Hydrostatic Drive System Overview on the Bobcat S175
The Bobcat S175 uses a closed-loop hydrostatic drive system where engine power is converted into hydraulic torque through a variable-displacement pump and two axial-piston drive motors. The engine can operate at full RPM and full load without any direct mechanical connection to the wheels.
As a result, normal engine sound, RPM stability, and throttle response do not indicate usable drive torque. Push power depends entirely on hydrostatic pressure generation and internal sealing efficiency.
How Push Power Is Created in a Skid-Steer Loader
Push power is a function of hydraulic pressure multiplied by motor displacement. The hydrostatic pump increases swash plate angle to raise loop pressure when resistance increases. The drive motors convert that pressure into torque at the wheels.
If pressure cannot rise to specification, torque output collapses even though flow and engine RPM remain normal.
Why the Engine Runs Fine but Push Power Is Weak
When a Bobcat S175 has weak push power but the engine runs fine, the failure is isolated to the hydrostatic drive loop. The engine is producing power, but the hydrostatic system cannot convert that power into torque due to internal leakage or pressure loss.
This condition is most noticeable when pushing into a pile, climbing ramps, or attempting to counter-rotate under load.
Pressure vs Flow in Push Power Complaints
Flow determines how fast the machine moves with no load. Pressure determines how much force the machine can generate against resistance. Weak push power is always a pressure problem, not a flow problem.
Machines with worn components often move at normal speed but stall immediately when resistance increases.
Root Causes Ranked by Probability
1. Excessive Internal Leakage in Drive Motors
Worn piston shoes, barrels, or valve plates inside the drive motors allow high-pressure oil to leak directly into the case drain circuit. Pressure cannot build beyond a low threshold, resulting in poor torque output.
2. Low Charge Pressure
Charge pressure maintains positive inlet pressure to the closed loop and prevents cavitation. Low charge pressure starves the pump and motors, reducing effective displacement and torque.
3. Hydrostatic Pump Efficiency Loss
Internal leakage within the pump reduces volumetric efficiency. Pump wear affects both sides equally and often presents as weak overall push power.
4. Cross-Port Relief Valves Opening Early
Relief valves that open below specification limit maximum loop pressure. This causes the machine to stall early under load.
5. Mechanical Drag Misinterpreted as Power Loss
Seized bearings, tight chains, or brake drag increase resistance. This is far less common and usually accompanied by abnormal noise or heat.
Hot vs Cold Push Power Behavior
Cold oil temporarily improves sealing and allows slightly higher pressure. As oil reaches operating temperature, leakage increases and push power drops sharply.
A machine that feels acceptable for the first few minutes but loses power quickly is exhibiting internal hydrostatic wear.
Professional Diagnostic Procedure
Step 1: Stall Pressure Test
Measure maximum loop pressure while pushing against a solid object. Compare results to manufacturer specifications.
Step 2: Case Drain Flow Measurement
Measure case drain flow from both drive motors under load. Excessive flow confirms internal motor leakage.
Step 3: Charge Pressure Verification
Measure charge pressure at idle and under load. Low charge pressure indicates charge pump or filter issues.
Step 4: Relief Valve Pressure Check
Test cross-port relief valve opening pressure to ensure they are not unloading prematurely.
Step 5: Mechanical Drag Inspection
Verify chaincase condition, bearing freedom, and brake drag only after hydraulic faults are ruled out.
What NOT to Do (Costly Mistakes)
- Replacing the engine to fix a hydrostatic problem
- Assuming weak push is normal for machine size
- Changing hydraulic oil repeatedly without testing pressure
- Ignoring case drain measurements
- Continuing operation while torque is limited
Realistic Repair Cost Ranges (€)
- Charge pressure diagnostics and service: €150 – €300
- Drive motor rebuild (each): €1,200 – €2,000
- Hydrostatic pump rebuild: €2,000 – €3,500
- Relief valve replacement: €120 – €250
- Complete drive system overhaul: €4,000 – €7,000
Final Technical Takeaway
Weak push power on the Bobcat S175 with a healthy engine is a classic hydrostatic pressure failure. Speed without torque indicates internal leakage or pressure unloading. Correct diagnosis focuses on stall pressure, case drain flow, and charge pressure, not engine performance.
FAQ
Can a Bobcat move fast but still have weak push power?
Yes. Speed requires flow; push power requires pressure.
Is weak push power always caused by worn motors?
Most commonly, yes, but charge pressure and relief valves must be checked first.
Does higher engine RPM increase push power?
Only marginally. Pressure limits torque, not RPM.
Is it safe to keep operating with weak push power?
No. Continued operation accelerates pump and motor damage.
Should both drive motors be rebuilt together?
Only if diagnostics show excessive leakage on both sides.
