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The EX120-2 and Its Electronic Throttle System
The Hitachi EX120-2 excavator was part of Hitachi’s second-generation lineup of mid-size hydraulic machines, introduced in the early 1990s. Built for reliability and precision, the EX120-2 featured an electronically controlled throttle system—a departure from the mechanical linkages of earlier models. This system used an EC motor (Electronic Control motor) to adjust engine RPM based on operator input, interfacing with the ECU (Electronic Control Unit), control pad, and cab harness.
While the design improved fuel efficiency and responsiveness, it also introduced a layer of complexity that could become a diagnostic challenge when components aged or wiring deteriorated.
Symptoms of Erratic Throttle Behavior
A common failure scenario involves the throttle jumping erratically between power and off when the EC motor is connected. Voltage readings at the motor terminals fluctuate between 15V and 25V, and the motor pulses without stabilizing. Despite replacing the control pad, ECU, cab harness, and EC motor, the throttle remains non-functional.
This behavior suggests a deeper electrical fault—likely a grounding issue, intermittent short, or signal interference within the harness or control logic.
Grey Market Machines and Diagnostic Limitations
The EX120-2 was widely imported as a grey market machine, especially in North America. These units, while mechanically identical to domestic models, often lack support from dealers due to regional software differences and undocumented wiring changes. As a result, finding a technician with the right diagnostic tools—such as the DR EX interface used by Hitachi and Deere—is critical.
Operators of grey market machines frequently report difficulty sourcing parts and service, especially for electronic components. In some cases, even experienced mechanics refuse to work on them due to the lack of schematics and support.
Grounding and Harness Damage Under the Hydraulic Pump
One of the most overlooked failure points is the main harness section that runs beneath the hydraulic pump. This area is exposed to heat, oil saturation, and physical abrasion from sticks, debris, and vibration. Over time, insulation can degrade, leading to shorts or open circuits that confuse the ECU.
Technicians have found that even when resistance tests show continuity, the wires may be internally corroded or intermittently shorting under load. Oil-soaked harnesses can conduct stray voltage, causing the EC motor to behave erratically.
Recommended steps include:
The control pad sends throttle position signals to the ECU, which then modulates voltage to the EC motor. If either unit is mismatched or improperly calibrated, the motor may receive unstable signals. Replacing both components without verifying firmware compatibility can lead to erratic behavior.
In some cases, aftermarket or remanufactured ECUs lack the correct throttle mapping, especially in grey market units. Technicians should confirm part numbers and software versions before installation.
Recommendations for Restoration
To restore throttle function:
Conclusion
The Hitachi EX120-2’s electronic throttle system offers precision—but only when every component communicates cleanly. Erratic voltage, pulsing motors, and failed replacements point to deeper electrical issues, often hidden beneath the hydraulic pump or within grey market wiring. With careful inspection, proper grounding, and matched components, the system can be revived. But in the world of aging excavators, sometimes the smartest fix is the simplest one: eliminate the gremlins and take back control.
The Hitachi EX120-2 excavator was part of Hitachi’s second-generation lineup of mid-size hydraulic machines, introduced in the early 1990s. Built for reliability and precision, the EX120-2 featured an electronically controlled throttle system—a departure from the mechanical linkages of earlier models. This system used an EC motor (Electronic Control motor) to adjust engine RPM based on operator input, interfacing with the ECU (Electronic Control Unit), control pad, and cab harness.
While the design improved fuel efficiency and responsiveness, it also introduced a layer of complexity that could become a diagnostic challenge when components aged or wiring deteriorated.
Symptoms of Erratic Throttle Behavior
A common failure scenario involves the throttle jumping erratically between power and off when the EC motor is connected. Voltage readings at the motor terminals fluctuate between 15V and 25V, and the motor pulses without stabilizing. Despite replacing the control pad, ECU, cab harness, and EC motor, the throttle remains non-functional.
This behavior suggests a deeper electrical fault—likely a grounding issue, intermittent short, or signal interference within the harness or control logic.
Grey Market Machines and Diagnostic Limitations
The EX120-2 was widely imported as a grey market machine, especially in North America. These units, while mechanically identical to domestic models, often lack support from dealers due to regional software differences and undocumented wiring changes. As a result, finding a technician with the right diagnostic tools—such as the DR EX interface used by Hitachi and Deere—is critical.
Operators of grey market machines frequently report difficulty sourcing parts and service, especially for electronic components. In some cases, even experienced mechanics refuse to work on them due to the lack of schematics and support.
Grounding and Harness Damage Under the Hydraulic Pump
One of the most overlooked failure points is the main harness section that runs beneath the hydraulic pump. This area is exposed to heat, oil saturation, and physical abrasion from sticks, debris, and vibration. Over time, insulation can degrade, leading to shorts or open circuits that confuse the ECU.
Technicians have found that even when resistance tests show continuity, the wires may be internally corroded or intermittently shorting under load. Oil-soaked harnesses can conduct stray voltage, causing the EC motor to behave erratically.
Recommended steps include:
- Removing the hydraulic pump to inspect the harness underneath
- Replacing or rewrapping damaged wires with heat-resistant loom
- Testing voltage under load, not just static resistance
- Verifying ground connections at the frame and battery
The control pad sends throttle position signals to the ECU, which then modulates voltage to the EC motor. If either unit is mismatched or improperly calibrated, the motor may receive unstable signals. Replacing both components without verifying firmware compatibility can lead to erratic behavior.
In some cases, aftermarket or remanufactured ECUs lack the correct throttle mapping, especially in grey market units. Technicians should confirm part numbers and software versions before installation.
Recommendations for Restoration
To restore throttle function:
- Inspect and clean all ground points, especially near the battery and cab mounts
- Replace the harness section under the hydraulic pump if any damage is found
- Use shielded wire for signal lines to reduce electrical noise
- Confirm ECU and control pad compatibility with the EC motor
- Test voltage at the motor while cycling the throttle to observe signal stability
- Consider installing a mechanical throttle override for temporary operation
Conclusion
The Hitachi EX120-2’s electronic throttle system offers precision—but only when every component communicates cleanly. Erratic voltage, pulsing motors, and failed replacements point to deeper electrical issues, often hidden beneath the hydraulic pump or within grey market wiring. With careful inspection, proper grounding, and matched components, the system can be revived. But in the world of aging excavators, sometimes the smartest fix is the simplest one: eliminate the gremlins and take back control.
