Komatsu PC150-6K Swing Control Failure and Hydraulic Diagnostics

[MikePhua]

The PC150-6K and Komatsu’s Mid-Class Excavator Lineage
The Komatsu PC150-6K is part of Komatsu’s sixth-generation hydraulic excavator series, designed for mid-size earthmoving, utility trenching, and forestry work. Komatsu, founded in Japan in 1921, has long been a global leader in construction equipment, with the PC series representing one of its most successful product lines. The PC150-6K, introduced in the 1990s, featured a robust hydraulic system, pilot-assisted controls, and a swing motor capable of precise rotational movement—critical for trenching and material placement.
While not as widely sold as the PC200 or PC300, the PC150-6K found favor in North America and Europe for its balance of power and transportability. Many units remain in service today, especially in owner-operator fleets and municipal yards.
Symptoms of Uncontrolled Swing Movement
A recurring issue in aging PC150-6K units involves uncontrolled swing motion. When the operator releases the swing control lever to neutral, the upper structure continues rotating for several seconds before gradually stopping. This behavior occurs in both directions, though often more severely in one. The swing brake engages after a delay, but the deceleration is sluggish and inconsistent.
Key symptoms include:

• Continued rotation after joystick release
  
• Swing brake engages only after several seconds
  
• No loss of swing power during actuation
  
• Issue persists despite replacement of swing motor and check valves
  

This points to a hydraulic control fault rather than a mechanical failure in the swing motor itself.
Terminology Clarification

• Swing Motor: A hydraulic motor that rotates the upper structure of the excavator.
  
• Pilot Control: A low-pressure hydraulic system that actuates main control valves via joystick input.
  
• Compensation Valve: A valve that balances pressure between circuits to prevent overrun or cavitation.
  
• Soft Swing Valve: A solenoid-controlled valve that modulates swing acceleration and deceleration for smoother operation.
  

Investigating the Hydraulic Control Circuit
The swing system on the PC150-6K relies on pilot pressure to actuate the swing spool, which directs flow to the swing motor. When the joystick is released, pilot pressure should drop to zero, allowing the swing brake to engage and the motor to decelerate. If pilot pressure lingers or the drain circuit is blocked, the swing spool may remain partially open, causing continued rotation.
Diagnostic steps include:

• Tee in pressure gauges on both pilot lines to the swing spool
  
• Observe pressure drop when joystick returns to neutral
  
• Inspect drain circuit from pilot controller for blockage or backpressure
  
• Verify swing brake solenoid function and fuse integrity
  
• Check for soft swing valve presence and operation (if equipped)
  

In one case, a technician discovered that the pilot drain line was partially blocked, preventing full pressure release. Cleaning the line restored normal swing behavior.
Component Replacement and System History
The affected machine had previously undergone extensive repairs:

• Swing motor replaced with OEM unit
  
• Check valves replaced
  
• Pilot control valve replaced
  
• Compensation valves on both sides of swing spool replaced
  

Despite these efforts, the issue persisted. This suggests that either the replacement components were not calibrated correctly, or an overlooked subsystem—such as the soft swing valve or pilot drain—was at fault.
Some Komatsu models include a soft swing valve mounted near the swing motor feed lines. This valve opens an orifice between the left and right drive ports, allowing smoother deceleration. If stuck open or miswired, it can cause delayed braking and overrun.
Field Anecdotes and Operator Experience
A foreman in Ontario recalled a similar issue on a PC300-6, where the swing brake switch had been disabled during winter operations to prevent freezing. The result was uncontrolled swing until the switch was reactivated. In another case, a PC200 exhibited identical symptoms due to a cracked pilot hose that allowed residual pressure to linger after joystick release.
Operators often describe the sensation as “coasting,” where the upper structure feels like it’s gliding past the intended stop point. This can be dangerous in tight quarters or near trench edges, especially when working around utilities or structures.
Preventive Maintenance and Long-Term Solutions
To prevent swing control issues:

• Inspect pilot lines and drain circuits annually
  
• Replace pilot filters every 500 hours
  
• Test swing brake solenoid and fuse quarterly
  
• Clean or replace soft swing valve if equipped
  
• Use OEM seals and valves to ensure proper pressure modulation
  

For machines operating in cold climates, consider installing hydraulic line heaters or using low-viscosity fluid to prevent valve sticking.
Conclusion
Uncontrolled swing on the Komatsu PC150-6K is typically caused by residual pilot pressure or faulty modulation valves. While mechanical components like the swing motor and brake may be in good condition, hydraulic control faults can lead to delayed braking and unsafe operation. By methodically testing pilot pressure, inspecting drain paths, and verifying valve function, technicians can restore precise swing control and extend the life of this reliable mid-size excavator.