A sand and gravel yard reported that a Liugong CLG856III wheel loader would stall whenever the operator applied the service brakes sharply while driving downhill. The engine would die immediately, requiring a restart. The issue did not happen during gentle braking.
The technician first verified the braking system components. The brake booster (air-over-hydraulic) was functioning. However, he noticed that the "Transmission Cut-Off Valve" (which disengages the transmission when brakes are applied) was staying engaged even after the brake pedal was released. On the CLG856III, an air signal from the brake valve activates a pneumatic cylinder on the transmission control valve to cut power to the wheels.
The technician removed the air line to the cut-off cylinder. He found that the check valve in the air line was installed backward, blocking the air exhaust. When the driver hit the brakes hard, the air pressure surged into the cut-off cylinder. Because the check valve was blocked, the air could not vent quickly. This kept the transmission in "Neutral" even as the driver released the brake and stepped on the accelerator. The engine, now under load but with the transmission effectively disconnected, stalled due to the sudden resistance change and the drop in RPM.
Furthermore, the brake pedal return spring was weak. The technician reversed the check valve to the correct orientation and replaced the pedal return spring. He also adjusted the stroke of the brake valve pushrod. After these adjustments, the transmission engaged immediately upon brake release, and the stalling issue was resolved.
