Locating and Replacing the DP Sensor on a Hitachi EX120 Excavator

[MikePhua]

The Hitachi EX120 and Its Hydraulic Control System
The Hitachi EX120 excavator, part of the EX series launched in the late 1980s, was designed for mid-size earthmoving tasks with a focus on mechanical reliability and hydraulic precision. Powered by an Isuzu 4BG1 engine and equipped with a load-sensing hydraulic system, the EX120 became a staple in construction fleets across Asia, Europe, and North America. Its control system includes a differential pressure (DP) sensor, which plays a key role in monitoring hydraulic flow and pressure balance across the control valve.
The DP sensor helps the machine’s controller determine how much hydraulic pressure is needed for various functions, such as boom lift, arm extension, and swing. A faulty DP sensor can lead to sluggish operation, erratic control response, or complete loss of function in certain circuits.

Where Is the DP Sensor Located
On the EX120-2 model, the DP sensor is mounted directly on the main control valve block. It appears as a small cylindrical component—often described as a “shiny can”—and is typically covered with a yellow or orange protective sleeve from the factory. The sensor is threaded into the valve body and secured with a single electrical connector.
To access the sensor:

• Locate the main hydraulic control valve beneath the operator’s cab
  
• Identify the cylindrical sensor near the center of the valve block
  
• Remove any protective covering or shielding
  
• Disconnect the electrical connector
  
• Unscrew the sensor using a wrench or socket
  

In some cases, a hydraulic line may obstruct access. Removing this line temporarily can provide better clearance. Always depressurize the system before disconnecting any hydraulic components.

Symptoms of a Faulty DP Sensor
When the DP sensor begins to fail, the machine may exhibit:

• Delayed or weak hydraulic response
  
• Inconsistent boom or arm movement
  
• Error codes on diagnostic display (if equipped)
  
• Reduced fuel efficiency due to poor load sensing
  
• Occasional stalling during multi-function operations
  

These symptoms often mimic pump wear or valve spool sticking, making diagnosis tricky. A technician in Louisiana once spent two days chasing a sluggish boom issue before discovering the DP sensor had failed internally, sending erratic voltage signals to the controller.

Testing and Replacement Tips
To verify sensor function:

• Use a multimeter to check voltage output during operation
  
• Compare readings against factory specifications (typically 0.5–4.5V range)
  
• Inspect connector pins for corrosion or looseness
  
• Replace the sensor if readings are unstable or out of range
  

When installing a new sensor:

• Use thread sealant rated for hydraulic systems
  
• Torque to manufacturer spec (usually 25–35 Nm)
  
• Reconnect the electrical plug securely
  
• Cycle hydraulic functions to confirm proper response
  

After replacement, some machines may require a brief calibration or system reset. Consult the service manual for model-specific procedures.

DP Sensor Availability and Aftermarket Options
Original Hitachi sensors are available through authorized dealers, but aftermarket versions can offer cost savings. Ensure compatibility by matching:

• Thread size and pitch
  
• Voltage range
  
• Connector type
  
• Pressure rating (usually up to 5,000 psi)
  

Some operators report success using sensors from other EX-series models, such as the EX100 or EX200, though minor wiring modifications may be needed.

Preventive Maintenance and Long-Term Reliability
To extend the life of the DP sensor and surrounding components:

• Keep hydraulic fluid clean and within spec
  
• Replace filters every 500 hours
  
• Inspect electrical connectors quarterly
  
• Avoid high-pressure washing near the valve block
  
• Monitor system response and address sluggishness early
  

A contractor in Georgia implemented a quarterly sensor check protocol and reduced downtime by 30% over two years. Early detection of sensor degradation can prevent cascading failures in the hydraulic system.

Conclusion
The DP sensor on a Hitachi EX120 excavator is a small but vital component in the hydraulic control system. Located on the main valve block and shielded by a factory sleeve, it monitors pressure differentials and informs the machine’s response to operator input. When symptoms of sluggishness or erratic movement appear, inspecting and replacing the DP sensor can restore performance and prevent costly repairs. With proper maintenance and attention to sensor health, the EX120 continues to deliver reliable service across a wide range of job sites.