A heavy civil contractor lodged a complaint regarding a John Deere 350G LC excavator experiencing severely sluggish boom and stick functions. The machine would take nearly ten seconds to raise the boom from the ground, and the stick would stall easily when digging in hard clay. The engine sounded healthy and did not lug down, but the hydraulics simply felt anemic. Initial service diagnostics showed no active fault codes for the main hydraulic pump or the ECM.
The diagnostic procedure began by teeing a gauge into the main pump outlet. At high idle with the boom lever activated, the pump pressure should have spiked to the system relief setting of roughly 5,000 psi. Instead, the gauge only registered 2,200 psi, indicating the pump was flowing fluid but failing to build pressure under load.
The main pump on the 350G LC is a variable displacement axial piston pump controlled by a load-sensing compensator valve. The technician removed and inspected the compensator valve. The spool was clean and moved freely, but the internal feedback spring had broken into two pieces. The broken spring prevented the pump from destroking properly in response to load pressure. However, replacing the compensator spring only slightly improved the response; the boom was still dangerously slow.
The breakthrough came when the pilot system pressure was checked. The excavator utilizes a separate gear pump mounted directly to the back of the main axial piston pump to supply pilot pressure. The pilot gauge registered a dismal 200 psi, far below the required 580 psi. Because the main pump's displacement is controlled by pilot pressure acting against the compensator, low pilot pressure meant the pump was stuck near maximum displacement but could not overcome the system resistance.
The pilot pump is driven by a short splined coupling that inserts into the main pump shaft. When the pilot pump housing was removed, the problem was obvious. The internal splines of the drive coupling had completely stripped out. The main pump shaft was spinning, but it was no longer turning the pilot pump. The 200 psi on the gauge was merely residual pressure back-feeding through the return lines. The pilot pump had been freewheeling, starving the main pump's control circuit of the pressure needed to function correctly. A new pilot pump coupling was fabricated and installed, and the pilot relief valve was recalibrated. The main pump was finally able to destroke and upstroke on demand, restoring the excavator's digging power to full factory specifications.
