A large quarry reported that a SEM 663D wheel loader experienced a violent "jerk" (shock) when shifting from 1st to 2nd gear. Additionally, the machine would not shift into 3rd gear (high speed) when traveling on flat haul roads. The transmission temperature was normal.
The technician connected a pressure gauge to the "Forward and Reverse Clutch" ports. The pressure gauge showed a rapid spike to 3.0 MPa (overshoot) during the 1st to 2nd shift, confirming the harsh engagement. The pressure for the 3rd gear clutch was non-existent.
He removed the "Transmission Control Valve." The 3rd gear shift valve spool was stuck in the "Disengaged" position by a small piece of silicone sealant used in a previous repair. This debris blocked the pilot pressure from moving the spool, preventing 3rd gear engagement.
The harsh shifting (jerk) was traced to the "Modulating Valve Accumulator." This accumulator uses a nitrogen-charged piston to dampen the pressure rise during shifts. He tested the accumulator charge. The nitrogen pressure was zero. The "O-ring" on the accumulator piston had failed, allowing the gas to escape. Without the accumulator cushion, the hydraulic pressure hit the clutch pistons instantly, causing the violent jerk.
Furthermore, upon inspecting the clutch pack, he found the "Piston Seals" for the 2nd gear clutch were worn. The worn seals caused a pressure lag, which the system tried to compensate for by ramping up pressure rapidly, exacerbating the jerk.
The repair involved cleaning the control valve spool, replacing the accumulator piston seal and re-charging the accumulator with nitrogen to 0.5 MPa. The worn clutch seals were also replaced. The valve body was reassembled with new gaskets (avoiding excess sealant). Afterward, the shifts were smooth, and the loader shifted into 3rd gear correctly.
