Fendt 5.1.99 (S026) – Main Tractor Electronic Control Unit (ECU / E-Box) / Enhanced Control module (A002): Auxiliary pump flow monitor — Minimum quantity is not reached

Triggers when the engine is running at over 1000 RPM for at least 1 second, but the S026 flow monitor switch does not detect sufficient hydraulic oil flow or pressure from the auxiliary constant displacement pump. The system automatically implements safety restrictions: control valve flow is reduced to a maximum of 20 l/min, the hydraulic oil preheater is switched off, and the fault requires a key reset/restart to clear.

• 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 series
• 409
• 410
• 411
• 412
• Favorit 700 series
• 700 Vario series
• 711
• 712
• 714
• 715
• 716
• 717
• Favorit 900 series
• 900 Vario series
• 916
• 920
• 924
• 926
• 930
• 300 Vario series
• 800 Vario series
• 815
• 817
• 818

1. 1

   Verify the trigger condition: ensure the fault only appears after the engine has been running at >1000 RPM for at least 1 second (immediate appearance at key-on indicates a hard electrical short).

2. 2

   Test the S026 switch voltage: must be < 1.1 V with engine OFF, and > 1.7 V at engine speeds > 1000 RPM.

3. 3

   Inspect the signal line wiring harness for the S026 switch for a short-circuit to ground (a short to earth results in a 0/0 V reading).

4. 4

   Check if the S026 switch is physically stuck open or closed without oil flow (plausibility error).

5. 5

   Verify auxiliary pump pressure using a manual hydraulic pressure gauge to rule out mechanical failure.

1. 1

   Locate the Component

   Turn off the tractor. The S026 switch is located on the right side of the tractor, at the control valves. You will need to remove the right-side panel to access it.

2. 2

   Inspect Wiring

   Disconnect the electrical connector to the S026 flow monitor switch. Inspect the pins for corrosion and test the wiring harness back to the ECU for continuity and shorts to ground. Repair any damaged wiring.

3. 3

   Replace the Switch

   If the wiring is intact but the voltage values are out of spec (or the switch is stuck open/closed), remove and replace the S026 flow monitor switch.

4. 4

   Test Hydraulic Pressure

   If the new switch does not resolve the code, the auxiliary hydraulic pump is failing to produce the minimum flow. Connect a flow/pressure meter to the auxiliary circuit to confirm.

5. 5

   Replace the Auxiliary Pump

   If mechanical failure is confirmed, drain the hydraulic fluid and replace the auxiliary constant displacement pump.

6. 6

   Reset the System

   This specific fault code cannot be cleared with a standard terminal reset. You must perform a key reset (turn the ignition completely off, wait, and restart the tractor).

7. 7

   Verify Repair

   Run the tractor engine above 1000 RPM for several seconds. Verify that the fault does not return and that the control valve flow is no longer restricted to 20 l/min.