JLG 450AJ Boom Drifts Down Overnight

Confirmed cause: The counterbalance valve for the main lift cylinder had an internal leak, allowing fluid to slowly bypass its poppet and seat. Repair performed: Replaced the counterbalance valve cartridge on the main lift cylinder valve block. The old valve was inspected and found to have a contaminated and worn poppet seat. Verification: The boom was elevated and left overnight. It held its position with no measurable drift. Technician notes: - This fault is often misdiagnosed as a cylinder piston seal failure. Always isolate components systematically. - Before condemning the valve, ensure pilot pressure is not being applied due to an electrical or mechanical fault in the control system. In this case, the machine was off, so pilot pressure was not a factor. - A small piece of debris was found in the old valve's seat. This highlights the importance of fluid cleanliness.