Intermittent Swing Failure in Terex TC35 Mini Excavators

[MikePhua]

Terex Compact Excavator Lineage and TC35 Overview
Terex Corporation, originally founded in 1933 as a division of General Motors, became a major player in compact construction equipment after acquiring German manufacturer Schaeff in the early 2000s. The TC35 mini excavator, part of the Terex compact series, was designed for urban excavation, utility trenching, and landscaping. With an operating weight of around 3.5 tons and a digging depth exceeding 3 meters, the TC35 offers a balance of maneuverability and hydraulic power.
Although Terex later sold its compact equipment division to Yanmar, the TC35 remains in use globally, particularly in Europe and North America. Its pilot-controlled hydraulics and swing motor configuration make it a capable machine, but like many compact excavators, it can suffer from intermittent control issues.
Terminology Annotation

• Swing Motor: A hydraulic motor responsible for rotating the upper structure of the excavator. It receives fluid from the main control valve and is actuated via pilot signals.
  
• Pilot Control: A low-pressure hydraulic system that sends signals from the joystick to the main valve block, triggering movement.
  
• Solenoid Valve: An electrically actuated valve that opens or closes hydraulic flow based on joystick input or system logic.
  

Symptoms of Intermittent Swing Failure
Operators have reported that the TC35 rotates normally one moment and then fails to respond the next, despite all other functions working correctly. The issue typically resolves itself within minutes, suggesting a transient fault rather than a mechanical failure.
This behavior points to several possible causes:

• Electrical interruption at the swing solenoid
  
• Contaminated pilot lines causing signal blockage
  
• Loose wiring or corroded connectors at the joystick or valve block
  
• Internal sticking of the swing spool due to debris or wear
  

In one case, the swing function failed mid-operation but returned after a brief idle period. The operator noted no warning lights or hydraulic noise, indicating that the issue was isolated to the swing circuit.
Recommended Diagnostic Steps
To isolate the fault, technicians should proceed with the following:

• Inspect the swing solenoid for secure connections and voltage during joystick actuation
  
• Check pilot pressure at the swing control port using a low-pressure gauge (typically 300–500 psi)
  
• Clean and reseat all electrical connectors at the joystick and valve block
  
• Remove and inspect the swing spool for scoring or contamination
  
• Test the joystick switch for intermittent contact using a multimeter
  

If the swing motor itself is suspected, it can be tested by manually actuating the valve spool and observing rotation. However, most intermittent faults are electrical or pilot-related rather than motor failure.
Field Anecdotes and Practical Advice
In Ohio, a contractor experienced similar issues with a TC35 during trenching work. After replacing the swing solenoid and cleaning the pilot lines, the problem disappeared. He later discovered that the original solenoid had internal corrosion due to water ingress through a cracked harness seal.
In Sweden, a fleet technician traced the fault to a joystick switch that had worn contacts. Replacing the switch restored consistent swing control, and the team added dielectric grease to all connectors to prevent future corrosion.
Preventive Measures for Control Reliability
To reduce the risk of intermittent swing failure:

• Inspect and clean electrical connectors every 250 hours
  
• Replace pilot filters annually or every 500 hours
  
• Use weatherproof seals on joystick wiring harnesses
  
• Flush hydraulic fluid every 1,000 hours to remove contaminants
  
• Train operators to report control lag immediately to prevent escalation
  

Industry Trends and Electronic Control Integration
Modern mini excavators increasingly rely on electronic control systems to manage hydraulic functions. While this improves precision and efficiency, it also introduces complexity. Faults that once required mechanical inspection now demand electrical diagnostics and software tools.
Manufacturers like Yanmar and Kubota have begun integrating CAN bus systems into compact machines, allowing real-time monitoring of joystick input, valve response, and motor feedback. While the TC35 predates these systems, retrofitting diagnostic ports and upgraded joysticks can improve reliability.
Conclusion
Intermittent swing failure in the Terex TC35 is typically caused by pilot signal disruption or solenoid malfunction. With structured diagnostics and preventive maintenance, operators can restore consistent rotation and avoid downtime. As compact excavators evolve, understanding the interplay between hydraulics and electronics becomes essential for field reliability. The TC35, despite its age, remains a capable machine when properly maintained and monitored.