A construction site reported that a Liugong CLG856H wheel loader would not move forward or backward after a routine service on the air-over-hydraulic braking system. The technician had replaced the brake pads and adjusted the parking brake, but now the engine would rev freely, and the machine remained stationary.
The CLG856H uses a transmission cut-off system to improve braking efficiency. When the operator presses the brake pedal, a pneumatic valve (the "Cut-off Valve") sends a signal to the transmission control valve to disengage the clutches, effectively putting the transmission in neutral to reduce the load on the brakes. The technician first checked the transmission oil level and filter, which were clean. He then measured the air pressure at the transmission cut-off valve. He found that there was constant air pressure at the valve, even when the brake pedal was released.
This indicated that the cut-off valve was stuck in the "brake applied" position. The technician inspected the brake pedal linkage. He discovered that during the brake pad replacement, the pushrod for the brake valve had been adjusted too long. This prevented the brake valve from fully returning to the "released" position. Consequently, the valve continued to send a small amount of air pressure to the transmission cut-off valve, enough to keep the clutches disengaged but not enough to apply the service brakes.
The repair involved shortening the brake pushrod to the manufacturer's specified height. Once adjusted, the brake pedal returned fully to its stop, the air signal to the transmission stopped, and the loader was able to engage gears and drive normally. This case highlights the importance of checking the interaction between safety systems and powertrain controls.
