An aggregates yard reported that a CAT 950M wheel loader would pull strongly in forward gears but would slip and eventually stop moving in reverse after 20 minutes of operation. The transmission temperature gauge was consistently reading high.
The technician connected the Electronic Technician (ET) software to monitor the clutch pressures. The forward clutch pressures were steady at 300 psi. When reverse was selected, the pressure would build to 300 psi initially, but then drop to 150 psi as the oil temperature rose. This pointed to an internal leak in the reverse clutch circuit that worsened as the oil thinned out with heat.
The transmission valve body was removed to access the clutch pressure regulation valves. The valves were clean, but the reverse clutch modulation valve showed signs of scoring. However, the real issue was found when the transmission sump was dropped. The suction screen was found to have chunks of black rubber on it. These were traced back to the reverse clutch piston seals inside the transmission.
A torn seal allows high-pressure oil to bypass the piston, preventing the clutch plates from engaging. The transmission was removed and disassembled. The reverse clutch piston seals were indeed torn and brittle. The source of the heat was traced to the transmission oil cooler. The cooler fins were completely blocked with a mixture of rock dust and hydraulic oil "mayonnaise" (emulsified oil from condensation). The blocked cooler caused the transmission fluid to overheat, accelerating the degradation of the rubber seals.
The repair involved rebuilding the reverse clutch pack with new seals and friction plates, flushing the cooler, and installing a new suction screen. After refilling, the loader operated perfectly in both directions, and the transmission temperature stabilized.
