Swing Speed Problem

[MikePhua]

When the swing function on a Kobelco SK70SR-1E exhibits sluggish rotation despite working hydraulics, the culprit often lies beyond the swing motor—it may point to hydraulic control logic, electronics, or RPM behavior rather than pure mechanical failure.
What Happens

• The swing brake deploys properly, but the rotation is lethargic—taking around 9–10 seconds per revolution despite no directional difference between clockwise and counter-clockwise movement.
  
• Wiring and sensor signals appear within specification, and swapping sensors has no effect.
  
• All other functions—boom, arm, drive—activate ample swing power when used, but swing alone yields meager performance.
  
• A hydraulic component known as the PSV-B spool was tested and replaced but did not resolve the issue.
  
• Notably, other operations like boom or stick movement cause the engine RPM to increase, but swing action does not trigger such an RPM boost.
  
• Upon accidental activation of a diagnostic “A programming mode,” the system lost stepper motor settings and began flashing an E-01 error—signifying controller misconfiguration.
  
• Recorded data shows that the expected RPM diverges from actual speed as the throttle increases—up to a 200 RPM deficit at higher settings, while low RPM remains reasonably accurate.
  
• Swing seems tied into a broader hydraulic-electronic system (the “Mechatro” system), which manages both throttle and spool control via stepper motor signals.
  

Drawing on Kobelco Specs
The SK70SR-2 version lists a nominal swing speed of 12.5 revolutions per minute (rpm) under ideal conditions (warm hydraulic oil, high idle, level ground) . This provides a baseline for expected swing timing in the 200–300 RPM engine speed range.
Possible Causes and Fixes

• Hydraulic priority logic
  The system’s swing spool redirects bypass flow to the swing motor when activated—without the correct proportional pilot signal, flow may be diverted elsewhere .
  
• RPM mismatch
  Since swing doesn’t raise engine speed, pump flow to the swing drive is starved. Diagnosticians should inspect throttle control circuits, stepper motor programming, and the Mechatro controller itself—especially after entering programming mode incorrectly.
  
• Electronic control issues
  With mismatched RPM readings and persistent error flags (like E-01), the controller may be misconfigured or faulty. Professional diagnostics or controller replacement could be warranted .
  
• Valve or pump flow faults
  In other Kobelco models, sluggish movement of several functions points to pump output flow loss or faults within the control valve stack .
  

Recommended Troubleshooting Steps

• Monitor and log swing performance:
  • Measure actual swing RPM versus engine RPM during swing-only operation.
    
  • Compare with expected 12.5 rpm baseline for evaluation.
    
• Test RPM response:
  • Manually increase engine RPM during swing movement to see if swing speed improves.
    
• Clear controller error:
  • Reset the Mechatro controller after resolving programming errors, and re-run calibration sequences reliably.
    
• Check hydraulic spool operation:
  • Use shop tests to verify proper shift of the swing spool, P2 bypass cut spool, and verify that oil flow is correctly directed to the swing motor.
    
• Evaluate controller hardware:
  • If diagnostic errors persist or stepper motor settings are unresolved, consult Kobelco service for ECU or board repair/replacement.
    

Terminology Clarified

• Swing spool – A hydraulic valve spool that diverts flow from the P2 rotor pump to the swing motor when swing is commanded.
  
• P2 bypass cut spool – A means of shutting off bypass flow to prioritize swing motor pressure for smoother motion.
  
• Mechatro system – Kobelco’s hybrid electronic-hydraulic control system mixing mechanical spools with electronic pilot pressure control via stepper motors and ECU.
  

A Field Anecdote
A heavy-equipment technician on a small-scale urban construction site recalled diagnosing a similar issue: the swing drive function lagged but activated fully when the operator moved the boom. Investigating, he found that the electronic controller had lost calibration after rough transport. Once recalibrated, RPM matched throttle settings and swing speed returned to normal—cutting rotation time from 8 seconds per revolution to the expected 4.8 seconds at 12.5 rpm.
Kobelco Background
Introduced in the late 1990s, the Kobelco SK70SR line was designed for narrow-tail, compact excavator applications. By the mid-2000s, the SK70SR-1E and later versions incorporated Mechatro control systems to improve efficiency. While exact sales figures are proprietary, Kobelco remains a major global excavator brand, known for blending robustness with advanced hydraulics.
Summary of Solutions

• Log actual swing vs expected speed (12.5 rpm baseline).
  
• Stimulate RPM during swing to test flow limitation.
  
• Reset or reprogram controller if calibration was lost.
  
• Test/hypothesize spool or pump malfunctions interfering with swing.
  
• Replace ECU board or controller if electronic errors persist.