COM III
• 927 Vario COM III
• 930 Vario COM III
• 933 Vario COM III
• 936 Vario COM III
• 200 Vario Series: 200 V/F/P
• 300 Vario Series
• 400 Vario Series
• 700 Vario Series (including SCR models)
• 800 Vario Series
• 900 Vario Series: 916, 920, 922, 924, 926, 927, 930, 933, 936 (COM III and newer)

1. 1

   Inspect the differential lock auto button on the MFA for physical damage, sticking, or liquid intrusion (e.g., spilled drinks).

2. 2

   Verify the symptom on the dash terminal to confirm that manual 100% diff lock works, but auto mode does not.

3. 3

   Connect the FENDIAS diagnostic tool and monitor live data for the MFA (A039/A100) to check if the button registers a state change or is permanently stuck open/closed.

4. 4

   Check the main harness connector underneath the Multifunction Armrest for loose pins, corrosion, or wiring damage interrupting CAN communication or power.

1. 1

   Disconnect Power

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

2. 2

   Access the MFA

   Carefully remove the retaining screws on the underside of the Multifunction Armrest to lift the top console cover.

3. 3

   Clean and Inspect

   If the button is simply sticking due to dirt or debris, clean the contact area with electrical contact cleaner.

4. 4

   Test the Switch

   If the button is a discrete switch, test it for continuity using a multimeter. If the button is integrated into a sealed membrane keypad or circuit board (as is common on newer Vario models), the individual button cannot be replaced.

5. 5

   Replace the Component

   If the switch/keypad has failed internally, replace the affected MFA keypad module or the entire Armrest Comfort Module.

6. 6

   Reassemble and Clear Codes

   Reconnect the MFA harness, reassemble the armrest, and reconnect the battery. Turn the ignition on and use the Vario terminal to clear the stored 05.1.52 fault code.

7. 7

   Test Operation

   Engage the tractor and test the automatic differential lock function to ensure the code does not return.