Fendt 18.1.02 – A039 - MFA (Multifunction Armrest): A039 - MFA, multifunction armrest: Auto-Guide full activation button faulty, Signal fault

The system has detected an invalid electrical signal from the Auto-Guide activation button located on the multifunction armrest. The underlying condition is typically a short circuit, an open circuit, or a physically stuck button that is sending a continuous, uninterrupted signal to the controller.

• 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, S4)
• Fendt 800 Vario (COM III, S4)
• Fendt 900 Vario (COM III, S4)

1. 1

   Visual & Physical Inspection: Inspect the Auto-Guide full activation button on the armrest/joystick. Check if the button is physically stuck in the depressed position due to dirt, spilled liquids, or mechanical wear.

2. 2

   Signal Testing via Terminal: Use the tractor's onboard diagnostic terminal or the FENDIAS dealer software to monitor the live state of the Auto-Guide button. Verify if the signal is permanently active (stuck closed) or entirely missing (open circuit) when pressed.

3. 3

   Wiring & Connection Check: If the button feels mechanically sound, inspect the internal ribbon cables and wiring harnesses within the MFA that connect the button matrix to the armrest's main circuit board. Look for pinched wires or loose connectors.

4. 4

   Check for Accompanying Codes: Ensure there are no broader CAN bus faults (such as 18.1.03 'MFA fails to report'). If 18.1.03 is present, the issue is a total communication failure of the armrest, not just the button.

1. 1

   Power Down

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

2. 2

   Clean the Switch

   If the button is physically sticking, carefully clean the edges of the button housing using a plastic-safe electrical contact cleaner and compressed air to free it.

3. 3

   Disassemble and Replace Component

   If the button is internally shorted or broken, carefully disassemble the upper housing of the multifunction armrest. Replace the specific faulty button switch or the affected joystick/keypad sub-assembly. (Note: Depending on the exact generation, AGCO may require replacing the joystick handle or the keypad cluster rather than a single microswitch).

4. 4

   MFA Replacement (If necessary)

   If the internal control board of the armrest has failed and cannot process the button signal, the entire A039 Multifunction Armrest assembly must be replaced.

5. 5

   EOL Programming

   If a completely new A039 Multifunction Armrest module is installed, End-of-Line (EOL) programming must be performed using the FENDIAS dealer software. This pairs the new armrest to the A050 Basic Control Unit.

6. 6

   Clear Codes & Test

   Reconnect the battery, turn on the ignition, clear the stored 18.1.02 fault code, and test the Auto-Guide activation to ensure the signal fault is resolved.