Fault Phenomenon: John Deere 850K dozer when using GPS grading, the blade height slowly drifts automatically.

Troubleshooting Process:
– Checked tilt sensor: zero-point drift.
– Checked solenoid valve: proportional valve null leakage.
– Checked hydraulic lock: internal leakage.
– Checked battery: unstable battery voltage.

Root Cause: The proportional control solenoid valve in the automatic grade control system has internal null leakage.

Required Parts: Replace proportional solenoid valve.