New Holland ENG 3987 – ENGINE: Fuel Pump Rack Position Sensor: Output Voltage Low

The engine control module is receiving a voltage signal from the fuel pump rack position sensor that is below the minimum expected threshold, which typically points to a short circuit to ground in the sensor's signal wire, a disconnected wiring harness, or an internal failure of the sensor itself.

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1. 1

   Inspect the wiring harness leading to the fuel injection pump and the rack position sensor for disconnected plugs, chafed wires, or corrosion in the connector pins.

2. 2

   Using a multimeter, verify that the sensor is receiving the correct reference voltage (typically 5V) and has a solid ground connection from the Engine ECU.

3. 3

   Test the signal wire between the sensor connector and the Engine ECU to ensure there is continuity and that the wire is not shorted to ground.

4. 4

   Monitor the sensor's output voltage while the key is on. If the voltage remains stuck below the minimum threshold regardless of engine state, the sensor has failed.

1. 1

   Turn off the machine and disconnect the battery

   This prevents electrical shorts during repair.

2. 2

   Locate the fuel pump rack position sensor

   The sensor is mounted on or integrated into the fuel injection pump assembly on the ISM-IDI engine.

3. 3

   Disconnect the electrical harness from the sensor

   If the connector is heavily corroded, clean it with contact cleaner and apply dielectric grease.

4. 4

   Repair or replace the wiring harness

   If the diagnostic tests revealed a short to ground in the wiring harness, repair or splice the damaged section of the wire, or replace the affected harness section.

5. 5

   Replace the sensor

   If the sensor itself is faulty, remove its mounting hardware and replace the sensor. Note: On some ISM-IDI injection pumps, the rack position sensor is integrated into the governor or pump assembly and cannot be serviced separately without specialized pump calibration.

6. 6

   Reconnect the battery

7. 7

   Clear the existing fault codes

   Use a diagnostic tool or Can-Bus DTC Eraser to clear the existing fault codes from the engine ECU.

8. 8

   Start the engine and verify

   Verify that code 3987 does not return and that the engine operates smoothly.