Independent Hydrostatic Drive Circuits on the Bobcat T200
The Bobcat T200 uses two independent hydrostatic drive loops, one for each track. Each loop consists of a variable displacement pump section and a dedicated axial piston motor. These circuits operate independently but are controlled simultaneously through the operator input system.
Straight-line travel depends on both loops producing equal pressure and flow. Any imbalance results in directional deviation, especially under load conditions.
Why the Machine Pulls to One Side
When the Bobcat T200 pulls hard to one side, one drive circuit is producing less torque than the other. This imbalance becomes more pronounced as load increases, such as when climbing, pushing, or turning under resistance.
The issue is not related to steering input, but to unequal hydrostatic output between left and right circuits.
Pressure vs Flow Imbalance in Hydrostatic Systems
Straight travel requires both equal flow and equal pressure. Flow differences cause speed variation, while pressure differences cause torque imbalance. In most real-world cases, pulling under load is caused by pressure loss on one side rather than flow variation.
A weakened side cannot generate sufficient torque, allowing the stronger side to dominate and steer the machine unintentionally.
Root Causes Ranked by Probability
1. Internal Leakage in One Drive Motor
Worn piston assemblies and valve plates allow high-pressure oil to leak internally into the case drain circuit. This reduces effective pressure and torque on that side.
This is the most common cause and becomes significantly worse under load.
2. Hydrostatic Pump Section Imbalance
The dual pump may have uneven wear between sections, resulting in unequal pressure output to each track.
3. Cross-Port Relief Valve Weakness
A relief valve opening too early on one side limits pressure and reduces torque output.
4. Control Linkage or Servo Calibration Issue
Improper centering of the control system can cause unequal displacement commands to the pump.
5. Mechanical Drag on Opposite Side
Increased resistance on one track (e.g., seized rollers or tight track) can simulate hydraulic imbalance by requiring more torque.
Hot vs Cold Behavior
Hydrostatic imbalance caused by internal leakage is more noticeable when the machine is warm. Oil viscosity decreases, increasing leakage and worsening torque imbalance.
A machine that tracks straight when cold but pulls when warm is typically experiencing internal hydraulic wear.
Professional Diagnostic Procedure
Step 1: Straight-Line Travel Test Under Load
Operate the machine on a flat surface and under load to confirm deviation direction and severity.
Step 2: Case Drain Flow Comparison
Measure case drain flow from both drive motors. The side with higher flow is losing pressure internally.
Step 3: Loop Pressure Measurement
Install gauges on both drive loops and compare pressure output under identical conditions.
Step 4: Relief Valve Testing
Check cross-port relief valves for proper opening pressure and consistent operation.
Step 5: Undercarriage Inspection
Inspect tracks, rollers, and idlers for resistance or mechanical drag.
Step 6: Control Calibration Verification
Ensure that control inputs result in equal pump displacement on both sides.
What NOT to Do (Common Expensive Mistakes)
- Replacing both motors without identifying the faulty side
- Adjusting controls to mask a hydraulic issue
- Ignoring case drain measurements
- Over-tightening tracks to correct pulling behavior
- Replacing the pump without confirming motor condition
Realistic Repair Cost Ranges (€)
- Diagnostic testing: €150 – €300
- Drive motor rebuild (single side): €1,500 – €2,500
- Hydrostatic pump rebuild: €2,500 – €4,000
- Relief valve replacement: €120 – €250
- Undercarriage repair: €500 – €2,000
Final Technical Takeaway
A Bobcat T200 that pulls to one side is almost always experiencing hydrostatic imbalance caused by pressure loss in one drive circuit. The most reliable diagnostic method is comparing case drain flow and loop pressure between both sides. Accurate identification of the weaker side prevents unnecessary replacement of functional components and reduces repair costs significantly.
FAQ
Is pulling always caused by hydraulic issues?
Most cases are hydraulic, but mechanical drag can contribute.
Can the issue be corrected by adjusting controls?
No. Adjustments only mask the underlying imbalance.
Why does the problem worsen when warm?
Lower oil viscosity increases internal leakage.
How do I identify the weaker side?
By comparing case drain flow and pressure output between both circuits.
Should both drive motors be replaced?
Only if both show excessive internal leakage.
