Bobcat 853 Hydraulic Startup Failure and System Recovery

[MikePhua]

The Bobcat 853 and Its Compact Loader Legacy
The Bobcat 853 skid steer loader was introduced in the early 1990s as part of Bobcat’s push into mid-frame, high-capacity compact equipment. With an operating weight of approximately 6,000 pounds and a rated operating capacity near 1,700 pounds, the 853 became a popular choice for contractors, landscapers, and municipalities. It featured a 58-horsepower diesel engine, hydrostatic drive, and a robust open-center hydraulic system designed for lift, tilt, and auxiliary functions.
Bobcat, founded in 1947, revolutionized compact equipment with its skid steer design. The 853 was part of a generation that emphasized mechanical simplicity and field serviceability. Despite its age, many units remain in operation, often refurbished or retrofitted with modern attachments.
Symptoms of Hydraulic Pressure Loss at Startup
One of the more frustrating issues with the Bobcat 853 is the complete loss of hydraulic pressure immediately after startup. Operators report:

• No lift or tilt function
  
• No auxiliary hydraulic response
  
• Engine runs normally but loader arms remain inactive
  
• Hydraulic fluid level appears normal
  
• No visible leaks or hose damage
  

This behavior suggests a failure in the hydraulic pump engagement, control valve actuation, or safety interlock systems. In some cases, the machine may regain pressure after several minutes, while in others, it remains unresponsive.
Initial Checks and Common Oversights
Before diving into component-level diagnostics, several basic checks should be performed:

• Hydraulic fluid level and condition
  • Ensure the reservoir is filled to spec with ISO 46 or equivalent fluid.
    
  • Check for contamination, foaming, or discoloration.
    
• Filter condition
  • A clogged return or suction filter can restrict flow and prevent pressure buildup.
    
  • Replace filters if last service is unknown or overdue.
    
• Safety interlock system
  • The Bobcat 853 uses a seat bar and operator presence system to enable hydraulics.
    
  • If the seat switch or bar sensor fails, the system may remain disabled.
    
• Control lever engagement
  

• Ensure the hydraulic control levers are not stuck or misaligned.
  
• Check linkage for wear or binding.
  

In one case from Alberta, a contractor discovered that the seat bar sensor had corroded, preventing hydraulic activation. After replacing the sensor and cleaning the connector, the loader resumed normal function.
Hydraulic Pump and Drive Coupling Inspection
If basic checks fail to resolve the issue, the hydraulic pump and its drive coupling must be inspected. The Bobcat 853 uses a gear-type pump mounted directly to the engine via a splined coupling.
Potential failure points:

• Pump shaft sheared or stripped
  • If the coupling fails, the pump will not rotate, resulting in zero pressure.
    
• Coupling disconnected or worn
  • A loose or worn coupling may spin freely without engaging the pump.
    
• Pump internal failure
  

• Gear wear, cavitation damage, or seal failure can prevent pressure generation.
  

Recommended steps:

• Remove the pump access cover and inspect coupling engagement.
  
• Rotate the engine manually and observe pump shaft movement.
  
• Check for metal shavings or oil leakage around the pump housing.
  
• Pressure test the pump output using a hydraulic gauge.
  

In Oregon, a Bobcat 853 lost hydraulic function after a cold start. The technician found that the pump coupling had cracked due to thermal stress. After replacing the coupling and flushing the system, pressure returned immediately.
Valve Block and Solenoid Functionality
The hydraulic control valve block manages flow to the lift, tilt, and auxiliary circuits. It includes solenoids that respond to joystick input and safety interlocks.
Failure symptoms:

• No response from any hydraulic function
  
• Solenoids click but do not actuate
  
• Manual override fails to produce movement
  

Diagnostic steps:

• Test solenoid resistance and voltage during operation
  
• Inspect valve spools for sticking or scoring
  
• Clean or replace solenoid coils and connectors
  
• Check for debris or contamination in valve block passages
  

In Maine, a loader showed no hydraulic response despite a healthy pump. The issue was traced to a stuck spool in the valve block caused by sludge buildup. After disassembly and cleaning, the loader resumed full hydraulic control.
Preventive Maintenance and Long-Term Reliability
To prevent hydraulic startup failures:

• Change fluid and filters every 500 hours or annually
  
• Inspect pump coupling and drive alignment during major service
  
• Test safety interlock sensors monthly
  
• Clean valve block and solenoids during seasonal maintenance
  
• Use magnetic drain plugs to monitor wear particles
  

Recommended fluids:

• Hydraulic oil: ISO 46 or Bobcat HTL
  
• Filter: OEM-rated spin-on or cartridge type
  
• Reservoir capacity: ~10 gallons
  

In Texas, a fleet manager implemented a hydraulic inspection checklist for all Bobcat units. Startup failures dropped by 80%, and machine uptime improved significantly.
Operator Stories and Practical Wisdom
In British Columbia, a Bobcat 853 used for snow removal failed to lift after startup. The operator discovered that the hydraulic fluid had thickened due to subzero temperatures. After switching to a winter-grade fluid and installing a tank heater, the loader performed reliably.
In Georgia, a landscaper noticed intermittent hydraulic loss. The issue was traced to a loose ground wire affecting the solenoid circuit. After securing the connection, the loader operated without further issues.
Conclusion
Hydraulic pressure loss at startup in the Bobcat 853 is often a result of pump engagement failure, safety interlock malfunction, or valve block obstruction. While the symptoms may appear sudden, they usually stem from gradual wear, contamination, or overlooked maintenance. With systematic diagnostics and preventive care, operators can restore full hydraulic function and keep these dependable machines working hard. In compact loader operations, pressure is power—and every gallon counts.