A cold-chain distribution warehouse reported that a Mitsubishi FG25N reach truck had developed a progressive reach-function fault over a three-week period. When the operator extended the carriage under load (a full pallet at approximately 3 meters of reach), the carriage extension was noticeably slower than normal - taking roughly 16 seconds to reach the desired extension point, versus the normal 12 seconds. More critically, when the operator stopped the reach function mid-extension and held the carriage position, the carriage would slowly retract - approximately 4 centimeters over 30 seconds - requiring the operator to continuously hold the reach lever forward to maintain reach position. The operator also reported a faint rhythmic "knocking" sound from the reach-cylinder area during extension, and the truck's mast channels showed visible uneven wear concentrated on the right side.
The technician first understood the FG25N reach truck's reach architecture. The reach truck uses a telescopic "Reach Cylinder" mounted within the mast's outer rails, with the cylinder's rod-end connected to the reach carriage and its cap-end connected to the mast's base. When the operator commands carriage extension, hydraulic pressure enters the cylinder's cap-end chamber, pushing the cylinder's piston - which pulls the reach carriage outward along the mast rails. The cylinder's rod is guided within a "Reach Cylinder Guide Bushing" mounted at the cylinder's rod-end - a bronze bushing that maintains the rod's alignment within the cylinder housing and prevents lateral rod movement under load. When the piston seal fails, high-pressure oil bypasses internally between the cap-end and rod-end chambers, producing the slow extension and the uncontrolled retraction the operator described. When the guide bushing wears, the rod is allowed to move laterally under load, producing the knocking sound and accelerating the wear on both the rod's surface and the cylinder's bore.
He verified the symptom by extending the carriage under a loaded test pallet to approximately 3 meters of reach, stopping the reach function, and monitoring the carriage's position over 5 minutes. The carriage retracted approximately 36 centimeters over 5 minutes - far beyond the 2-centimeter allowable limit, confirming internal bypass in the reach circuit.
He isolated the fault by first checking the "Reach Cylinder" with the carriage fully retracted and the truck on level ground. He disconnected the cylinder's rod-end hydraulic line, capped it, and pressurized the cylinder's cap-end side to its working pressure using an external test pump. The cylinder's rod drifted outward by 8 millimeters over 5 minutes - beyond the 1-millimeter allowable limit, confirming the cylinder's "Piston Seal" had failed and was allowing high-pressure oil to bypass internally between the cap-end and rod-end chambers.
He then inspected the "Reach Cylinder Guide Bushing" mounted at the cylinder's rod-end. The bronze bushing showed visible wear - the bushing's inner surface had worn into an oval profile, with the oval's long axis roughly 2.2 millimeters wider than the bushing's nominal diameter (spec maximum is 1 millimeter of clearance). The worn bushing had allowed the cylinder's rod to move laterally under load, producing the rhythmic "knocking" the operator described and accelerating the wear on both the rod's surface and the cylinder's bore. The cylinder's rod itself showed visible scoring along its stroke length - consistent with the worn guide bushing allowing metal-to-metal contact between the rod and the cylinder's end cap.
The root cause was traced to piston seal failure compounded by guide bushing wear. The reach cylinder's piston seal is subjected to continuous high-pressure load cycles during carriage extension and retraction, with peak pressures occurring when the truck extends the carriage while carrying a heavy load. Over extended service (the truck had approximately 7,200 hours), the seal's elastomer had hardened and cracked, eventually allowing high-pressure oil to bypass internally. The worn guide bushing had allowed lateral rod movement under load, which had placed asymmetric stress on the piston seal - accelerating the seal's wear and producing the knocking sound the operator described. The asymmetric rod movement had also placed uneven lateral loads on the mast's right channel, producing the uneven channel wear the operator observed.
The repair involved honing the cylinder bore to remove the scoring, replacing the complete piston seal assembly with a new OEM seal kit (including the primary lip seal, secondary seal, and wear ring), replacing the reach cylinder's guide bushing with a new unit, and replacing the rod-end seal and wiper. The cylinder's rod was inspected for scoring - the scoring was shallow and the rod was polished to restore a smooth surface rather than replaced. The reach circuit's hydraulic oil and return filter were replaced with fresh specified products, and the reach circuit was bled to remove trapped air. After reassembly, the carriage retraction under load was eliminated - the carriage held position with no measurable drift over a 5-minute loaded test. Carriage extension time was restored to its normal 12 seconds under full load, the rhythmic knocking was gone, and the mast channel wear was expected to stabilize. Reach cylinder seal inspection and guide bushing replacement were added to the truck's annual inspection checklist.
