A quarry operator reported that a Caterpillar 966M wheel loader would slowly creep forward when the transmission was in neutral and the parking brake was released. Additionally, the machine felt like it was dragging a brake when driving at high speed. The operator had to constantly apply the service brake to keep the machine stationary while idling.
The technician first checked the transmission oil and found it to be clean and at the correct level. The neutral creep issue is often caused by residual pressure in the clutch packs or a stuck spool in the control valve. However, pressure gauges showed zero pressure in all clutch circuits when in neutral.
The investigation shifted to the parking brake assembly, which is located on the output shaft of the transmission. Upon disassembly, the technician found that the key connecting the parking brake hub to the transmission output shaft had sheared off. The hub was rotating slightly on the shaft due to friction, but the position sensor was reporting the shaft as stationary (neutral).
The dragging sensation was caused by the forward clutch plates being slightly warped. Even though hydraulic pressure was released, the warped plates were dragging against each other due to fluid viscosity, creating a "viscous coupling" effect that caused the creep. The parking brake hub key was replaced, and the forward clutch pack was rebuilt with new friction plates and seals. The transmission was reassembled, and the creep disappeared, restoring neutral stability.
