Detailed Analysis of Hydraulic Issues on the Bobcat S185: Causes, Diagnostics, and Solutions

[MikePhua]

Understanding the Hydraulic System in the Bobcat S185
The Bobcat S185 skid steer loader is equipped with a hydraulic system that powers the loader arms, auxiliary hydraulics, and travel functions. This system relies on hydraulic pumps, control valves, hydraulic lines, cylinders, and various safety interlocks to provide smooth and precise operation. Problems in the hydraulic circuit often manifest as loader arm drift, loss of lifting power, auxiliary hydraulic anomalies, or unexpected machine behavior.

Common Symptoms Observed in the Bobcat S185 Hydraulic System

• Loader arm drifting down rapidly without operator input.
  
• Raising the safety lap belt or shutting down the machine stops the drift.
  
• Loader arms hold position when lifting the machine but cannot stay up under load continuously.
  
• Auxiliary hydraulics (such as snow blade controls) slightly lift the loader arms if cycled repeatedly.
  
• Lift and tilt controls continue to be active even when the seat belt is raised, with the brake engaged.
  
• Occasional stalls or unintended movement if controls are bumped with the lap belt lifted.
  
• Hydraulic noises such as whining or unusual sounds during operation.
  
• Fluid leaks or potential internal valve leaks suspected.
  

Potential Causes of these Hydraulic Symptoms

• Relief Valve Stuck Open or Faulty Check Valve
  If the loader arm drifts downward rapidly, a common cause is a relief valve stuck open inside the loader control valve assembly or a malfunctioning check valve. This allows hydraulic fluid to bypass the cylinder, causing the arms to lose pressure and drop.
  
• Cracked or Damaged Control Valve Body
  Cracks in the valve body can lead to internal leaks between hydraulic circuits. This may explain why auxiliary hydraulics slowly lift the loader arms when cycled: fluid is inadvertently redirected between circuits, lifting the arms slightly over time.
  
• Faulty or Improper Safety Interlocks
  The lap belt sensor is designed to engage the brake and lock out controls for safety. If controls remain active with the belt raised, it suggests a malfunction in the control logic or wiring, potentially causing dangerous unintended motion.
  
• Hydraulic Pump or Drive Belt Issues
  Insufficient hydraulic pressure due to a failing pump or loose/ worn drive belts can cause erratic operation, reduced lift capacity, and noise.
  
• Air or Contamination in Hydraulic System
  Air pockets or contamination inside the hydraulic fluid reduce system responsiveness and may cause drifting or erratic movement.
  

Diagnostic and Troubleshooting Approaches

• Visual and Operational Checks:
  Observe the loader arm behavior starting and stopping the machine; note whether raising the lap belt changes response. Check for hydraulic fluid leaks, abnormal noises, or overheating.
  
• Control Valve Inspection:
  Remove covers or panels to access the loader control valve assembly. Look for signs of damage or cracks. Testing or rebuilding the valve may be necessary, focusing on relief and check valves.
  
• Hydraulic Pressure Testing:
  Using a pressure gauge, measure hydraulic pressures at lift circuits and auxiliary outlets to ensure they meet manufacturer specifications.
  
• Electrical and Safety System Testing:
  Verify the lap belt sensor and brake interlocks function correctly. Inspect wiring and connectors for corrosion or faults.
  
• Hydraulic Fluid Analysis:
  Check fluid levels, and condition, replacing oil and filters if contaminated or degraded.
  
• Auxiliary Hydraulic Circuit Test:
  Isolate auxiliary valve functions and test flow and pressure to diagnose unintended cross-flow or blockages.
  

Practical Solutions and Recommendations

• Valve Repair or Replacement:
  Repair or rebuild control valves with a focus on replacing faulty relief or check valves. If the valve body is cracked, replace it to prevent cross-circuit leaks.
  
• Safety System Repair:
  Repair or replace faulty lap belt sensors or wiring to ensure that controls properly lock out when safety devices are triggered.
  
• Hydraulic Pump and Belt Maintenance:
  Check the drive belt tension and condition; replace if worn. Service or replace the hydraulic pump to restore pressure if necessary.
  
• System Flushing and Filtration:
  Flush the hydraulic system to remove air and contaminants. Replace hydraulic filters regularly to maintain fluid cleanliness.
  
• Operator Training and Usage:
  Ensure operators are aware of safety protocols and proper use of lap belts and controls. Discourage manipulating controls with the safety belt raised.
  
• Regular Inspection Routine:
  Implement a maintenance schedule to regularly check hydraulic hoses, fittings, valve assemblies, and safety interlocks to catch problems early.
  

Terminology Explained

• Relief Valve: A valve that limits maximum hydraulic pressure, protecting components from damage.
  
• Check Valve: Prevents backflow of hydraulic fluid, helping maintain pressure in cylinders.
  
• Control Valve Body: The main assembly directing hydraulic fluid to various circuits.
  
• Lap Belt Interlock: Safety mechanism that disables machine controls when the operator is not properly seated and belted.
  
• Hydraulic Pump: Converts engine power into hydraulic flow and pressure.
  
• Auxiliary Hydraulics: Additional hydraulic circuits for attachments beyond standard loader functions.
  

Case Story: Resolving Loader Arm Drift on a Bobcat S185
An owner of a 2004 Bobcat S185 experienced rapid loader arm drift despite attempts to hold the arms raised. The machine’s auxiliary hydraulics also had strange effects on arm position. After diagnostic testing, they discovered the relief valve in the loader control valve was stuck open. Additionally, a cracked control valve body allowed fluid to leak internally between circuits.
The repair involved rebuilding the main valve, replacing the relief valve, and sealing the cracked valve body. The lap belt sensor wiring was also repaired to restore safety lockout functions. Post-repair, the loader arms held firm under load, auxiliary hydraulics operated independently without leakage, and controls locked out properly with the seat belt raised.
This case highlights the importance of thorough diagnosis including mechanical, electrical, and hydraulic components to resolve complex symptoms on Bobcat skid steers.

Summary Checklist for Bobcat S185 Hydraulic Issue Resolution

• Inspect valve body for cracks or damage.
  
• Test and repair relief valves and check valves.
  
• Verify lap belt sensor and control interlocks.
  
• Check hydraulic pump pressure and drive belt condition.
  
• Flush hydraulic system and replace filters.
  
• Train operators on safety protocols.
  
• Establish regular inspection and maintenance schedule.
  

Conclusion
Hydraulic issues on the Bobcat S185 such as loader arm drift and control anomalies often stem from faulty relief valves, leaking control valve bodies, or malfunctioning safety interlocks. Detailed troubleshooting and repairs targeting these areas restore smooth operation, safety, and reliability. Combining mechanical valve servicing with electrical system checks and routine hydraulic maintenance ensures durable performance. Operator awareness and preventive care further minimize downtime and enhance machine safety.
This comprehensive review draws on operator experiences, hydraulic theory, and practical repair approaches to offer a clear roadmap for diagnosing and solving hydraulic problems on Bobcat S185 skid steer loaders. It serves as a valuable reference for equipment owners, technicians, and users aiming to keep their machines fully operational and safe.