Case 544, 656 Hydrostatic Drive (H70, H80) Tractors Service Manual

Pull the drive control valve and check the ball and seat sealing surfaces for scoring before assuming pump failure. Verify charge pump flow first on the test port; charge pressure below 500 PSI points to a worn charge pump or a relief valve stuck open. Check drive pressure separately, it should hold above 5800 PSI under load, per the checks on page 98.

Check for aeration first, a whining pump almost always means the system is pulling air rather than wearing out. Inspect suction line fittings, the dipstick tube seal, and pump inlet O-rings for leaks that let air in without letting oil out. Confirm charge pressure at the test port; readings under 500 PSI confirm cavitation is starving the pump. Bleed the system per the air leak diagnosis on page 84.

Drain the reservoir and inspect the fluid for metal fines, dark burnt oil usually means thrust plates or the ball guide are wearing and shedding material into the circuit. Replace both hydraulic filters immediately regardless of hours logged; the regular interval is 100 hours for the second change and 250 hours for every change after that, per page 63. Check oil cooler flow and clean any restriction before refilling, since restricted cooling drives the overheating on page 102.

Check servo pump pressure before touching the drive control valve, a weak servo signal is the usual reason the swashplate won't stroke fast enough to hit transport speed. Verify the neutral adjustment on the servo control linkage isn't dragging the swashplate off center, per the adjustment steps on page 77. Confirm charge pump flow is adequate under load; a pump barely keeping up will show sluggish direction changes and no power at speed, covered on page 100.

Adjust the neutral setting on the servo control and linkage before suspecting internal wear, most creep complaints trace back to a linkage that's drifted out of true center. Loosen the adjustment clevis, center the lever, and reset per the procedure on page 77. Recheck under load since creep that persists after adjustment points to a leaking servo cylinder seal rather than linkage misalignment.

Drain and flush the cooling system anytime antifreeze looks rusty or has gone past its service life, corrosion damage on these engines almost always traces back to neglected coolant rather than a manufacturing defect. Refill with the correct antifreeze/water mix and pressure test the system for external leaks before returning the tractor to work. Inspect the block and head for pitting at every coolant service going forward, since caught early it's a flush, caught late it's a casting replacement.