A wheel loader was brought in with a dangerous symptom. Every time the operator pushed into a tough gravel bank, the transmission would violently pop out of gear into neutral. They could drive across the lot fine, but under high torque, it dropped out. The shop replaced the shift solenoids, but the ghost remained.
The problem was in the transmission's neutralizer valve system. A powershift transmission uses the neutralizer to momentarily dump clutch pressure to allow smooth shifts and prevent shock loads. When the operator moves the direction lever, the neutralizer spool shifts, bleeding off main pressure to the directional clutches, and then slowly closes to re-apply pressure. The speed of this re-application is controlled by a tiny orifice-a hole often less than 0.020 inches in diameter-in the spool housing.
We pulled the neutralizer valve and found the micro-orifice was packed with a tiny piece of Teflon seal debris. With the orifice plugged, the neutralizer spool couldn't close properly under high flow conditions. When the operator drove into the gravel bank, the torque spike created a momentary pressure fluctuation in the system. The stuck neutralizer spool reacted to the fluctuation by dumping all the clutch pressure, instantly dropping the machine into neutral. We cleared the orifice with a single strand of copper wire from a welding brush, and the loader pulled hard through the bank without dropping a gear.
