A wheel loader was brought in with a dangerous and annoying symptom. Whenever the operator put the machine in gear, it would violently lurch forward, even with the engine at low idle. The operator had to stand heavily on the brake pedal to keep the truck from taking off. The shop assumed the transmission clutch packs were welded together, but a stall test showed the clutches were releasing fine in Neutral.
The problem wasn't in the transmission; it was in the inching valve. On a loader, the brake pedal acts as an "inching" valve for the transmission. As you press the brake pedal, a spool valve in the hydraulic line dumps the modulation pressure to the transmission clutches, smoothly disengaging them before the mechanical brake pads bite.
We pulled the inching valve spool and found severe scoring on the hard-chrome plating. The loader had been operating in a highly contaminated environment, and microscopic grit had scored the spool bore. Because of this scoring, when the transmission was in Neutral, high-pressure pilot oil was "cross-porting"-bleeding past the spool directly into the forward clutch circuit. The forward clutch was slowly applying itself even though the shifter was in Neutral. The moment the operator shifted to Forward, the clutch was already halfway engaged, causing the violent lurch. We honed the valve body and replaced the scored spool. The loader shifted smoothly again, proving that a violent shift isn't always a transmission problem; it's often a tiny valve leaking pressure where it shouldn't.
