1) Safety & prep (do this first)
- Isolate machine: battery disconnect, relieve hydraulic pressure, block loader and boom, chock wheels, secure machine on stands. Theory: hydraulic systems store energy; failure to de-pressurize causes uncontrolled motion or spray-injection injuries.
- Gather OEM manual, pressure gauge kit, leak-detection dye, sealant, torque tools, hoist. Theory: accurate gauges and correct torques ensure components operate within designed limits; guessing increases failure risk.

2) System overview — how the drivetrain works (theory)
- Power path: engine → hydrostatic pump(s) → directional/control valve(s) → wheel motors or travel motors → reduction gearbox/final drives → wheels/tracks.
- Hydrostatic principle: engine drives a variable or fixed displacement pump that produces flow; flow and pressure applied to a hydraulic motor generate torque and speed. Displacement controls torque vs speed; relief valves limit maximum pressure.
- Final drives reduce motor rpm and increase torque via planetary gears or gear sets; bearings and seals transmit load and contain lubricant.
- Common failure modes map to components: low flow/loss of motion (pump or contamination), low pressure/poor torque (pump or relief valve or internal leakage in motors), one-side loss (control valve or directional spool or hose), noise/overheat (cavitation, aeration, or bearing failure), oil contamination and seal failure (wear/overpressure).

3) Symptom-driven diagnostic sequence (in order) — what to do and why
1. Visual and basic checks
- Inspect hydraulic fluid level, color, smell, and filter condition; check for external leaks, flexible lines, hose chafing, and loose fittings. Theory: low/contaminated fluid is the most common cause and can cause cavitation, overheating, and component wear.
2. Function check (no-load)
- With operator controls, run engine at idle and slightly above, cycle drive forward/reverse, note noises, sluggishness, one-side behavior, overheating indicators. Theory: isolates gross failures before pressurizing system under load.
3. Pressure and flow testing (pump/valve isolation)
- Connect a calibrated pressure gauge at pump outlet and test pressures at specified RPM and control positions. Measure flow if possible. Theory: differentiates pump output issues (low pressure or flow) from downstream restrictions or motor leakage.
4. Meter-in/meter-out valve checks and spool behavior
- Monitor pressure drop across control valve and check spools for dumping/bleed-off. Theory: a leaky or stuck control spool will bypass flow internally causing loss of drive despite good pump output.
5. Wheel-motor isolation tests
- Block drive to one wheel/motor at a time (as OEM procedure directs) and measure motor-case pressure and motor inlet/outlet behavior. Theory: detects internal motor leakage (loss of displacement/sealing) or mechanical binding in final drive.
6. Final-drive inspection
- Remove wheel or final-drive cover to inspect reduction gears, bearings, splines, and lubricant condition. Theory: metal particles in oil, bearing play, or gear pitting indicate mechanical failure that reduces torque and can overload hydraulics.
7. Thermal and contamination checks
- Check oil cooler, radiator, and hydraulic oil temperature under load. Sample oil for particulate/ferrous content. Theory: contamination and overheating accelerate wear and cause cavitation/noise.

4) Repairs in logical order — what you repair and why it fixes the fault
(Do simpler fixes first; more invasive repairs only after diagnostics confirm.)

1. Replace hydraulic fluid and filter(s)
- When to: oil dark, burnt smell, high particle count, or after contamination event.
- Theory/fix: removes abrasive contaminants and degraded oil that cause internal leakage, scoring of pistons/rotors, and valve wear. Restores lubrication and reduces cavitation/heat.

2. Repair or rebuild the hydrostatic pump
- When to: pressure/flow tests show low output, abnormal suction cavitation noise, excessive internal leakage.
- Theory/fix: pump wear (worn pistons, valve plate, or bushings) reduces volumetric efficiency causing low flow or loss of pressure. Rebuilding restores correct clearances and displacement, restoring pump output and re-establishing system pressure/torque.

3. Service or replace directional/control valve(s) and pressure relief settings
- When to: pump tests OK but machine lacks drive or drifts; spool sticking or internal leakage observed.
- Theory/fix: worn spools or seats let flow bypass to tank or between circuits. Cleaning, resealing, or replacing restores proper metering and directional control; adjusting relief valves sets safe maximum pressure and prevents blow-by that reduces delivered torque.

4. Repair/replace travel/wheel motors
- When to: one wheel has poor torque or stalls; motor-case pressure increases; internal leakage confirmed.
- Theory/fix: motor internal seals, pistons, or swash plate wear reduces mechanical/hydraulic efficiency, causing loss of torque and internal bypass. Rebuilding corrects clearances, replaces seals, restoring motor torque and speed control.

5. Overhaul final drives (gearbox/planetary)
- When to: unusual gear noise under load, metal particles in oil, axial play, or seized output.
- Theory/fix: worn gears or bearings lower mechanical transmission efficiency and can overload hydraulic components. Replacing bearings, gears, seals, and fresh lubricant restores mechanical advantage and reduces parasitic losses.

6. Replace hoses/fittings and correct routing
- When to: visible damage or internal collapse suspected or pressure pulsation.
- Theory/fix: collapsed hoses restrict flow and cause cavitation and pressure spikes. Correct hoses & routing restore flow capacity and prevent aeration.

7. Repair cooling and suction restrictions
- When to: overheating, low inlet pressure (pump suction), cavitation.
- Theory/fix: clogged coolers or suction screens starve pump, causing cavitation which damages pump/motors. Cleaning or replacing cooling elements and suction lines restores continuous flow and reduces temperature/cavitation.

5) Reassembly, bleeding, calibration, and verification
- Reassembly: follow OEM torques and sealant procedures. Theory: correct fastener preload and sealing prevent leaks and misalignment that cause premature failure.
- Bleeding and fill: purge air from system per OEM method (operative cycles and pump flow lines). Theory: trapped air compresses and causes spongy controls, noise, and reduced effective flow.
- Set relief pressures and calibrate controls: use pressure gauges with engine at working RPM. Theory: correct pressures ensure pumps/motors operate in the designed torque/speed envelope.
- Functional load test: drive loader under a controlled load over full forward/reverse range, monitor temp, pressures, and oil cleanliness. Theory: validates repairs under real conditions and reveals any residual internal leakage or overheating.

6) How each repair step directly fixes faults — short mapping
- Replace fluid/filter → removes wear agents and restores lubrication/cooling; reduces internal leakage and cavitation.
- Pump rebuild → restores displacement and pressure; fixes low speed/poor torque due to pump inefficiency.
- Valve repair → stops internal bypass; fixes drifting/one-side/inefficient power delivery despite pump good.
- Motor rebuild → restores hydraulic-to-mechanical conversion; fixes weak or non-rotating wheel despite correct pressure/flow.
- Final-drive overhaul → fixes mechanical loss and noises; restores correct torque multiplication so motors aren’t overloaded.
- Hose/cooler service → eliminates flow restriction and overheating; prevents cavitation and pump/motor damage.

7) Final checks & preventive actions
- After repair, monitor oil condition and pressure for 10–20 hours; check for metal particles and rising temps. Theory: early re-failures show up quickly if root cause incomplete.
- Preventive schedule: regular fluid/filter changes, periodic pressure checks, keep cooling system clean, inspect hoses and fittings. Theory: prevents contamination-induced wear and extends life of pump/motors/final drives.

Follow the OEM workshop manual for torque values, gasket/seal part numbers, hydraulic test ports, and pump/motor rebuild tolerances.
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