Diagnosing Left Swing Slowdown on the Bobcat 337 Excavator

[MikePhua]

The Bobcat 337 and Its Hydraulic Swing System
The Bobcat 337 is a compact excavator produced in the early 2000s, designed for utility trenching, site prep, and light demolition. With an operating weight of around 7,500 lbs and a digging depth exceeding 11 feet, it balances maneuverability with breakout force. Bobcat, a brand under Doosan since 2007, has long been known for its compact equipment innovations, and the 337 was part of its push into mid-size excavators with full-featured hydraulic systems.
The swing function on the 337 is hydraulically actuated via a dedicated swing motor and valve assembly. It allows the upper structure to rotate left or right, typically powered by pilot-operated joystick controls. The swing circuit is pressure-sensitive and shares fluid with other hydraulic functions, such as the boom, bucket, and travel motors.
Terminology Annotation
- Swing Motor: A hydraulic motor that rotates the upper structure of the excavator.
- Pilot Pressure: Low-pressure hydraulic signal used to control high-pressure valves and actuators.
- Flow Sharing: A hydraulic system design where multiple functions draw from a common pump, with priority or proportional distribution.
- Thermal Expansion: The increase in fluid volume and viscosity change due to rising temperature, affecting system performance.
Symptoms of Left Swing Slowdown
Operators have reported that the Bobcat 337 swings normally to the right but slows significantly when swinging left, especially as hydraulic temperature rises. Interestingly, activating the bucket simultaneously improves left swing speed. This suggests a pressure or flow imbalance rather than a mechanical fault.
The issue typically emerges after the machine has warmed up, indicating that heat-induced viscosity changes or valve behavior may be contributing factors. The swing motor itself is unlikely to be the root cause if it performs well in one direction.
A Story from the Field
In Bavaria, a contractor using a Bobcat 337 for roadside ditching noticed that the machine’s left swing would lag after 30 minutes of operation. Initially suspecting a weak swing motor, he consulted a technician who advised checking the pilot valve and swing spool. Upon inspection, they found that the left swing spool had minor scoring and was sticking under heat expansion. Replacing the spool and flushing the hydraulic fluid resolved the issue. The contractor later added a hydraulic cooler to reduce operating temperatures and prevent recurrence.
Diagnostic Strategy and Inspection Points
To isolate the cause of left swing slowdown:

• Monitor hydraulic fluid temperature during operation
  
• Check pilot pressure to the swing valve in both directions
  
• Inspect swing valve spool for wear, scoring, or sticking
  
• Test swing motor performance under load in both directions
  
• Observe system behavior when other functions (e.g., bucket) are engaged
  
• Verify that the hydraulic filter is clean and fluid is within spec
  

If swing improves when another function is used, it may indicate a pressure relief or flow-sharing issue. The system may be prioritizing other functions or compensating for a weak spool spring.
Preventative Maintenance and Recommendations
To maintain swing performance:

• Change hydraulic fluid every 1,000 hours or annually
  
• Replace filters at each fluid change and inspect for debris
  
• Use fluid with proper viscosity index for temperature stability
  
• Clean swing valve block and inspect spool movement during service
  
• Install auxiliary cooling if operating in hot climates or under continuous load
  

For machines with over 5,000 hours, consider resealing the swing motor and inspecting valve springs for fatigue.
Industry Trends and Hydraulic System Evolution
As of 2025, compact excavators increasingly use load-sensing hydraulics and electronic valve control to optimize flow distribution. Newer Bobcat models feature integrated diagnostics and temperature-compensated valves, reducing the likelihood of heat-related performance drops.
Aftermarket upgrades include proportional pilot valves, enhanced cooling packages, and remote monitoring systems that track hydraulic pressure and temperature in real time.
Conclusion
A left swing slowdown on the Bobcat 337 is often a symptom of thermal imbalance or spool wear rather than a motor failure. By analyzing pilot pressure, valve behavior, and fluid condition, operators can pinpoint the issue and restore full swing functionality. With proper maintenance and attention to hydraulic temperature, the 337 remains a reliable and responsive machine for compact excavation tasks.