Komatsu PC400-6 CLSS (Closed-Center Load Sensing System): How It Works, How It Fails, and How To Fix It

[MikePhua]

Overview
The PC400-6 excavator uses Komatsu’s CLSS—Closed-center Load Sensing System—to deliver smooth, fuel-efficient multi-function hydraulics. CLSS marries a pair of variable-displacement axial-piston pumps with a load-sensing (LS) feedback line from the main control valve. The pump controller constantly targets a small pressure margin above actual load so the machine supplies “only what is needed,” preserving feel and speed while lowering heat.
Key Components and What They Do

• Twin Main Pumps (P1/P2)
  Variable-displacement axial-piston units supplying the implement/swing circuits. Swash plate angle determines flow; a servo piston and control module modulate that angle.
  
• Gear Pump (Pilot/Service)
  Feeds the PPC (proportional pilot control) system and auxiliary services.
  
• Main Control Valve (MCV)
  Multi-section valve bank with meter-in/meter-out spools, individual port reliefs, regeneration checks, and shuttle logic for priority functions.
  
• LS (Load-Sense) Line
  Summarizes the highest downstream load pressure at the MCV and returns it to the pump controller. The controller drives pump outlet to “LS + margin.”
  
• Pump Control Module (PC/EPC)
  A hydromechanical controller (with electric trim on many -6 machines) that sets standby pressure, margin, and power-match characteristics.
  
• PPC Pilot Manifold
  Provides stable pilot pressure to the joysticks and to proportional solenoids for features like boom/swing priority, straight-travel, and auto-decel.
  

Why CLSS Feels Different

• Constant Margin Control
  The pump does not blast full pressure; it sits roughly one small step above the heaviest active load. This is why simultaneous boom-arm-swing feels coordinated instead of starved.
  
• Meter-In Priority
  Spools meter the flow precisely; the pump follows demand rather than forcing it. This enables fine grading and low-jerk starts.
  
• Energy Efficiency
  Less excess flow across relief valves equals less heat and lower fuel burn.
  

Reference Numbers (Typical, For Warm Oil 50–60 °C)

• Main system relief (implement): ~33–35 MPa (4,800–5,100 psi)
  
• Swing relief: ~24–28 MPa (3,500–4,000 psi)
  
• Travel relief (per motor): ~34–36 MPa (4,900–5,200 psi)
  
• Pilot pressure (PPC): ~3.5–4.5 MPa (500–650 psi), commonly ~3.9–4.2 MPa
  
• Standby (no-load pump outlet): ~3–4 MPa (435–580 psi)
  
• LS margin (pump outlet minus LS): ~2.5–3.5 MPa (360–510 psi)
  

These are “working ballpark” values used by many field techs on comparable -6 machines. Always verify the exact spec for your serial number before adjusting.
How CLSS Prioritizes and Regenerates

• Boom/Swing Priority
  Shuttle/check logic and solenoid-controlled orifices bias flow for faster boom raise or more responsive swing as needed.
  
• Arm Regeneration
  On arm-in, a regen check routes return oil to the cylinder’s other side to increase speed and reduce cavitation.
  
• Straight-Travel Function
  A bypass/pre-comp circuit helps keep both tracks supplied evenly when using implements during travel.
  

Symptoms, Root Causes, and First Checks

• Slow All Functions When Warm
  • Likely Causes: Low LS margin (misadjusted pump), airated or degraded oil, clogged case-drains/strainers, worn pump.
    
  • Quick Checks: Confirm pilot pressure; measure pump outlet (Pp) and LS pressure (PLS) simultaneously; inspect suction screens and return filters.
    
• Good Single Function, Poor Multi-Function
  • Likely Causes: Sticking shuttle in the LS network, internal leakage at spools/port reliefs, weak pilot supply, incorrect priority solenoid orifice.
    
  • Quick Checks: Compare LS with single vs. simultaneous functions; command swing + boom and watch if margin collapses.
    
• Harsh Starts or Jerky Micro-Movements
  • Likely Causes: Air in pilot circuit, contaminated or torn spool seals, pilot damping orifices blocked, low pilot pressure.
    
  • Quick Checks: Gauge the pilot manifold; verify damping orifices are clear; bleed pilot lines at the manifold.
    
• Overheating Without Obvious External Leaks
  • Likely Causes: Excess relief flow (mis-set relief too low or sticky), excessive internal leakage (worn spools/pump), over-tight LS margin adjustment.
    
  • Quick Checks: Infrared the cooler; check if main relief is cracking during normal digs; confirm LS margin isn’t cranked beyond ~3.5 MPa.
    
• Travel Weak Under Load but Implements OK
  • Likely Causes: Travel relief drifted low, motor case pressure high (blocked case return), straight-travel valve leakage.
    
  • Quick Checks: Tee into travel relief test port; read case pressure at the travel motors during stall (keep within maker’s limit).
    

Step-By-Step Diagnostic Workflow (Field-Proven)

1. Warm Up the Oil
   Run until hydraulic tank reaches 50–60 °C; cold oil hides problems.
   
2. Verify Engine and Pilot Baselines
   • Set engine at rated no-load rpm, confirm it is stable.
     
   • Measure pilot pressure at the PPC manifold: target ~3.9–4.2 MPa.
     
3. Measure Standby and LS Margin
   • Install gauges at: Pump outlet (Pp), LS line (PLS), and a convenient work port.
     
   • No-load standby: expect ~3–4 MPa at Pp with PLS near zero.
     
   • Command a moderate load (e.g., boom raise against a pile): Pp should rise to PLS + ~3 MPa.
     
4. Single- vs Multi-Function Test
   • Command boom only, then boom + arm + swing.
     
   • If margin (Pp – PLS) collapses under multi-function, suspect LS shuttle blockage, pump control lag, or insufficient pilot.
     
5. Relief Health Check
   • Slowly stall a function (briefly, and safely).
     
   • Confirm relief cracking close to spec. Re-center immediately to avoid heat.
     
6. Priority & Solenoid Check
   • Activate boom or swing priority.
     
   • If behavior does not change, test the solenoid coil resistance, voltage supply, and look for blocked orifices.
     
7. Leakage and Case Drains
   • Check pump and swing/travel motor case drain rates vs. limits.
     
   • Elevated case drain signals internal wear.
     

Adjustment Guidelines (Proceed Only With Specs and Clean Oil)

• Pilot Pressure
  Adjust regulator at PPC manifold to spec before touching anything else.
  
• LS Margin (Pump Controller)
  Adjust the margin screw so that under a controlled moderate load the pump holds roughly 2.5–3.5 MPa above LS. Too high = heat; too low = sluggish combined functions.
  
• Main and Port Reliefs
  Set main system relief first, then individual port reliefs in the sequence specified for your serial range. Use incremental turns, record baseline, and lock nuts to torque.
  
• Electronic Trim (If Equipped)
  Calibrate EPC signals (boom/swing priority, power modes) per service procedure. Confirm correct voltage at coils and correct current maps.
  

Preventive Maintenance That Pays Off

• Hydraulic Oil and Filtration
  • Change intervals based on hours and contamination readings.
    
  • Use the correct viscosity grade for ambient temperatures.
    
  • Replace return and pilot filters on schedule; inspect suction strainers every oil change.
    
• Pilot Circuit Cleanliness
  • Tiny orifices drive big behaviors. Keep pilot oil pristine.
    
  • Bleed air after any hose/component replacement.
    
• Hose Routing and Clamps
  • Vibrations can fatigue LS and pilot lines. Inspect clamps, replace chafed sleeves, correct tight bend radii.
    
• Cooler Cleanliness
  • Blow out coolers from the reverse airflow direction.
    
  • A 5–10 °C drop in tank temperature after cleaning is common.
    

Common CLSS Myths, Clarified

• “Crank Up the Relief for More Power.”
  Reliefs are last-resort safety—raising them masks problems and creates heat. Fix LS margin and leakage first.
  
• “Weak Pilot Means Only Slow Controls.”
  Low pilot also starves priority logic and LS shuttles; it can cripple multi-function performance.
  
• “Pump Replacement Solves Everything.”
  A new pump with a dirty LS network or mis-set reliefs will act like the old one. Clean, calibrate, then judge.
  

Field Story: The Machine That Hunted and Heated
A quarry PC400-6 arrived with complaints of surging boom speed and rising tank temps after 40 minutes. Pilot was a touch low at 3.4 MPa; standby a bit high; LS margin drifted from 3.0 down to 1.5 MPa during boom + swing. The fix was not a pump. Cleaning two varnished LS shuttle orifices in the MCV, restoring pilot to 4.0 MPa, and resetting margin to 3.0 MPa stabilized the machine. Reliefs were re-set to spec. Oil temps dropped ~8 °C and the “hunting” disappeared.
Upgrades and Operator-Facing Tweaks

• Anti-Cavitation Checks
  Refresh tired checks in arm/boom sections to tame chatter on quick reversals.
  
• Fine-Meter Kits
  For precision tasks, some shops fit spool-end shims or updated damping orifices that soften initial spool opening.
  
• Auto-Decel Health
  Ensure the auto-decel signal and engine speed control work; consistent rpm helps CLSS maintain a steady margin.
  

Safety Notes You Shouldn’t Skip

• Always depressurize pilot and implement circuits before cracking lines.
  
• Cap/plug ports immediately to prevent dust entry—pilot contamination ruins days.
  
• Use two technicians for stall tests: one on the machine, one on gauges. Keep stalls brief.
  

Owner/Operator Checklist (No Special Tools Required)

• Listen for pump “whistle” changes when adding a second function.
  
• Feel hoses: one circuit running abnormally hot points to relief waste.
  
• Watch for slow-return cylinders—can indicate meter-out issues or internal leakage.
  
• Track fuel burn/shift: sudden increases may be hydraulic inefficiency, not just engine.
  

Quick Wins When Time Is Tight

• Verify pilot to 4.0 MPa and re-test.
  
• Clean cooler pack thoroughly.
  
• Replace return and pilot filters; inspect suction screen.
  
• Exercise all functions fully for 5–10 minutes to purge micro-air from pilot branches after service.
  

Bottom Line
CLSS on the PC400-6 is elegantly simple once you focus on three pillars: clean oil, correct pilot, correct margin. Measure margin (Pp – PLS) with warm oil and real loads. If multi-function sags, chase the LS path and pilot first, not the pump. Set reliefs last. Do this, and you’ll recover the trademark Komatsu smoothness, precise feel, and lower heat that CLSS is famous for.