Hydrostatic Charge Circuit Role on the Bobcat S650
The Bobcat S650 uses a closed-loop hydrostatic drive system supported by a charge pump. The charge circuit maintains positive pressure at the inlet of the hydrostatic pump and drive motors, prevents cavitation, and replaces internal leakage losses within the loop.
Without stable charge pressure, the hydrostatic system cannot build loop pressure, resulting in complete or partial loss of drive even though the engine and implement hydraulics function normally.
What Happens During a Filter Change
Hydraulic filter replacement interrupts oil continuity within the charge circuit. If the filter housing is not properly pre-filled or sealed, air enters the system. This creates a compressible medium that prevents the charge pump from maintaining pressure.
Unlike open systems, hydrostatic systems are extremely sensitive to air. Even small amounts of air can collapse pressure generation completely.
Why Drive Is Lost After Filter Replacement
When a Bobcat S650 experiences sudden loss of drive after filter change, the most likely cause is charge circuit failure due to air ingestion or flow restriction.
The hydrostatic pump receives insufficient oil supply, leading to cavitation, pressure collapse, and loss of torque in both drive loops.
Pressure vs Flow in Charge Circuit Failures
The charge circuit is responsible for maintaining pressure, not high flow. Even though flow demand is low, stable pressure is critical. Air in the system reduces effective pressure because it compresses under load.
This results in a situation where oil appears to be circulating, but usable pressure cannot be established.
Root Causes Ranked by Probability
1. Air Trapped in the Charge Circuit (Most Common)
Improper filter installation without pre-filling allows air pockets to remain in the system. The charge pump cannot purge air effectively, leading to cavitation.
2. Incorrect or Collapsed Hydraulic Filter
Using a non-approved filter or a filter with incorrect flow characteristics restricts oil supply to the charge pump.
3. Filter Seal or Housing Leak
Damaged seals allow air to be drawn into the system under suction conditions.
4. Charge Pump Cavitation Damage
Extended operation with air in the system damages the charge pump, reducing its ability to generate pressure even after air is removed.
5. Blocked Suction Line or Debris Ingress
Contamination introduced during filter change can restrict oil flow and starve the charge pump.
Hot vs Cold Behavior
Air-related issues typically appear immediately after service and are not temperature-dependent initially. However, as the system warms, cavitation damage may worsen and lead to permanent loss of charge pressure.
If drive partially returns after repeated cycling, trapped air is still present in the system.
Professional Diagnostic Procedure
Step 1: Verify Oil Level and Filter Installation
Check oil level and ensure the filter is properly installed, sealed, and of correct specification.
Step 2: Measure Charge Pressure
Install a gauge and confirm whether charge pressure is present at idle and under load.
Step 3: Bleed Air from System
Cycle controls repeatedly to purge air, ensuring continuous oil supply to the pump.
Step 4: Inspect for Air Leaks
Check suction lines, filter housing, and seals for leaks allowing air ingress.
Step 5: Evaluate Charge Pump Condition
If pressure does not recover, inspect or replace the charge pump.
What NOT to Do (Common Expensive Mistakes)
- Replacing drive motors for a charge circuit issue
- Running the machine at high RPM with cavitation present
- Ignoring filter specification requirements
- Failing to pre-fill filters during installation
- Assuming pump failure without verifying air ingestion
Realistic Repair Cost Ranges (€)
- Hydraulic oil and filter service: €150 – €300
- Charge circuit diagnostics: €150 – €300
- Charge pump replacement: €800 – €1,500
- Hydrostatic pump replacement (severe damage): €4,000 – €7,000
- Suction line repair: €100 – €300
Final Technical Takeaway
Sudden loss of drive on the Bobcat S650 after filter change is almost always a charge circuit failure caused by air ingestion or flow restriction. Hydrostatic systems require stable, air-free oil supply to function. Correct installation and proper bleeding prevent one of the most common and costly service errors.
FAQ
Can air in the system completely stop movement?
Yes. Air prevents pressure buildup in the hydrostatic loop.
Should filters always be pre-filled?
Yes. Pre-filling minimizes air entry during installation.
Can cavitation damage the pump quickly?
Yes. Even short operation under cavitation conditions can cause permanent damage.
Is drive loss always immediate after filter change?
Often yes, but partial symptoms may appear first.
Can the system self-bleed?
Partially, but manual bleeding procedures are usually required for full recovery.
