A wheel loader was suffering from a complete loss of hydraulics the moment the bucket touched a pile. The engine wouldn't lug down, and the pressure gauge would drop to zero. The main pump had been swapped twice, and the suction hoses checked. The problem persisted.
We focused on the charge pump. In a hydrostatic transmission and closed-center hydraulic system, the charge pump is a small gear pump that maintains a constant "base pressure" to keep the main piston pistons pressed against the swashplate. Without charge pressure, the main pistons float, and the pump cannot generate torque.
We removed the metal spin-on filter (strainer) for the charge pump. It was crushed inward like a soda can. The bypass valve in the filter head had failed to open, and the pump was trying to suck oil through a plugged filter. The vacuum created by the pump was so strong that it physically collapsed the metal filter housing. Once the housing collapsed, it completely cut off the flow of oil to the charge pump. With no charge pressure, the main pump pistons retracted from the swashplate, and the loader lost all drive and lift the instant the system was loaded. Replacing the $20 filter and the stuck bypass valve restored the machine. If a main pump is swapped but the machine has no power, always check if the charge pump filter is collapsed.
