Fault Phenomenon: John Deere 772G grader hydraulic pump cannot vary displacement, system flow fixed at maximum or minimum.

Troubleshooting Process:
– Checked actuator: swashplate actuating piston stuck.
– Checked control oil: insufficient pilot pressure.
– Checked spring: return spring broken.
– Checked oil quality: severely contaminated fluid.

Root Cause: The swashplate actuator of the variable piston pump is stuck due to oil contamination, unable to adjust displacement based on load.

Required Parts: Hydraulic pump overhaul kit or replace pump.