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Solution to April’s snow plow problem
Turner Hydraulics felt that regardless of the possible problems in the valve assembly, the pump should compensate. The PAVC65 Parker pump is a little different from most load sensing, pressure compensated pumps because it does not have a case drain line. The case is part of the suction connection, and the drain line is actually a control drain. In addition to this difference, most pumps have the compensator and load sense controls assembled in a parallel circuit setup so the control with the lowest setting causes the pump to compensate.
The Parker pump has its load sense differential spring area relieved by the maximum pressure compensator. They are confi gured in series, and changing the load sense differential spring increases or decreases the maximum compensator setting.
It was discovered that the control drain and load sense connections were switched — causing the pump to respond like a fi xed-displacement pump without a relief. The two identical looking hoses were switched, and the pump began compensating. However, an additional problem was discovered. When the system was running in the neutral position, pressure dropped to about 125 psi. The system pressure should have gone to 2000 psi and compensated the pump. Further inspection found the directional valve outlet section was set up with a pipe plug isolating the pressure from the tank, making the valve a blocked-center type necessary for these types of pumps. They found the pipe plug threads were stripped and the plug was lodged in the tank cavity. This probably caused the “thump” noise when they fi rst started the system up. After this was corrected, the pump now compensates at the 2000 psi setting.