Fendt 05.1.91 – Multifunction Armrest (MFA) module: Signal fault with the "rear auto mode" button

The system detects that the "rear auto mode" button on the multifunction armrest is faulty, sending an invalid signal, or stuck.

• 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, S4)
• Fendt 400 Vario (COM III)
• Fendt 500 Vario (SCR, S4)
• Fendt 700 Vario (COM III, SCR, S4)
• Fendt 800 Vario (COM III, SCR, S4)
• Fendt 900 Vario (COM III, SCR, S4)
• Fendt 922
• Fendt 924
• Fendt 927
• Fendt 930
• Fendt 933
• Fendt 936
• Fendt 939

1. 1

   Inspect the rear auto mode button on the armrest keypad for physical damage, sticky residue, or debris that might cause the button to stick in the depressed position.

2. 2

   Inspect the main wiring harness and connectors leading to the MFA (A039/A100/A141) for loose pins, corrosion, or chafing.

3. 3

   Use a 160-pin adapter box to test the MFA module's inputs and outputs to verify if the signal fault is internal to the armrest or a wiring issue.

4. 4

   Verify that the MFA is communicating properly with the basic control unit ECU (A050) to rule out a broader CAN bus or ground issue.

1. 1

   Disconnect Power

   Turn off the tractor and disconnect the battery to prevent electrical shorts or accidental CAN bus spikes.

2. 2

   Access the Keypad

   Carefully remove the trim and retaining screws on the multifunction armrest to access the internal keypad module.

3. 3

   Clean the Contacts

   If the button is mechanically stuck due to dirt, use a specialized electronic contact cleaner to clean the membrane/switch area. Actuate the button several times to free it up.

4. 4

   Test Continuity

   If accessible, test the microswitch for the rear auto mode button for proper continuity when pressed and released.

5. 5

   Replace the Module

   If the button's internal microswitch or the printed circuit board (PCB) has failed, the individual button usually cannot be serviced separately. You will need to replace the affected keypad sub-assembly or the entire MFA module.

6. 6

   Reassembly and EOL Programming

   Install the new keypad/armrest and reconnect the battery. Note: If the entire MFA module (A039/A100) is replaced, Fendt End-of-Line (EOL) programming and calibration via the FENDIAS diagnostic tool will be required to pair the new armrest with the tractor's basic control unit (A050).