Hitachi EX120 Throttle Linkage and Auxiliary Cable Identification

[MikePhua]

The Hitachi EX120 and Its Mechanical Control Layout
The Hitachi EX120 excavator was introduced in the early 1990s as part of Hitachi’s push into the mid-size hydraulic excavator market. With an operating weight around 12 metric tons and powered by the reliable Isuzu 4BG1 engine, the EX120 became a staple in utility trenching, forestry, and demolition work. Unlike newer models with electronic throttle actuators, the EX120 uses a mechanical throttle cable system routed from the operator’s control lever to the engine governor.
Terminology annotation:

• Throttle Cable: A mechanical linkage that adjusts engine RPM by pulling on the governor lever.
  
• Governor Lever: A pivoting arm on the fuel injection pump that regulates fuel delivery based on throttle input.
  
• Auxiliary Cable: A secondary cable often routed near the throttle, used for cold-start enrichment, fuel shutoff, or diagnostic override.
  
• Cable Sheath: The protective outer casing that houses the inner cable, preventing wear and contamination.
  

In Ontario, a contractor restored a 1994 EX120 and discovered two cables running to the governor. One was clearly the throttle, but the second—wrapped in heat-resistant tape—was disconnected and its function unclear. This led to a deeper investigation into auxiliary cable roles in older Hitachi excavators.
Identifying the Secondary Cable Under the Throttle
On many EX120 units, a second cable is routed beneath or alongside the throttle cable. This cable may serve one of several purposes depending on the machine’s configuration, year, and regional emission requirements.
Common functions:

• Cold-start fuel enrichment: Temporarily increases fuel delivery during startup
  
• Manual fuel shutoff: Used in lieu of solenoid shutoff in some export models
  
• Diagnostic override: Allows technicians to manually control fuel delivery during testing
  
• Engine stop cable: Found in early mechanical pump variants without electronic shutoff
  

Inspection tips:

• Trace the cable from the cab lever or firewall to its termination point
  
• Check for connection to the injection pump, governor housing, or solenoid bracket
  
• Look for labeling, color coding, or part numbers on the cable sheath
  
• Gently unwrap protective tape to inspect the end fitting—ball joint, clevis, or hook
  

Recommendations:

• If the cable is disconnected and the machine starts, runs, and shuts down normally, it may be redundant or legacy
  
• If the cable connects to a spring-loaded lever, it may be a manual shutoff or cold-start assist
  
• Avoid removing or cutting the cable until its function is confirmed
  
• Consult service manual or parts diagram for cable routing and identification
  

In Queensland, a technician found that the second cable on an EX120 was a cold-start enrichment device. It had been disconnected years earlier when the machine was retrofitted with a block heater, rendering the cable obsolete.
Cable Routing and Governor Interface Considerations
The EX120’s governor lever is sensitive to cable tension and alignment. Improper routing or interference from auxiliary cables can cause erratic throttle response, binding, or premature wear.
Routing guidelines:

• Maintain smooth cable curvature with no sharp bends
  
• Use rubber grommets at firewall and bracket pass-throughs
  
• Secure cables with clamps spaced every 30–40 cm
  
• Avoid contact with exhaust manifold, turbo, or hydraulic lines
  
• Ensure cable sheath is intact and inner cable slides freely
  

Recommendations:

• Lubricate cable ends with graphite or silicone-based lubricant
  
• Replace frayed or kinked cables with OEM-grade replacements
  
• Adjust throttle lever stop to prevent over-pulling the governor
  
• Test full throttle range after any cable adjustment
  

In British Columbia, a forestry crew experienced throttle lag due to a misrouted auxiliary cable pressing against the throttle sheath. Repositioning the cable and replacing worn clamps restored smooth RPM control.
Preventative Maintenance and Cable System Longevity
Mechanical throttle and auxiliary cables require regular inspection to prevent failure in the field. Unlike electronic systems, cable-driven controls are exposed to vibration, heat, and debris.
Maintenance checklist:

• Inspect cable sheath for cracks, splits, or abrasion
  
• Check cable ends for corrosion or loose fittings
  
• Verify throttle lever travel and spring return
  
• Clean cable routing path during engine bay service
  
• Replace cables every 3,000–4,000 hours or if stiffness is noted
  

Recommendations:

• Keep spare throttle and auxiliary cables in field inventory
  
• Label cables during disassembly to prevent misrouting
  
• Use stainless steel cable clamps for corrosion resistance
  
• Document cable replacements and routing changes in service log
  

In Alaska, a contractor added cable inspection to his monthly maintenance routine. This prevented throttle failures during winter trenching and improved operator confidence.
Conclusion
The secondary cable beneath the throttle on a Hitachi EX120 is often overlooked but may serve critical functions depending on the machine’s configuration. Whether it’s a manual shutoff, cold-start assist, or diagnostic override, identifying and maintaining this cable ensures safe and predictable engine control. In the world of mechanical excavators, every cable has a purpose—and clarity begins with tracing, testing, and thoughtful inspection.