10-19-2025, 01:43 PM
Background of the EX120-3 and Its Hydraulic System
The Hitachi EX120-3 is a mid-sized excavator released in the late 1990s, part of Hitachi’s third-generation EX series. Known for its reliability and compact power, it was widely adopted in construction, forestry, and utility sectors. The EX120-3 features a dual hydraulic pump system, electronically controlled via pressure sensors and solenoids, with a load-sensing valve bank that adjusts flow based on operator input and system demand.
Hitachi, founded in 1910, entered the hydraulic excavator market in the 1960s and became a global leader by the 1990s. The EX series was a commercial success, with tens of thousands of units sold worldwide. The EX120-3, in particular, was praised for its fuel efficiency and responsive hydraulics—until age and wear began to reveal systemic vulnerabilities.
Terminology Notes
Operators of aging EX120-3 units often report sluggish hydraulic response, especially when cold. In some cases, slew and travel functions only operate at full speed when another function is simultaneously activated and deadheaded. This suggests the pump is not receiving proper load signals unless forced by pressure buildup.
Frequent root causes include:
Some operators have explored converting the EX120-3 from electronic pump control to manual. This involves bypassing the ECU and installing mechanical linkages or pilot valves to directly modulate pump displacement. While this can restore basic functionality, it sacrifices efficiency and may violate safety standards.
Real-World Cases and Operator Insights
A contractor in Wisconsin reported that his EX120-3 would bog the engine immediately upon startup, even with all levers in neutral. After extensive testing, he discovered that oil was bypassing internally in the valve bank, causing constant load. Replacing the valve seals and recalibrating the pump resolved the issue.
In another case, a UK-based operator found that his machine only responded when another function was deadheaded. He traced the fault to a broken P sensor wire, which had been chewed by rodents. After replacing the connector and shielding the harness, the machine returned to normal operation.
Preventive Measures and Long-Term Solutions
The Hitachi EX120-3 remains a capable excavator, but its aging electronic-hydraulic interface can become a source of persistent trouble. By understanding the interplay between sensors, solenoids, and pump logic—and by applying methodical diagnostics—operators can restore performance and extend the machine’s working life. In the world of hydraulics, pressure is power—but only when properly controlled.
The Hitachi EX120-3 is a mid-sized excavator released in the late 1990s, part of Hitachi’s third-generation EX series. Known for its reliability and compact power, it was widely adopted in construction, forestry, and utility sectors. The EX120-3 features a dual hydraulic pump system, electronically controlled via pressure sensors and solenoids, with a load-sensing valve bank that adjusts flow based on operator input and system demand.
Hitachi, founded in 1910, entered the hydraulic excavator market in the 1960s and became a global leader by the 1990s. The EX series was a commercial success, with tens of thousands of units sold worldwide. The EX120-3, in particular, was praised for its fuel efficiency and responsive hydraulics—until age and wear began to reveal systemic vulnerabilities.
Terminology Notes
- Pressure Sensor (P Sensor): Monitors hydraulic pressure and sends signals to the controller to adjust pump output.
- Solenoid Valve: Electrically activated valve that controls hydraulic flow direction and volume.
- Deadheading: When hydraulic flow is blocked at the actuator, causing pressure buildup and potential overheating.
- Load Sensing: A system that adjusts pump output based on the resistance encountered by the hydraulic actuator.
Operators of aging EX120-3 units often report sluggish hydraulic response, especially when cold. In some cases, slew and travel functions only operate at full speed when another function is simultaneously activated and deadheaded. This suggests the pump is not receiving proper load signals unless forced by pressure buildup.
Frequent root causes include:
- Broken or corroded pressure sensor wiring, leading to incorrect or missing signals
- Faulty solenoids, which fail to open or close properly under voltage
- Internal leakage in the valve bank, causing pressure loss and inefficient flow
- Electronic control unit (ECU) degradation, resulting in erratic pump commands
- Contaminated hydraulic fluid, which affects valve response and sensor accuracy
- Inspect the pressure sensor harness for continuity and corrosion
- Test solenoids with a multimeter for resistance and actuation
- Monitor pump output pressure during various functions using a hydraulic gauge
- Check for excessive heat buildup in the valve bank, indicating internal leakage
- Flush and replace hydraulic fluid and filters if contamination is suspected
Some operators have explored converting the EX120-3 from electronic pump control to manual. This involves bypassing the ECU and installing mechanical linkages or pilot valves to directly modulate pump displacement. While this can restore basic functionality, it sacrifices efficiency and may violate safety standards.
Real-World Cases and Operator Insights
A contractor in Wisconsin reported that his EX120-3 would bog the engine immediately upon startup, even with all levers in neutral. After extensive testing, he discovered that oil was bypassing internally in the valve bank, causing constant load. Replacing the valve seals and recalibrating the pump resolved the issue.
In another case, a UK-based operator found that his machine only responded when another function was deadheaded. He traced the fault to a broken P sensor wire, which had been chewed by rodents. After replacing the connector and shielding the harness, the machine returned to normal operation.
Preventive Measures and Long-Term Solutions
- Replace pressure sensor connectors every 2,000 hours or during major service
- Shield wiring harnesses with braided sleeving to prevent rodent damage
- Use high-quality hydraulic fluid with anti-foaming and anti-corrosion additives
- Install auxiliary pressure gauges for real-time monitoring
- Document all electrical repairs and sensor replacements for future diagnostics
The Hitachi EX120-3 remains a capable excavator, but its aging electronic-hydraulic interface can become a source of persistent trouble. By understanding the interplay between sensors, solenoids, and pump logic—and by applying methodical diagnostics—operators can restore performance and extend the machine’s working life. In the world of hydraulics, pressure is power—but only when properly controlled.
