Fendt 5.1.99 (S025, S026) – E-Box / Basic Control Unit (A050): Short-circuit to earth (ground) in the signal line for S025 or S026

Short-circuit to earth (ground) in the signal line for either the S025 (LS pump pressure-operated switch / variable displacement pump pressure monitoring switch) or the S026 (Auxiliary pump flow monitor / steering pump flow controller). The system registers a 0/0 V test sequence, resulting in a complete loss of monitoring for either pump. The control system reacts by reducing the control valve flow to 20 l/min, shutting off the hydraulic oil preheater, and locking the fault code. Triggers after the engine has been running for at least 1 second at >1000 rpm.

• 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
• Farmer 400 Vario Series: 409
• Farmer 400 Vario Series: 410
• Farmer 400 Vario Series: 411
• Farmer 400 Vario Series: 412
• Favorit 700 / 700 Vario Series: 711
• Favorit 700 / 700 Vario Series: 712
• Favorit 700 / 700 Vario Series: 714
• Favorit 700 / 700 Vario Series: 715
• Favorit 700 / 700 Vario Series: 716
• Favorit 700 / 700 Vario Series: 717
• Favorit 800 / 800 Vario Series: 815
• Favorit 800 / 800 Vario Series: 817
• Favorit 800 / 800 Vario Series: 818
• Favorit 900 / 900 Vario Series: 916
• Favorit 900 / 900 Vario Series: 920
• Favorit 900 / 900 Vario Series: 924
• Favorit 900 / 900 Vario Series: 926
• Favorit 900 / 900 Vario Series: 927
• Favorit 900 / 900 Vario Series: 930
• Favorit 900 / 900 Vario Series: 933
• Favorit 900 / 900 Vario Series: 936
• Favorit 900 / 900 Vario Series: 939

1. 1

   Voltage Test: Perform a voltage test on the signal lines for S025 and S026. The fault condition is confirmed if the test sequence reads 0 / 0 V (indicating a dead short to ground).

2. 2

   Wiring Harness Inspection: Consult the 'Spool valves 1' circuit diagram (often found on Sheet 7 or Sheet 26 depending on the model year). Trace the wiring harness from the E-Box to the S025 and S026 switches.

3. 3

   Connector Check: Disconnect the sensors and check if the short to ground persists on the harness side. If the 0V reading disappears when the sensor is unplugged, the sensor itself is internally shorted. If the short remains, the wiring harness is chafed or pinched against the chassis.

1. 1

   Isolate the Short

   Based on your diagnostic checks, determine whether the short is in the wiring harness or internal to the S025/S026 switches.

2. 2

   Repair Wiring (If Applicable)

   If the harness is chafed (commonly where it routes near the valve block or transmission housing), repair the damaged section of the wire, apply heat-shrink tubing, and re-secure the harness away from pinch points.

3. 3

   Replace Sensor (If Applicable)

   If the sensor is internally shorted, relieve hydraulic system pressure, unscrew the faulty S025 pressure switch or S026 flow monitor, and install the replacement part.

4. 4

   System Reset

   Because this specific fault code is hard-coded to lock, you cannot clear it via standard terminal clearing. You must perform a 'Key reset / restart' (turn the ignition completely off, wait for the E-Box to power down, and restart the tractor) to clear the code and restore full hydraulic flow.

5. 5

   Verify Operation

   Run the tractor above 1000 rpm for several seconds to ensure the code does not return and that hydraulic flow exceeds the restricted 20 l/min limit.