Hitachi EX120-2 Electrical Resistor Overheating and Circuit Integrity

[MikePhua]

The EX120-2 and Hitachi’s Excavator Evolution
The Hitachi EX120-2 hydraulic excavator was introduced in the early 1990s as part of Hitachi’s second-generation compact-medium lineup. Built for trenching, grading, and light demolition, the EX120-2 combined mechanical durability with increasingly sophisticated electrical and hydraulic systems. With an operating weight of approximately 12 metric tons and a dig depth exceeding 5.5 meters, it became a popular choice for contractors seeking reliability in confined or urban job sites.
Hitachi Construction Machinery, founded in 1970, rapidly expanded its global footprint through partnerships and innovation. The EX series sold tens of thousands of units worldwide, with the EX120-2 remaining in service across Asia, Europe, and North America due to its robust frame and accessible parts network.
Electrical Resistor Function and Overheating Risks
Within the EX120-2’s electrical system, resistors are used to regulate voltage and current flow to various components, including sensors, relays, and control modules. One common resistor type found in these machines is the PVC-coated ceramic resistor, often mounted near the fuse box or relay cluster.
Its primary functions include:

• Voltage drop for low-current circuits
  
• Load balancing across relays
  
• Surge protection during startup or shutdown
  
• Heat dissipation in regulated loops
  

When a resistor overheats or burns, symptoms may include:

• Smoke or melting of PVC insulation
  
• Electrical odor near the cab or relay panel
  
• Loss of function in related circuits (e.g., fuel solenoid, glow plugs)
  
• Blown fuses or intermittent power faults
  
• Visible charring or discoloration on the resistor body
  

In one case, a contractor in Malaysia noticed smoke from the relay panel after startup. Inspection revealed a scorched PVC resistor linked to the fuel shutoff circuit. Replacing the resistor and cleaning the terminals restored normal operation.
Root Causes of Resistor Failure
Resistor overheating is typically caused by:

• Excessive current due to shorted components downstream
  
• Corroded terminals increasing resistance and heat
  
• Incorrect resistor rating during replacement
  
• Poor ventilation or mounting near heat sources
  
• Aging insulation and thermal fatigue
  

To diagnose the issue:

• Disconnect the resistor and test continuity
  
• Measure voltage drop across the resistor under load
  
• Inspect connected circuits for shorts or overloads
  
• Check fuse ratings and relay function
  
• Verify resistor specifications (ohms and wattage) against factory diagrams
  

A technician in Ontario traced repeated resistor failure to a faulty glow plug relay that remained energized after startup, causing continuous current draw through the resistor.
Replacement Strategy and Circuit Protection
When replacing a burned resistor:

• Use a ceramic or metal oxide resistor rated for high temperature
  
• Match resistance (ohms) and power rating (watts) precisely
  
• Mount away from heat-generating components
  
• Use crimped terminals with heat-shrink insulation
  
• Add inline fuses or thermal cutoffs if circuit design allows
  

Recommended specs for EX120-2 resistors vary by circuit, but common values include:

• 10–20 ohms
  
• 10–25 watts
  
• Flame-retardant coating or ceramic casing
  

Avoid using generic PVC resistors in high-load circuits. In one retrofit project, a fleet manager in Texas replaced all PVC resistors with ceramic units and added ventilation slots to the relay panel, reducing heat buildup and improving reliability.
Preventive Maintenance and Electrical Health
To prevent future resistor failure:

• Inspect electrical connectors quarterly
  
• Clean terminals with contact cleaner and apply dielectric grease
  
• Check relay function and replace aging units
  
• Monitor voltage drops across key circuits
  
• Document resistor replacements and circuit behavior
  

Use of thermal imaging during service inspections can help detect hotspots before failure. A restoration shop in New Zealand added a small cooling fan to the relay box of an EX120-2 used in tropical conditions, extending component life significantly.
Operator Stories and Practical Fixes
In 2022, a demolition crew in Florida experienced intermittent shutdowns on their EX120-2. After multiple fuse replacements, they discovered a burned resistor linked to the fuel solenoid. Replacing the resistor and isolating the solenoid circuit with a relay resolved the issue permanently.
Another example comes from a vineyard in Chile, where an EX120-2 was used for trenching. The operator noticed a burning smell during long idle periods. Inspection revealed a resistor mounted too close to the hydraulic control valve. Relocating the resistor and upgrading to a ceramic unit eliminated the problem.
Conclusion and Recommendations
Resistor failure in the Hitachi EX120-2 is often a symptom of deeper electrical imbalance or circuit overload. With proper diagnosis, targeted replacement, and preventive care, the machine’s electrical system can be stabilized and protected from future damage.
Recommendations include:

• Match resistor specs precisely during replacement
  
• Inspect related circuits for shorts or overloads
  
• Upgrade to ceramic or flame-retardant resistors
  
• Improve ventilation around electrical panels
  
• Monitor voltage and current behavior during operation
  

With attention to detail and proactive maintenance, the EX120-2 remains a dependable and responsive excavator—capable of performing in harsh environments without compromising electrical integrity.