John Deere HMM3509 – HMM: Moisture Sensor Low Voltage Alert

The HMM (Moisture Sensor) control unit is experiencing a low voltage condition (less than 8V) on the AMS power circuit (cc# 912), triggering a service alert (FMI 18). This low voltage compromises the control unit's ability to retain data when the machine's key switch is turned off, which can lead to the loss of crucial stored information such as yield maps and affect overall system functionality.

• 9560 STS
• 9570 STS
• 9650 STS
• 9660 STS
• 9670 STS
• 9750 STS
• 9760 STS
• 9770 STS
• 9860 STS
• 9870 STS
• S560
• S670
• S690
• C670
• T-series
• W-series

1. 1

   Verify the presence of the active HMM 3509.18 fault code in the diagnostic menu on the machine's CommandCenter.

2. 2

   Check overall system voltage at the batteries and alternator, as well as switched and unswitched voltage on the display, to rule out a general charging issue.

3. 3

   Visually inspect the wiring harness and connectors leading to the moisture sensor in the grain tank for rodent damage, chafing, or corrosion (green scale) on the pins.

4. 4

   Check for blown fuses associated with the moisture sensor circuit, which can be caused by a failing motor or shorted circuit board.

5. 5

   Listen for the moisture sensor actuator to cycle when the key is first turned on. If it tries to move then stops, or makes repeated attempts, it indicates a mechanical bind or failing motor.

6. 6

   Test the 12VDC actuator motor by bypassing the control board/proximity switch and applying 12V directly to the motor to see if it runs continuously.

1. 1

   Access and clean sensor

   Access the moisture sensor located in the clean grain elevator (typically requiring entry into the grain tank). Thoroughly clean the assembly to remove any built-up grain, chaff, or debris that could be causing a mechanical obstruction.

2. 2

   Replace faulty components

   Based on diagnostic findings, replace the failed component. This may include internal relays, the circuit board (if corroded or blowing fuses), the motor/actuator assembly (if it failed the direct power test), or the complete moisture sensor module.

3. 3

   Update software

   If the moisture sensor module or circuit board was replaced, a dealer technician may need to perform a software download to the new HMM controller using a laptop, as replacement units often arrive blank from the factory.

4. 4

   Calibrate sensor

   Following any component replacement, perform a calibration through the CommandCenter. This must be done with the engine off and the sensor empty of grain to ensure correct mechanical operation and positioning of the plunger.