John Deere HMM523313 – HMM: Massflow Sensor Signal Too Low

The mass flow sensor signal (circuit 939) is below the acceptable threshold. The mass flow sensor converts the force of harvested grain hitting an impact plate into an electronic signal for the GreenStar Yield Monitoring System. A 'signal too low' condition indicates that the HMM control unit is receiving a voltage from the sensor that is below the expected operational range, which can lead to inaccurate or zero yield readings.

• 9660 STS
• 9670 STS
• 9760 STS
• 9770 STS
• STS series combines
• WTS series combines

1. 1

   Check for any visible damage to the mass flow sensor and its wiring harness at the top of the clean grain elevator

2. 2

   Ensure the mass flow sensor is securely plugged into the wiring harness connector

3. 3

   Inspect the mass flow sensor impact plate for an accumulation of dirt, mud, or other debris

4. 4

   Review the diagnostic trouble code history for other codes related to the HMM controller or power supply (e.g., low voltage to the controller)

5. 5

   Confirm that the correct header width and crop type are selected in the combine's display

1. 1

   Access the Sensor

   Locate the mass flow sensor at the top of the clean grain elevator housing inside the grain tank.

2. 2

   Inspect Wiring and Connectors

   Carefully examine the wiring harness leading to the mass flow sensor for any signs of damage, corrosion, or loose connections. Repair or replace any damaged wiring.

3. 3

   Test Sensor Signal

   With the sensor connected and the machine running, back-probe the signal wire at the sensor's connector using a multimeter. The voltage should fluctuate as the force on the impact plate changes. A consistently low or zero voltage reading indicates a faulty sensor.

4. 4

   Calibrate the Mass Flow Sensor

   After cleaning, repair, or replacement of any components, perform a 'Mass Flow Vibration Calibration' through the combine's CommandCenter display. Run the machine at harvesting RPM with the separator and feeder house engaged to establish a baseline with no crop flow. A yield calibration should also be performed for each crop type.

5. 5

   Replace the Sensor or Controller

   If the sensor fails to produce a signal and the wiring is intact, the mass flow sensor itself is likely faulty and requires replacement. If the sensor and wiring are confirmed to be good, the issue may lie with the HMM controller or its circuit board.