John Deere 1639 – Engine Control Unit (ECU): Fan Speed High — Most Severe Level

Issue description

Triggered when the controller detects that the cooling fan speed is operating at a higher RPM than the commanded or desired speed.

Determine if the DTC periodically goes away, which highly indicates an intermittent loose connection or chafed wire rather than a failed sensor.
Check the harness connection at the fan speed sensor (B53) and the Engine Control Unit (ECU)-to-engine interface harness connector 3 (X16). Ensure connectors are in good condition and free of corrosion and debris.
Check the wiring at the fan speed sensor and at the solenoid for continuity on the signal wires (e.g., check for continuity between fan speed sensor pin B and ECU connector X16 pin 33, and sensor pin C to the ECU). Look specifically for a broken signal wire going to the speed sensor.

Access the Fan Drive
Locate the hydraulic fan drive motor where the fan speed sensor (B53) is mounted.
Disconnect and Inspect
Disconnect the harness from the fan speed sensor and the ECU. Inspect the pins for damage, moisture, or pushed-back terminals. Repair or replace the connector/pins if necessary.
Repair Wiring
If the continuity test reveals an open circuit or a broken signal wire, splice and repair the wire according to OEM wiring repair standards.
Replace the Sensor
If the wiring harness, ground, and connectors all test good, the fan speed sensor itself has likely failed internally. Unbolt the old sensor from the hydraulic fan motor and install the new one.
Clear and Verify
Reconnect all harnesses, clear the ECU fault codes using a diagnostic tool, and run the machine to verify that the fan speed regulates properly and the code does not return.

(3)

Fan Speed Sensor
AN306751
Fits Excavators (470GLC, 470P) and Sprayers (R4023, 4630, Hagie STS12, STS16).
AN306751

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