Hydraulic System Separation on the Bobcat S185
The Bobcat S185 uses two fundamentally different hydraulic systems operating from the same oil reservoir: an open-center implement hydraulic system and a closed-loop hydrostatic drive system. These systems share oil but are otherwise hydraulically independent.
This separation explains why lift arms, bucket tilt, and auxiliary hydraulics can function normally while the machine is completely unable to move.
How the Drive System Is Supplied
The hydrostatic drive system on the S185 consists of a variable-displacement pump, two axial-piston drive motors, a charge pump, and associated control valves. The charge pump supplies make-up oil and maintains positive inlet pressure for the closed loop.
If charge pressure collapses or the drive pump cannot generate loop pressure, travel is lost even though the implement hydraulics remain unaffected.
Why the Machine Will Not Move Despite Working Hydraulics
When a Bobcat S185 has fully functional hydraulic implements but no travel in either direction, the fault is isolated to the hydrostatic drive system supply or control.
This symptom indicates that no usable pressure is being generated in either drive loop, or that the pump swash plate is not being commanded out of neutral.
Pressure vs Flow in This Failure Mode
Implement hydraulics are flow-dominated and operate at moderate pressure. The drive system is pressure-dominated. Losing pressure in the drive loop has no effect on lift or auxiliary function speed.
This is why operators often misinterpret the problem as electronic or mechanical, when it is purely hydraulic.
Root Causes Ranked by Probability
1. Charge Pressure Loss (Most Common)
A clogged charge filter, air ingestion after service, or charge pump failure prevents the hydrostatic pump from receiving make-up oil. Without charge pressure, the closed loop cavitates and produces no torque.
2. Drive Pump Servo Control Failure
If the servo piston cannot tilt the swash plate, the pump remains in neutral. This can be caused by pilot pressure loss, contamination, or failed control valves.
3. Drive Enable or Safety Interlock Active
Seat bar, seat switch, or parking brake logic can disable drive command while leaving implement hydraulics functional.
4. Charge Relief Valve Stuck Open
A stuck relief valve dumps charge pressure directly to tank, starving the drive loop.
5. Internal Pump Failure
Catastrophic pump failure may result in zero pressure generation. This is less common but must be considered if all other checks pass.
Hot vs Cold Behavior
Charge pressure-related failures often appear immediately after service or startup. Internal pump failures show identical behavior hot and cold.
If travel returns briefly when revving the engine, charge pressure deficiency is strongly indicated.
Professional Diagnostic Procedure
Step 1: Verify Safety Interlocks
Confirm seat bar, seat switch, and parking brake status before hydraulic testing.
Step 2: Measure Charge Pressure
Install a gauge at the charge pressure test port and verify pressure at idle and under command.
Step 3: Inspect Charge Filter and Lines
Check for clogged filters, collapsed hoses, or air ingestion after recent service.
Step 4: Servo Pressure Verification
Verify that pilot pressure reaches the pump servo during joystick input.
Step 5: Loop Pressure Testing
Confirm whether any pressure develops in the drive loops when command is given.
What NOT to Do (Expensive Mistakes)
- Replacing drive motors without verifying charge pressure
- Ignoring interlock systems and chasing hydraulic faults
- Assuming pump failure without pressure testing
- Continuing operation with cavitation noise
- Skipping charge circuit inspection after filter changes
Realistic Repair Cost Ranges (€)
- Charge filter and oil service: €120 – €250
- Charge pump replacement: €600 – €1,200
- Servo control repair: €300 – €600
- Hydrostatic pump replacement: €3,500 – €6,000
- Electrical interlock repair: €100 – €300
Final Technical Takeaway
When hydraulic functions work but a Bobcat S185 will not move, the failure lies in the hydrostatic drive supply or control, not the implement system. Charge pressure must be verified before any major component replacement. This diagnostic step alone prevents the most expensive misdiagnoses.
FAQ
Can low hydraulic oil cause no travel?
Yes. Low oil level starves the charge pump first.
Does air in the system stop travel?
Yes. Air ingestion collapses charge pressure.
Can electronics disable travel only?
Yes. Safety interlocks often disable drive but not implements.
Is pump failure common in this scenario?
No. Charge circuit issues are far more common.
Should the machine be moved while diagnosing?
No. Operating without charge pressure damages the pump rapidly.
