Why Does the Hitachi EX120-5 Keep Running After the Key Is Turned Off

[MikePhua]

The EX120-5 and Its Electrical Backbone
The Hitachi EX120-5 hydraulic excavator, part of the Dash-5 series introduced in the late 1990s, was designed to meet the growing demand for electronically controlled, fuel-efficient machines in the 12-ton class. With a 4-cylinder Isuzu engine, advanced hydraulic modulation, and a refined cab layout, the EX120-5 became a popular choice for contractors working in urban infrastructure, utility trenching, and light demolition. Hitachi, founded in 1910, had by then established itself as a global leader in construction machinery, with the Dash-5 series marking a transition toward integrated electronic control systems.
One of the lesser-known but critical components in this system is the alternator’s role in managing electrical flow—not just for charging the battery, but also for signaling engine shutdown. When the alternator malfunctions or backfeeds current, it can cause the engine to continue running even after the ignition key is turned off.
The Role of the Alternator Diode
The alternator in the EX120-5 includes a small wire—often referred to as the “exciter” or “field” wire—that supplies initial current to energize the rotor when the engine starts. Once the alternator begins generating power, this wire also carries voltage back to the ignition circuit. If not properly isolated, this backfeed can keep the fuel solenoid energized, preventing the engine from shutting down.
Terminology Annotation:

• Exciter Wire: A low-current wire that provides initial voltage to the alternator’s field coil, enabling it to begin charging.
  
• Backfeed: Unintended reverse flow of electrical current into a circuit, often causing components to remain powered.
  
• Diode: A semiconductor device that allows current to flow in one direction only, used to prevent backfeed in electrical systems.
  
• Fuel Solenoid: An electrically actuated valve that controls fuel flow to the engine; when de-energized, it shuts off fuel and stops the engine.
  

In the EX120-5, if the diode protecting the exciter wire fails or is missing, the alternator can send voltage back into the ignition circuit. This keeps the fuel solenoid energized even after the key is turned off, causing the engine to continue running.
Field Diagnosis and Simple Solutions
To confirm the issue, technicians can perform a basic test:

• Start the machine and let it idle
  
• Turn off the key and observe whether the engine continues running
  
• Disconnect the small wire from the alternator and repeat the test
  

If the engine shuts down properly after the wire is removed, the problem is confirmed: the alternator is backfeeding current through the exciter wire.
The solution is straightforward—install a diode in-line with the exciter wire. The diode should be rated for at least 3 amps and 50 volts, with the cathode (marked end) facing the alternator. This allows current to flow into the alternator during startup but blocks reverse flow once the engine is running.
Anecdotes from the Field
In rural Ireland, a contractor operating an EX120-5 noticed the machine refused to shut down after a long day of ditching. Initially suspecting a faulty key switch, he replaced the ignition barrel to no effect. After tracing the wiring, he discovered the exciter wire was feeding voltage back into the system. Installing a diode resolved the issue instantly.
In another case in Queensland, Australia, an operator reported that his EX120-5 would occasionally stay running even after the key was removed. The issue was intermittent, occurring more often in humid conditions. Upon inspection, the diode had corroded internally, allowing partial backfeed. Replacing it with a sealed automotive-grade diode restored normal operation.
Preventative Measures and Electrical Best Practices
To avoid similar issues in other machines:

• Always install a diode when replacing or upgrading alternators
  
• Use heat-shrink tubing and weatherproof connectors to protect the diode
  
• Inspect exciter wires for abrasion or corrosion during routine service
  
• Test voltage at the fuel solenoid with the key off to confirm proper shutdown
  
• Label wiring clearly to aid future diagnostics
  

For machines operating in wet or corrosive environments, consider using a diode with an integrated fuse and waterproof housing. This adds an extra layer of protection and simplifies troubleshooting.
Conclusion
The EX120-5’s engine shutdown issue is a classic example of how a small electrical oversight can lead to confusing mechanical behavior. By understanding the function of the exciter wire and the importance of diode isolation, technicians can quickly diagnose and resolve the problem. The Hitachi EX120-5 remains a reliable and capable excavator—provided its electrical pathways are kept clean, dry, and properly directed. In the world of heavy equipment, even a single diode can make the difference between frustration and flawless performance.