10-17-2025, 02:42 PM
Quick Answer
Flash codes 34 and 36 on a Hitachi EX120-2 typically indicate open circuits in the Boom Up Pilot Pressure (BPI) and Arm In Pressure (ABP) sensors. These faults often trace back to wiring issues or shorts in the shared 5V reference circuit, which can burn resistors in the hydraulic control unit if fuse ratings are incorrect.
Hitachi EX120-2 Background and Diagnostic System
The Hitachi EX120-2 excavator, introduced in the early 1990s, was part of Hitachi’s push toward electronically managed hydraulic systems. Built for mid-size earthmoving and utility work, the EX120-2 featured a hydraulic computer (PVC) that monitored sensor inputs and adjusted pump stroke and valve timing accordingly. Hitachi sold thousands of units globally, with many rebadged as John Deere 490E models in North America, sharing identical electronics but different engines.
Unlike modern CAN-based diagnostics, the EX120-2 uses a flash code system triggered by placing the computer into diagnostic mode. Codes are displayed via blinking lights, each sequence corresponding to a specific fault. The factory manual typically lists codes up to 32, but field experience and cross-referencing with Deere documentation reveal additional codes like 34 and 36.
Flash Code Definitions and Implications
The EX120-2’s sensors operate on a shared 5V reference signal. If any sensor or wire intermittently shorts to ground, the resulting current spike can overload the circuit and burn out a resistor in the PVC. This is especially likely if a fuse rated higher than 1A is installed in the F7 slot, which protects the valve control unit.
Key points:
A practical approach involves:
Anecdote from the Field
In Washington State, an EX120-2 rented to a contractor returned with sluggish hydraulics and a burnt resistor. The owner discovered a 5A fuse had been installed in place of the original 1A, likely by the renter. After replacing the resistor and restoring the correct fuse, the machine still showed flash codes. Systematic sensor testing revealed the angle sensor was intermittently shorting. A new sensor fixed the issue, and the owner now keeps spares on hand.
Recommendations and Preventive Measures
Flash codes on the Hitachi EX120-2 offer vital clues to hydraulic system faults, especially when factory manuals fall short. Understanding the shared 5V circuit and its vulnerabilities is key to preventing costly damage. With careful diagnosis and proper fuse management, operators can keep these legacy machines running reliably in the field.
Flash codes 34 and 36 on a Hitachi EX120-2 typically indicate open circuits in the Boom Up Pilot Pressure (BPI) and Arm In Pressure (ABP) sensors. These faults often trace back to wiring issues or shorts in the shared 5V reference circuit, which can burn resistors in the hydraulic control unit if fuse ratings are incorrect.
Hitachi EX120-2 Background and Diagnostic System
The Hitachi EX120-2 excavator, introduced in the early 1990s, was part of Hitachi’s push toward electronically managed hydraulic systems. Built for mid-size earthmoving and utility work, the EX120-2 featured a hydraulic computer (PVC) that monitored sensor inputs and adjusted pump stroke and valve timing accordingly. Hitachi sold thousands of units globally, with many rebadged as John Deere 490E models in North America, sharing identical electronics but different engines.
Unlike modern CAN-based diagnostics, the EX120-2 uses a flash code system triggered by placing the computer into diagnostic mode. Codes are displayed via blinking lights, each sequence corresponding to a specific fault. The factory manual typically lists codes up to 32, but field experience and cross-referencing with Deere documentation reveal additional codes like 34 and 36.
Flash Code Definitions and Implications
- Code 34
Indicates an open circuit in the Boom Up Pilot Pressure (BPI) sensor or its wiring. This sensor monitors pilot pressure during boom lift operations and feeds data to the hydraulic computer for stroke control.
- Code 36
Points to an open circuit in the Arm In Pressure (ABP) sensor or its wiring. Though not officially listed for the EX120-2, it appears in later models and rebadged variants. Its presence may suggest a retrofit or wiring crossover.
The EX120-2’s sensors operate on a shared 5V reference signal. If any sensor or wire intermittently shorts to ground, the resulting current spike can overload the circuit and burn out a resistor in the PVC. This is especially likely if a fuse rated higher than 1A is installed in the F7 slot, which protects the valve control unit.
Key points:
- All sensors (angle, pump delivery pressure, DP sensor) share the same 5V line.
- A short in any sensor can affect the entire circuit.
- Using a fuse above 1A increases the risk of resistor failure.
- Burnt resistors cause the pump to default to full stroke, leading to erratic hydraulics.
A practical approach involves:
- Disconnecting all sensors and observing whether the computer resumes flashing.
- Reconnecting sensors one by one to identify the fault source.
- Measuring voltage at each sensor to confirm proper 5V reference.
- Replacing the faulty sensor or repairing damaged wiring.
- Ensuring the correct fuse rating is restored.
Anecdote from the Field
In Washington State, an EX120-2 rented to a contractor returned with sluggish hydraulics and a burnt resistor. The owner discovered a 5A fuse had been installed in place of the original 1A, likely by the renter. After replacing the resistor and restoring the correct fuse, the machine still showed flash codes. Systematic sensor testing revealed the angle sensor was intermittently shorting. A new sensor fixed the issue, and the owner now keeps spares on hand.
Recommendations and Preventive Measures
- Always use the correct fuse rating—1A for the F7 slot.
- Inspect wiring harnesses for abrasion, especially near the pump and frame.
- Keep spare resistors and angle sensors in inventory.
- Use Deere 490E manuals for extended code references.
- Consider installing inline diagnostic LEDs or voltage testers for faster fault tracing.
Flash codes on the Hitachi EX120-2 offer vital clues to hydraulic system faults, especially when factory manuals fall short. Understanding the shared 5V circuit and its vulnerabilities is key to preventing costly damage. With careful diagnosis and proper fuse management, operators can keep these legacy machines running reliably in the field.
