A wheel loader was brought in with a terrifying high-speed symptom. In first and second gear, it operated perfectly fine, pushing dirt and loading trucks. But when the operator shifted into third or fourth gear to travel across the lot, the machine violently surged, bucking back and forth as if someone was turning the key off and on. The shop had replaced the fuel filters and the fuel transfer pump, assuming the engine was starving under high-speed load.
We hooked up a hydraulic pressure gauge to the transmission's torque converter outlet. In first and second gear, the pressure was steady. The moment it shifted into third, the gauge needle bounced wildly. The issue wasn't the engine; it was the torque converter lock-up clutch.
At high speeds, the ECU commands a solenoid valve to apply hydraulic pressure to a friction clutch inside the torque converter, physically locking the engine to the transmission for fuel efficiency. The diagnostic laptop showed that the lock-up solenoid was electrically commanding engagement, but the clutch was rapidly slipping in and out. We pulled the transmission control valve and found the lock-up solenoid spool was heavily contaminated with microscopic clutch debris. The debris was preventing the spool from fully shifting, causing the lock-up clutch to grab and release rapidly. This violent engagement and disengagement was shock-loading the drivetrain, causing the engine RPMs to surge up and down violently. We cleaned the spool bore and replaced the solenoid, and the loader drove smoothly at high speed.
