John Deere 1639.18 – ECU: Fan Speed Lower Than Expected

This code triggers when the engine controller commands a specific cooling fan speed, but the fan speed sensor detects that the fan is rotating at a moderately or significantly lower speed than requested. If left unresolved, this condition can cause the engine to overheat and the system to trigger an engine power derate to protect the machine.

• 5085E
• 5090E
• 5090EL
• 5100E
• 5085M
• 5100M
• 5100MH
• 5100ML
• 5115ML
• 5090M
• 5115M
• 9230
• 9330
• 9430
• 9530
• 9630
• 9360R
• 9370R
• 8120
• 8220
• 8320
• 8420
• 8520
• 8230T
• 8330T
• 8430T
• 7610
• 7710
• 7810
• 7830
• S660
• S670
• S760
• S770
• R4023
• 4630
• 7760
• 870G

1. 1

   Check the fan drive bleed screw to verify it is fully closed and no service cap screw is left in the vent hole

2. 2

   Inspect the variable speed fan drive pulley for external oil leakage

3. 3

   Inspect the drive belt and spring-loaded automatic belt tensioner for hardness, stretching, glazing, slipping, or weak tension

4. 4

   Test the fan speed sensor and wiring harness for a broken signal wire

1. 1

   Close the Bleed Screw

   If the bleed screw was left open from a previous service, simply close it, clear the ECU codes, and test run the machine at full RPM to see if the code returns.

2. 2

   Repair Sensor Wiring

   If the diagnostic tests reveal a broken signal wire, splice and repair the wiring harness leading to the fan speed sensor.

3. 3

   Replace the Fan Belt and Tensioner

   If the belt is hard or slipping, replace it. Note for large tractors (e.g., 9330/9630): This requires a helper. You must take the outer belts and the oil line off. With a second person helping, use a pry bar to split the clutch and pop the belt off the bottom pulley. Replace the belt tensioner at the same time if it is weak.

4. 4

   Replace the Variable Speed Fan Drive

   If the fan drive pulley is leaking oil or internally failed, the entire variable speed fan drive assembly must be replaced.