Gehl 6635 Hydrostatic Pump Controller Troubleshooting and System Behavior

[MikePhua]

The Gehl 6635 and Its Hydraulic Architecture
The Gehl 6635 SXT skid steer loader, produced in the late 1990s and early 2000s, was part of Gehl’s high-performance compact equipment lineup. Known for its robust frame, high lift capacity, and responsive controls, the 6635 featured a hydrostatic drive system powered by dual variable displacement pumps. These pumps were electronically modulated via a controller that adjusted swash plate angles based on joystick input, enabling smooth forward and reverse motion.
Gehl, founded in 1859 in Wisconsin, evolved from agricultural machinery into compact construction equipment. By the time the 6635 was released, Gehl had become a respected name in the skid steer market, competing with Bobcat, Case, and New Holland. Thousands of 6635 units were sold across North America, many of which remain in service today due to their mechanical simplicity and ease of repair.
Hydrostatic Drive and Controller Function
The hydrostatic system in the 6635 uses two axial piston pumps, each driving a wheel motor on either side of the machine. These pumps are controlled by an electronic module that receives input from the hand or foot controls and translates it into voltage signals that adjust the pump displacement.
Terminology annotation:
- Hydrostatic drive: A propulsion system using hydraulic fluid to transmit power from a pump to a motor, allowing variable speed and direction.
- Swash plate: A component in axial piston pumps that tilts to vary piston stroke and thus control fluid flow.
- Electronic controller: A module that interprets operator input and sends signals to actuators or solenoids to modulate pump behavior.
- PWM (Pulse Width Modulation): A method of controlling voltage to a device by varying the duty cycle of electrical pulses.
The controller ensures that the pumps respond proportionally to joystick movement. When the joystick is centered, the swash plates remain neutral, and the machine stays stationary. As the joystick moves forward or backward, the controller increases voltage to the pump actuators, tilting the swash plates and increasing flow to the drive motors.
Common Symptoms and Diagnostic Pathways
Operators may encounter issues where the machine fails to move, moves erratically, or only responds in one direction. These symptoms often point to faults in the controller, wiring harness, or pump actuators.
Typical failure points include:
• Broken or corroded wires in the harness between the joystick and controller
• Faulty potentiometers in the joystick assembly
• Damaged pump actuators or solenoids
• Controller failure due to moisture intrusion or voltage spikes
• Poor ground connections or low battery voltage affecting signal integrity
Diagnostic steps:
• Test voltage output from the joystick at full forward and reverse positions
• Inspect harness connectors for corrosion, pin damage, or loose terminals
• Use a multimeter to verify continuity between controller and pump actuators
• Check for diagnostic LEDs or fault codes on the controller (if equipped)
• Swap joystick or controller with known-good units to isolate the fault
Pump Behavior and Manual Override
In some cases, the pumps may remain in neutral despite proper input signals. This can occur if the swash plate actuators are seized or if the controller fails to send voltage. Operators can manually override the system by applying 12V directly to the actuator terminals to test pump response.
Safety tip: Always disconnect the controller before applying direct voltage to avoid backfeeding and damaging internal circuits.
If the pump responds to manual voltage, the issue lies upstream—likely in the controller or wiring. If the pump remains unresponsive, internal mechanical failure may be present, requiring disassembly and inspection of the swash plate mechanism.
Controller Replacement and Compatibility
The original controller used in the Gehl 6635 may be difficult to source due to age and manufacturer discontinuation. Some operators retrofit newer controllers or build custom harnesses using off-the-shelf PWM modules. However, compatibility with the pump actuators and joystick signals must be verified.
Recommendations:
• Match controller output voltage and signal type (PWM vs analog)
• Confirm actuator resistance and current draw to avoid overload
• Use shielded wiring to prevent signal interference
• Mount the controller in a dry, vibration-isolated location
Field Anecdotes and Repair Wisdom
One technician in Ontario recalled a 6635 that would only drive in reverse. After hours of tracing wires, he discovered a broken ground strap near the battery that disrupted signal return. Replacing the strap restored full directional control.
Another operator in Texas bypassed the controller entirely using toggle switches and relays to manually control pump displacement. While crude, the system allowed him to finish a grading job before sourcing proper parts.
Tips for long-term reliability:
• Seal all connectors with dielectric grease
• Replace worn joystick assemblies with OEM or high-quality aftermarket units
• Install a surge protector or fuse between the battery and controller
• Periodically inspect harness routing for abrasion or pinch points
Preventative Maintenance and System Care
To maintain hydrostatic performance:
• Change hydraulic fluid every 500 hours using manufacturer-recommended viscosity
• Replace filters and inspect for metal particles or discoloration
• Test battery voltage and charging system to ensure stable power supply
• Clean cooling fins on the oil cooler to prevent overheating
• Monitor actuator response during startup and warm-up cycles
For machines operating in wet or dusty environments, consider adding a sealed enclosure around the controller and rerouting harnesses through protective loom.
Conclusion
The hydrostatic pump controller in the Gehl 6635 is a critical component that translates operator intent into motion. When faults arise, understanding the interplay between joystick input, electronic modulation, and pump mechanics is essential for effective troubleshooting. With careful diagnostics, wiring integrity, and component testing, even aging systems can be restored to full functionality—keeping the 6635 moving with the precision and power it was built to deliver.