Diagnosing Low Charge Pressure on the Bobcat T200

[MikePhua]

Charge Pressure and Hydrostatic Drive Fundamentals
The Bobcat T200 is a mid-size compact track loader introduced in the early 2000s, designed for grading, lifting, and material handling. With a rated operating capacity of 2,000 lbs and a turbocharged diesel engine, it features a hydrostatic drive system powered by a gear-type charge pump. This pump supplies low-pressure hydraulic fluid to maintain system integrity, lubricate components, and release the drive motor brakes. When charge pressure drops below threshold, drive motors may lock up and the machine may trigger fault codes or shut down.
Terminology Notes

• Charge Pressure: Low-pressure hydraulic supply that feeds the hydrostatic system and brake release circuits.
  
• Case Drain Filter: A filter that captures internal leakage and debris from hydraulic motors.
  
• Relief Valve: A pressure-regulating valve that prevents over-pressurization and allows excess fluid to bypass.
  
• Deadheading: Blocking flow at a test port to measure pressure without allowing fluid to escape.
  

Symptoms and Initial Observations
After replacing a bottom roller and a worn drive hose on the right side, the machine began locking up after moving 50 feet. Inspection revealed a damaged brake disc with missing teeth. The brake was rebuilt with new seals, discs, and piston, but the issue persisted. Charge pressure measured 315 psi at idle, then dropped to 250 psi within two minutes, continuing to fall until the low pressure warning appeared.
Key symptoms included:

• Right-side drive motor locking under load
  
• Charge pressure dropping rapidly after startup
  
• No fluid returning to case drain during brake piston tests
  
• 800 psi at drive hose on right side, left side functioning normally
  

Possible Causes of Pressure Loss

• Contaminated Hose Replacement: Dirt introduced during hose replacement can damage pump internals or clog filters.
  
• Brake Piston Seal Damage: Improper installation may allow internal leakage, reducing pressure.
  
• Plugged Case Drain Filter: Debris from brake failure can restrict flow and raise backpressure.
  
• Stuck Relief Valve: Debris under the relief seat may cause premature bypassing of fluid.
  
• Internal Pump Wear: Gear pump may be starving due to suction-side blockage or cavitation.
  

Recommended Diagnostic Steps

• Inspect and clean the case drain filter. Even a few specs of metal can indicate deeper contamination.
  
• Locate the charge pressure sending unit and tee in a manual gauge to verify readings.
  
• Cap off drive hoses one side at a time to isolate pressure loss. A significant change may indicate motor leakage.
  
• Remove and flush the charge relief valve. Check for debris under the seat and inspect shim stack.
  
• Check suction-side screens or filters for blockage. Starvation can mimic pump failure.
  
• Inspect wiring and connectors at the pressure sender. Oil intrusion or loose pins can cause false readings.
  

Operator Anecdotes and Practical Advice
One technician noted that a plugged case drain filter caused low charge pressure and motor lockup on a similar model. Another shared that a leaking control valve and low hydraulic oil level had previously damaged his charge pump. A mechanic recommended shimming the charge relief valve to temporarily boost pressure and test system response.
An operator in Tennessee capped off the drive hoses and found only 800 psi on the right side, confirming a localized issue. He suspected debris had entered the system during hose replacement and considered removing the pump block for direct inspection.
Preventive Measures and Long-Term Solutions

• Always flush hoses before installation to prevent contamination.
  
• Replace case drain filters after brake or motor failure.
  
• Monitor charge pressure at full throttle, not idle, for accurate readings.
  
• Use clean oil and maintain proper levels to avoid cavitation and pump wear.
  
• Document pressure readings and component replacements for future diagnostics.
  

Conclusion
Low charge pressure on the Bobcat T200 can stem from contamination, valve malfunction, or internal pump starvation. Brake failure and hose replacement often introduce debris that clogs filters or damages seals. By isolating components, verifying pressure at key points, and inspecting relief valves and suction screens, operators can restore system integrity. The hydrostatic drive depends on clean, consistent charge pressure—any disruption can lock motors, trigger shutdowns, and lead to costly repairs.