9 hours ago
The Bobcat 863 and Its Hydrostatic Drive System
The Bobcat 863 skid steer loader was introduced in the late 1990s as part of Bobcat’s push into higher-capacity machines with improved hydraulic performance. With a rated operating capacity of 1,900 lbs and a turbocharged 73-hp Deutz diesel engine, the 863 quickly became a favorite in construction, agriculture, and rental fleets. Bobcat, founded in North Dakota in the 1950s, pioneered the compact loader market and has sold millions of units globally. The 863 featured a hydrostatic drive system, cable-controlled steering, and a robust frame designed for heavy-duty use.
Hydrostatic drive systems use variable displacement pumps to control hydraulic motors at each wheel. The operator’s steering levers modulate pump swash plates, directing oil flow to drive motors. These systems are responsive and compact but require clean oil, precise pressure regulation, and intact seals to function reliably.
Terminology Clarification
A recurring issue with the Bobcat 863 involves the right side wheels losing drive power after 30–60 seconds of operation. Initially, the wheels respond well—especially in reverse—but gradually become unresponsive. Restarting the engine temporarily restores function, suggesting a thermal or pressure-related fault.
Notably, the machine performs well on slopes and under load during the first minute of operation. However, sudden lever movements—such as quick forward-reverse transitions—cause the right side to fail faster. This points to a pressure drop or internal leakage that worsens with heat and demand.
Initial Inspections and Findings
Using a 4000 psi gauge, charge pressure was measured at idle and under throttle. Results showed:
Potential Causes and Solutions
Several factors can contribute to right side drive failure:
In Trinidad, a technician restoring a Bobcat 863 discovered that gradual lever movements preserved drive function longer than abrupt inputs. This insight helped isolate the issue to pressure regulation rather than mechanical failure. In another case, a contractor in Chicago replaced a hydrostatic motor with a rebuilt unit for $750 and restored full functionality after 300 hours of trouble-free operation.
A mining crew in South Dakota reported similar symptoms on a Bobcat 742B. Their solution involved replacing the entire hydro pump assembly after discovering internal leakage and worn seals. The lesson: hydrostatic systems are sensitive to contamination and pressure loss, and small faults can cascade into major failures.
Preventive Maintenance Recommendations
The Bobcat 863’s right side drive failure is a classic example of hydrostatic vulnerability. While the machine’s design is robust, its reliance on clean oil, precise pressure, and intact seals means that even minor issues—like a collapsed hose or missing shim—can cause intermittent failure. By combining pressure testing, visual inspection, and methodical troubleshooting, technicians can restore performance and extend the life of these dependable machines. The 863 remains a workhorse in the field, and with proper care, its drive system can deliver years of reliable service.
The Bobcat 863 skid steer loader was introduced in the late 1990s as part of Bobcat’s push into higher-capacity machines with improved hydraulic performance. With a rated operating capacity of 1,900 lbs and a turbocharged 73-hp Deutz diesel engine, the 863 quickly became a favorite in construction, agriculture, and rental fleets. Bobcat, founded in North Dakota in the 1950s, pioneered the compact loader market and has sold millions of units globally. The 863 featured a hydrostatic drive system, cable-controlled steering, and a robust frame designed for heavy-duty use.
Hydrostatic drive systems use variable displacement pumps to control hydraulic motors at each wheel. The operator’s steering levers modulate pump swash plates, directing oil flow to drive motors. These systems are responsive and compact but require clean oil, precise pressure regulation, and intact seals to function reliably.
Terminology Clarification
- Hydrostatic Pump: A variable displacement pump that controls oil flow to drive motors.
- Charge Pressure: Low-pressure oil supplied to the hydrostatic pump to prevent cavitation and maintain lubrication.
- Case Drain Filter: A filter that captures return oil from hydraulic motors, often used to detect wear debris.
- Relief Valve: A pressure-limiting valve that protects components from overload.
- Orifice Plug: A small, often screened, restriction in hydraulic circuits that regulates flow or pressure.
A recurring issue with the Bobcat 863 involves the right side wheels losing drive power after 30–60 seconds of operation. Initially, the wheels respond well—especially in reverse—but gradually become unresponsive. Restarting the engine temporarily restores function, suggesting a thermal or pressure-related fault.
Notably, the machine performs well on slopes and under load during the first minute of operation. However, sudden lever movements—such as quick forward-reverse transitions—cause the right side to fail faster. This points to a pressure drop or internal leakage that worsens with heat and demand.
Initial Inspections and Findings
- The steering linkages were confirmed to be mechanically sound, with proper spool valve actuation.
- The traction lock system was removed, and the machine was started manually via direct battery activation.
- Filters and hydraulic oil were replaced, and the case drain filter was cleaned. A fragment of hydraulic hose was found inside the filter canister, raising concerns about contamination.
- The right drive motor was disassembled and showed no visible damage. However, the relief valve lacked factory shims, which may have reduced pressure capacity.
Using a 4000 psi gauge, charge pressure was measured at idle and under throttle. Results showed:
- 0 psi at idle
- 100–120 psi at moderate throttle
- Pressure dropped to near zero when drive levers were engaged
Potential Causes and Solutions
Several factors can contribute to right side drive failure:
- Collapsed Suction Hose: Internal delamination can restrict oil flow, especially when warm.
- Air Ingress: Leaks in suction lines or seals can introduce air, causing cavitation and pressure loss.
- Contaminated Orifice Plug: A blocked orifice restricts charge pressure to the control valve. Removing the plug and flushing the port may restore flow.
- Worn Charge Pump: If the shared charge pump cannot maintain pressure, both sides may lose power simultaneously.
- Missing Relief Valve Shims: Without shims, the motor’s relief valve may open prematurely, bleeding off pressure.
- Inspect all suction hoses for internal collapse or soft spots.
- Check for air leaks at fittings, seals, and pump interfaces.
- Remove and clean the orifice plug and screen beneath the control valve hex plug.
- Measure charge pressure at various throttle levels and during drive engagement.
- Compare pressure readings to manufacturer specifications and test both sides independently.
In Trinidad, a technician restoring a Bobcat 863 discovered that gradual lever movements preserved drive function longer than abrupt inputs. This insight helped isolate the issue to pressure regulation rather than mechanical failure. In another case, a contractor in Chicago replaced a hydrostatic motor with a rebuilt unit for $750 and restored full functionality after 300 hours of trouble-free operation.
A mining crew in South Dakota reported similar symptoms on a Bobcat 742B. Their solution involved replacing the entire hydro pump assembly after discovering internal leakage and worn seals. The lesson: hydrostatic systems are sensitive to contamination and pressure loss, and small faults can cascade into major failures.
Preventive Maintenance Recommendations
- Replace hydraulic oil and filters every 500 hours or sooner in dusty environments.
- Inspect case drain filters for debris during each service interval.
- Use pressure gauges with fine increments (50 psi or less) for accurate diagnostics.
- Avoid sudden lever movements during warm-up to reduce stress on the system.
- Label and document relief valve shim configurations during disassembly.
The Bobcat 863’s right side drive failure is a classic example of hydrostatic vulnerability. While the machine’s design is robust, its reliance on clean oil, precise pressure, and intact seals means that even minor issues—like a collapsed hose or missing shim—can cause intermittent failure. By combining pressure testing, visual inspection, and methodical troubleshooting, technicians can restore performance and extend the life of these dependable machines. The 863 remains a workhorse in the field, and with proper care, its drive system can deliver years of reliable service.
