A aggregates processing yard reported that a Case 621G XR wheel loader would intermittently lose hydraulic cooling during heavy loading cycles. The operator noticed the hydraulic oil temperature gauge climbing rapidly past 100°C within 15 minutes of heavy V-pattern bucket work, then abruptly recovering to 78°C when the machine was idled. On several occasions, the reversing cooling fan-a hydraulic-driven fan used to periodically reverse airflow to purge debris from the cooler core-appeared to stop entirely during operation.
The technician first verified the hydraulic oil level and condition (correct), and checked the cooler core, which was moderately plugged with dust and seed debris but not severely blocked. He then located the "Hydraulic Cooling Fan Drive Motor" mounted to the cooler frame. The Case 621G XR uses a variable-displacement hydraulic motor to drive the cooling fan, and the "Reversing Fan Diverter Valve"-an electronically controlled three-position valve-reverses the motor's direction to blow debris off the cooler core for several seconds every 15 minutes.
He installed a flow meter on the fan motor's supply line and ran the engine at full speed. Under normal conditions, the fan motor should receive a steady 90 liters per minute at 210 bar. The flow meter read only 45 liters per minute, and the pressure fluctuated between 120 and 210 bar. He then cycled the reversing-fan diverter valve through its three positions (forward, neutral, reverse) using the diagnostic tool. In the reverse position, the flow dropped to 10 liters per minute and the pressure spiked to the relief setting, indicating the valve was not fully shifting.
He removed the diverter valve from the fan-drive manifold. The internal spool was sticky, and the "Fan Motor Coupling"-a flexible spline coupling between the hydraulic motor's output shaft and the fan hub-was found severely worn, with the spline teeth rounded off and the coupling occasionally slipping under high torque load. The rounded spline caused the motor to intermittently slip under load, wasting hydraulic energy as heat and reducing fan speed. Simultaneously, the sticky diverter valve spool-coated with varnish from thermally degraded oil-was not fully shifting into position, restricting the flow to the fan motor. Both faults had created a cascade: the reduced fan speed caused the cooler to be under-cooled, the resulting oil overheating degraded the oil and generated varnish, which further stuck the diverter valve spool.
The repair involved replacing the worn fan motor coupling and the fan drive hydraulic motor, which had suffered internal wear from the excessive slip and heat. The diverter valve was disassembled, cleaned with brake cleaner, and the spool was polished to restore free travel. The hydraulic oil was sampled and found to be thermally degraded; the system was flushed, and fresh ISO 68 anti-wear hydraulic oil was introduced with new return and pilot filters. The cooler core was back-flushed with compressed air to remove the accumulated debris. After reassembly, the fan motor received full 90 liters per minute flow, the diverter valve shifted cleanly through all three positions, and the loader ran a continuous 2-hour heavy bucket cycle with the hydraulic oil stabilized at 82°C.
