A heavy-duty, diesel-powered sit-down forklift was experiencing violent bucking and jerking, but only when driving over rough concrete dock plates or railroad tracks. On smooth floors, it drove perfectly. The shop had rebuilt the transmission valve body and replaced the torque converter, assuming the clutches were slipping under shock loads. The problem remained untouched.
Because the failure was entirely shock-related, we knew the issue had to be a physical connection breaking contact momentarily. We looked at the electrical diagram for the traction system. Modern diesel forklifts have an Operator Presence Switch (OPS) built into the seat. If the seat senses no weight, the computer kills the transmission solenoids to prevent a runaway machine.
We had an operator drive the forklift while we watched the dash indicator. Every time the truck hit a bump and bucked, the "Seat" indicator light on the dash flashed for a fraction of a second. We took the seat off and found that the metal bracket holding the heavy-duty plunger switch had a microscopic crack in the weld, allowing it to flex upward. When the forklift hit a bump, the operator's body lifted slightly off the seat. The cracked bracket flexed, the plunger switch popped open for a millisecond, and the engine ECU instantly cut the power to the transmission. The operator would then slam back into the seat, the switch closed, the power hit instantly, and the truck violently lurched forward.
It felt exactly like a transmission slipping and grabbing, but it was actually the engine management system performing its designed safety shutdown hundreds of times a shift. We welded the cracked bracket and added a heavy-duty zip tie as a strain relief. The bucking disappeared completely. When a machine cuts power only over bumps, always verify the seat switch is staying closed before you touch the drivetrain.
