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The Hitachi EX120-2 and Its Mid-Size Excavator Legacy
The Hitachi EX120-2 hydraulic excavator, introduced in the early 1990s, was part of Hitachi’s second-generation lineup aimed at improving reliability, serviceability, and electronic integration. With an operating weight of approximately 12 metric tons and a bucket capacity of 0.5–0.6 cubic meters, the EX120-2 was widely adopted for utility trenching, site preparation, and light demolition. Its success stemmed from a balance of mechanical durability and emerging electronic control systems, including warning indicators, hour meters, and sensor-driven diagnostics.
Hitachi Construction Machinery, founded in 1970, became a global leader in excavator design by the late 1980s. The EX series sold extensively across Asia, Europe, and North America, with the EX120-2 serving as a bridge between analog simplicity and digital monitoring.
Core Specifications
An operator performed a full harness replacement on his EX120-2 after discovering extensive damage—broken wires, splices, and corrosion. The retrofit included both inner and outer harnesses, restoring signal integrity and eliminating intermittent faults. The machine’s performance improved significantly, with smoother startup and consistent sensor readings.
However, a new issue emerged: all warning lights and the buzzer remained active when the engine was running, and the hour meter continued to count even after shutdown. Disconnecting the relay stopped the meter, but reconnecting it prevented restart. This pointed to a diode fault in the alternator, where residual voltage was feeding back into the system.
Anecdote from the Field
In Queensland, a contractor swapped the original Hitachi alternator for a generic aftermarket unit. Shortly after, he noticed the hour meter running continuously and warning lights staying on. After testing the diode pack and replacing the alternator with a Hitachi-compatible model, the issue resolved. He now checks alternator part numbers and diode orientation before installation.
Diagnostic Strategy and Repair Recommendations
The Hitachi EX120-2 remains a reliable excavator when its electrical systems are properly maintained. Harness replacement can restore performance, but care must be taken when installing aftermarket alternators. Diode faults can cause persistent warning lights and hour meter run-on, leading to confusion and unnecessary downtime. By understanding the interplay between alternator signals, relay behavior, and harness integrity, technicians can ensure smooth operation and accurate diagnostics. In modern excavators, every wire carries more than current—it carries confidence.
The Hitachi EX120-2 hydraulic excavator, introduced in the early 1990s, was part of Hitachi’s second-generation lineup aimed at improving reliability, serviceability, and electronic integration. With an operating weight of approximately 12 metric tons and a bucket capacity of 0.5–0.6 cubic meters, the EX120-2 was widely adopted for utility trenching, site preparation, and light demolition. Its success stemmed from a balance of mechanical durability and emerging electronic control systems, including warning indicators, hour meters, and sensor-driven diagnostics.
Hitachi Construction Machinery, founded in 1970, became a global leader in excavator design by the late 1980s. The EX series sold extensively across Asia, Europe, and North America, with the EX120-2 serving as a bridge between analog simplicity and digital monitoring.
Core Specifications
- Engine: Isuzu 4BG1, 4-cylinder diesel
- Power output: ~90 hp
- Operating weight: ~12,000 kg
- Hydraulic flow: ~180–200 L/min
- Electrical system: 24V with integrated warning panel
- Alternator: Hitachi OEM or aftermarket 24V unit with diode feedback
- Inner/Outer Harness: Refers to the electrical wiring looms inside the cab and extending to the engine and hydraulic compartments.
- Conversion Kit: A retrofit package used to replace or upgrade wiring, sensors, or control modules.
- Diode Fault: A failure in the alternator’s diode pack that can cause backfeed, improper signal grounding, or continuous current flow.
- Hour Meter Run-On: A condition where the hour meter continues to count even when the engine is off, often due to electrical backfeed.
An operator performed a full harness replacement on his EX120-2 after discovering extensive damage—broken wires, splices, and corrosion. The retrofit included both inner and outer harnesses, restoring signal integrity and eliminating intermittent faults. The machine’s performance improved significantly, with smoother startup and consistent sensor readings.
However, a new issue emerged: all warning lights and the buzzer remained active when the engine was running, and the hour meter continued to count even after shutdown. Disconnecting the relay stopped the meter, but reconnecting it prevented restart. This pointed to a diode fault in the alternator, where residual voltage was feeding back into the system.
Anecdote from the Field
In Queensland, a contractor swapped the original Hitachi alternator for a generic aftermarket unit. Shortly after, he noticed the hour meter running continuously and warning lights staying on. After testing the diode pack and replacing the alternator with a Hitachi-compatible model, the issue resolved. He now checks alternator part numbers and diode orientation before installation.
Diagnostic Strategy and Repair Recommendations
- Alternator Diode Test: Use a multimeter in diode mode to check for reverse leakage. Replace if voltage flows in both directions.
- Relay Function Check: Test relay coil and contacts for proper switching. Replace if sticking or shorted.
- Harness Ground Verification: Confirm all ground points are clean and secure. Poor grounding can mimic diode faults.
- Sensor Signal Isolation: Disconnect sensors one by one to identify false triggers.
- Hour Meter Circuit Review: Trace wiring from meter to ignition and alternator. Install diode isolator if needed.
- Aftermarket alternator with incorrect diode configuration
- Ground loop or poor chassis grounding
- Relay failure causing constant current flow
- Harness misrouting or pin misalignment
- Sensor backfeed due to shared power circuits
- Use OEM alternators or verified replacements with correct diode packs
- Inspect harness connectors quarterly for corrosion or pin damage
- Label all harness ends during installation to prevent miswiring
- Install surge protectors or diode isolators on sensitive circuits
- Document all wiring changes and part numbers for future reference
- Keep a wiring diagram and multimeter in the service kit
- Test alternator output and diode function during annual service
- Use heat-shrink connectors and dielectric grease on all terminals
- Train operators to report unusual warning light behavior
- Maintain a log of electrical repairs and component swaps
The Hitachi EX120-2 remains a reliable excavator when its electrical systems are properly maintained. Harness replacement can restore performance, but care must be taken when installing aftermarket alternators. Diode faults can cause persistent warning lights and hour meter run-on, leading to confusion and unnecessary downtime. By understanding the interplay between alternator signals, relay behavior, and harness integrity, technicians can ensure smooth operation and accurate diagnostics. In modern excavators, every wire carries more than current—it carries confidence.
