Why Does the Ignition Switch Get Hot on a Hitachi EX120-2 Excavator

[MikePhua]

Hitachi’s Excavator Legacy and the EX120-2 Platform
Hitachi Construction Machinery, founded in 1970, has built a global reputation for reliable hydraulic excavators. The EX120-2, introduced in the early 1990s, was part of Hitachi’s second-generation lineup, designed for mid-size excavation, trenching, and utility work. With an operating weight of around 12 metric tons and powered by the six-cylinder Isuzu 6BG1 engine producing approximately 90 horsepower, the EX120-2 featured a robust hydraulic system and straightforward electrical architecture. Thousands of units were sold across Asia, North America, and the Middle East, and many remain in service due to their mechanical durability and ease of repair.
Terminology Annotation

• Ignition Switch: The electrical component that activates the machine’s starter circuit and powers auxiliary systems.
  
• Voltage Drop: A reduction in electrical potential across a circuit, often caused by resistance or poor connections.
  
• Current Draw: The amount of electrical current consumed by a component or system.
  
• Thermal Load: Heat generated by electrical resistance or excessive current flow in a circuit.
  

Symptoms of a Hot Ignition Switch
Operators of the EX120-2 may notice the ignition switch becoming unusually warm or hot during operation. This can present as:

• Heat radiating from the key cylinder or surrounding panel
  
• Difficulty turning the key or intermittent starter engagement
  
• Faint burning smell near the dashboard
  
• Flickering instrument panel or erratic electrical behavior
  

While the machine may continue to run, this symptom often signals an underlying electrical issue that could lead to component failure or fire risk if ignored.
Root Causes and Diagnostic Pathways
Several factors can contribute to ignition switch overheating:

• High Current Load Through the Switch: On older machines, the ignition switch may carry power for multiple systems—starter solenoid, fuel shutoff, glow plugs, and accessories. Over time, this load can exceed the switch’s design limits.
  
• Corroded Terminals: Oxidation or moisture ingress at the switch terminals increases resistance, generating heat during operation.
  
• Loose or Undersized Wiring: Poor connections or wires not rated for the current draw can cause voltage drop and thermal buildup.
  
• Internal Switch Wear: The contacts inside the switch may degrade, leading to arcing and heat generation.
  
• Bypassed Relays: In some field repairs, technicians may bypass relays and route power directly through the ignition switch, increasing its load.
  

Anecdote from a Field Mechanic
In 2020, a mechanic in Queensland was called to inspect an EX120-2 that had a hot ignition switch and intermittent starting. After removing the dashboard panel, he found that the glow plug circuit had been wired directly through the ignition switch without a relay. The glow plugs were drawing over 30 amps during cold starts, far beyond the switch’s rating. He installed a dedicated relay and replaced the switch with a heavy-duty aftermarket unit. The heat issue disappeared, and the machine started reliably even in winter conditions.
Recommended Diagnostic Procedure

• Remove dashboard panel and inspect ignition switch terminals for corrosion or discoloration
  
• Measure voltage drop across the switch during startup and accessory operation
  
• Use a clamp meter to measure current draw through each circuit connected to the switch
  
• Check for bypassed relays or direct wiring to high-load components
  
• Replace switch if resistance exceeds manufacturer spec or if contacts show signs of arcing
  

Preventative Maintenance Strategies

• Clean and inspect ignition switch terminals every 500 hours
  
• Use dielectric grease to protect connectors from moisture
  
• Install relays for high-current systems like glow plugs and fuel solenoids
  
• Upgrade wiring to match current draw, especially if accessories have been added
  
• Replace ignition switch every 2,000 hours or if heat symptoms appear
  

Design Reflections and Suggested Improvements
While the EX120-2 was built for simplicity, its electrical system could benefit from:

• Modular relay blocks for accessory circuits
  
• Heat-resistant switch housings with integrated fusing
  
• Diagnostic ports for current monitoring
  
• Improved wire routing and shielding near the dashboard
  

Conclusion
A hot ignition switch on the Hitachi EX120-2 is often a symptom of excessive current load, poor connections, or internal wear. With proper diagnostics and targeted upgrades—especially the use of relays and clean wiring—operators can restore safe and reliable electrical performance. The EX120-2 remains a dependable excavator, but like all aging machines, it rewards those who pay attention to the small details hidden behind the key.