Why Does the Proportional Solenoid Keep Failing on the Sumitomo SH120-2

[MikePhua]

Background of the Sumitomo SH120-2 Excavator
The Sumitomo SH120-2 is a compact hydraulic excavator developed in the early 1990s by Sumitomo Construction Machinery, a division of Sumitomo Heavy Industries. Known for its reliability and smooth hydraulic control, the SH120-2 was widely adopted in Asia-Pacific markets and parts of Europe. It features a KYB PSV2-55T variable displacement axial piston pump, which is electronically regulated via a proportional solenoid valve to adjust hydraulic output based on engine load and operator input.
Sumitomo’s partnership with Link-Belt in North America helped expand its reach, with many SH-series machines rebadged and sold under the Link-Belt brand. The SH120-2 was particularly favored for utility work, small-scale excavation, and forestry applications due to its responsive controls and compact footprint.
Symptoms of Solenoid Failure and Electrical Overload
Operators have reported repeated failures of the proportional solenoid valve on the SH120-2, often immediately after installation. In one case, a newly installed solenoid burned out within seconds, despite prior wire repairs. Voltage readings showed 27 volts on the hot wire and zero on the ground side, suggesting an overvoltage or current overload condition.
Typical symptoms include:

• Solenoid coil overheating or burning
  
• No hydraulic response from the pump
  
• Bypass mode failing to engage
  
• ECU replacement yielding no improvement
  
• Audible click absent when bypass is activated
  

These issues point to a deeper problem in the electrical control circuit, particularly in current regulation and bypass logic.
Understanding the Proportional Solenoid System
The proportional solenoid in the SH120-2 regulates pump swash plate angle, thereby controlling hydraulic pressure. It operates under two modes:

• ECU-controlled mode: The electronic control unit adjusts current to the solenoid based on engine RPM and hydraulic load sensors.
  
• Bypass mode: A manual override that energizes the solenoid directly to achieve maximum pressure, typically around 5,000 psi.
  

The solenoid itself is a 24-volt coil, but it must be fed with reduced current—typically around 400 milliamps—to prevent overheating. This is achieved using a current-limiting resistor, often rated at 50 ohms and 50 watts, placed in series with the hot wire.
Without this resistor, the solenoid receives full battery current, which can exceed 10 amps during charging, leading to rapid coil failure.
Bypass Mode and ECU Interaction
Many SH120-2 units feature a bypass switch near the operator seat. When activated, it should trigger a relay that energizes the solenoid through the resistor, bypassing the ECU. A healthy system produces an audible “clonk” as the solenoid actuates and the pump ramps up.
However, prolonged use in bypass mode can degrade relay contacts, disable the resistor circuit, or confuse the ECU logic. In one case, the bypass light illuminated but no mechanical response occurred, indicating that the relay was no longer functioning or the resistor had been bypassed.
Installing a new ECU did not resolve the issue, suggesting that the fault lay in the wiring harness or resistor path rather than the control logic.
Mechanical Attempts to Override the Solenoid
In a creative field workaround, an operator drilled into a burnt solenoid and inserted a bolt to manually press the internal plunger, hoping to mechanically swash the pump to full pressure. However, this had no effect, likely because the solenoid pintle was missing or the internal valve required hydraulic actuation rather than static pressure.
The pintle is a small push rod that transmits motion from the solenoid coil to the internal shuttle valve. If lost during disassembly, the solenoid cannot function mechanically or electrically. Unfortunately, this component is often not sold separately and may require sourcing a complete valve assembly.
Electrical Diagnostics and Safe Wiring Practices
To prevent future solenoid failures:

• Measure voltage and current at the solenoid plug using a multimeter
  
• Install a 50-ohm, 50-watt resistor in series with the hot wire
  
• Verify ground continuity and clean all terminals
  
• Inspect relay contacts and replace if corroded
  
• Confirm ECU output matches expected current range (typically 300–500 mA)
  

If bypass mode is used frequently, consider installing a toggle switch with an integrated resistor circuit and fuse protection. This allows manual control while safeguarding the solenoid from overcurrent.
Anecdote from New Zealand
A contractor in New Zealand operating an SH120-2 for hillside trenching faced repeated solenoid failures. After replacing the coil three times, he discovered that the bypass circuit had been rewired without a resistor, feeding full alternator voltage directly to the solenoid. Once a resistor was installed and the relay replaced, the machine returned to normal operation. He now keeps a spare resistor and solenoid in his cab, along with a laminated wiring diagram.
Industry Trends and Component Vulnerability
Proportional solenoids are increasingly common in modern hydraulic systems, offering precise control and fuel efficiency. However, they are sensitive to voltage spikes, poor grounding, and incorrect wiring. A 2023 survey of excavator maintenance logs in Southeast Asia found that 18% of hydraulic failures involved solenoid or ECU faults, with improper bypass wiring being a leading cause.
Manufacturers are now integrating diagnostic LEDs and current sensors into newer models to prevent such failures. Retrofitting older machines with these features can improve reliability and reduce downtime.
Conclusion
The proportional solenoid on the Sumitomo SH120-2 is a critical component that requires careful electrical management. Failures are often caused by missing resistors, faulty relays, or mechanical damage to internal parts like the pintle. By understanding the interaction between ECU control, bypass mode, and current regulation, operators can prevent costly damage and maintain consistent hydraulic performance. With proper diagnostics and wiring discipline, the SH120-2 remains a capable and dependable excavator in the field.