Fendt 5.1.52 – A004 - ECU, Control Panel / Control Console: Diff. lock automatic system key - Key / A004 signal fault.

Electrical signal failure or plausibility error originating from the button/key that activates the automatic differential lock. The system is receiving an invalid, missing, or stuck signal from the switch. The automatic differential lock function is disabled, but manual 100% differential lock remains active.

• 409 Vario
• 410 Vario
• 411 Vario
• 412 Vario
• 711 Vario
• 712 Vario
• 714 Vario
• 715 Vario
• 716 Vario
• 717 Vario
• 718 Vario
• 916 Vario
• 920 Vario
• 924 Vario
• 926 Vario
• 930 Vario
• 411 Vario COM III
• 412 Vario COM III
• 413 Vario COM III
• 414 Vario COM III
• 415 Vario COM III
• 712 Vario COM III
• 714 Vario COM III
• 716 Vario COM III
• 718 Vario COM III
• 818 Vario COM III
• 820 Vario COM III
• Farmer 400 Vario series
• Favorit 700 and 900 Vario series
• Vario 300, 400, 700, 800, and 900 series (including COM III generations)

1. 1

   Check the automatic differential lock button on the control console to ensure it is not physically stuck, jammed by debris, or damaged.

2. 2

   Remove the switch and perform an electrical continuity test with a multimeter to ensure internal contacts open and close properly.

3. 3

   Inspect the wiring harness between the differential lock button and the A004 control console board for pinched wires, chafing, or disconnected terminals.

4. 4

   Verify the A004 control console is receiving proper voltage by checking the CAN-bus voltage supply on fuse board A013 (fuses F040 and F041).

5. 5

   Use the FENDIAS diagnostic tool to monitor the live state of the switch and see if the A004 ECU registers button presses.

1. 1

   Power Down

   Turn off the tractor and disconnect the battery to prevent short circuits while working on the control console.

2. 2

   Access the Switch

   Carefully pry up or unscrew the trim panel on the A004 control console/multifunction armrest to access the underside of the differential lock switches.

3. 3

   Disconnect

   Unplug the electrical connector from the back of the automatic differential lock button.

4. 4

   Replace the Switch

   If the switch failed the continuity test, depress the retaining tabs on the side of the switch body and push it out of the console bezel. Snap the new OEM switch into place.

5. 5

   Repair Wiring

   If the switch is fine but the wiring is damaged, repair the broken wire using solder and heat shrink, ensuring the connection to the A004 board is secure.

6. 6

   Reassembly

   Reconnect the switch harness, seat the control console trim back into place, and reconnect the battery.

7. 7

   Clear Codes

   Turn the ignition on. The fault may move to a stored state. Use the tractor's terminal or the FENDIAS tool to clear the stored 5.1.52 fault code.

8. 8

   Test

   Start the tractor, drive at a low speed, and engage the automatic differential lock to verify the system functions normally and the code does not return.