Troubleshooting Electrical Failures in HKX Pilot Valve Systems

[MikePhua]

HKX and Its Role in Excavator Hydraulic Control
HKX Inc., based in Washington State, specializes in hydraulic kits and control systems for excavators, particularly for auxiliary functions like thumbs, hammers, and grapples. Their pilot valve systems are designed to integrate with OEM hydraulic circuits, allowing operators to activate attachments via electric switches that trigger solenoid-controlled pilot valves. These valves modulate low-pressure pilot oil, which in turn actuates high-pressure control valves downstream.
HKX systems are widely used in retrofits, especially on machines that lack factory-installed auxiliary hydraulics. Their kits often include pilot valves, solenoids, relays, wiring harnesses, and control switches, tailored to specific excavator models.
Identifying Electrical Short Circuits in Pilot Valve Activation
A common issue encountered in field installations is a dead short when activating the pilot valve via the electric control button. This typically occurs after rewiring or retrofitting the system. In one case, an operator discovered shorted wires in the thumb circuit and rewired the entire harness. Upon activating the pilot valve, the system produced a dead short, blowing fuses and disabling the circuit.
The root cause was traced to the relay coil connected to the pilot valve solenoid. The relay schematic revealed diodes and a resistor between terminals, designed to suppress voltage spikes and protect the machine’s electronic control module (ECM). However, if the diode fails—especially in reverse bias—it can short internally and create a direct path to ground.
Recommendations for diagnosis:

• Inspect the relay for internal diode failure using a multimeter in diode mode
  
• Replace the relay with a known-good unit before bypassing
  
• Check for continuity between power and ground when the switch is activated
  
• Use fused test leads during troubleshooting to prevent damage
  

Understanding the Role of Diodes and Resistors in Relay Circuits
Diodes in relay circuits serve as flyback protection. When the relay coil is de-energized, the collapsing magnetic field generates a voltage spike that can damage sensitive electronics. The diode absorbs this spike by providing a path for current to dissipate safely. Resistors may be added to limit inrush current or stabilize voltage across the coil.
If the diode shorts internally, it effectively creates a direct connection between power and ground, causing fuse blowouts or ECM faults. In older machines without ECMs, the damage may be limited to wiring and relays, but in newer excavators, a failed diode can corrupt control logic or damage input circuits.
Best practices:

• Use relays with integrated flyback diodes rated for the system voltage
  
• Avoid mixing relays from different kits or manufacturers
  
• Label wires during installation to prevent polarity reversal
  
• Test relays outside the machine before installation
  

Field Repairs and Bush Engineering
In remote locations, operators often resort to improvised solutions. One technician bypassed the relay entirely and wired the pilot valve solenoid directly to the switch. While this restored function, it eliminated the protective diode and exposed the system to voltage spikes. In the bush, such fixes are sometimes necessary, but they should be considered temporary.
To improve reliability:

• Install an external flyback diode across the solenoid terminals
  
• Use a weatherproof inline fuse rated for the solenoid’s amperage
  
• Document the wiring changes for future service
  

A similar issue was reported in a forestry excavator operating in Alaska. After multiple relay failures, the operator installed a diode-protected toggle switch and ran the thumb circuit manually. The machine continued working for months without issue, but the ECM eventually flagged a voltage irregularity, prompting a full rewire.
Preventative Measures and Long-Term Solutions
To prevent future electrical failures in HKX pilot valve systems:

• Use marine-grade wiring with tinned copper conductors
  
• Seal all connectors with dielectric grease and heat-shrink tubing
  
• Mount relays in vibration-isolated enclosures
  
• Periodically inspect wiring for abrasion, corrosion, and loose terminals
  

If the machine operates in wet or corrosive environments, consider upgrading to sealed relays and waterproof connectors. HKX offers support for diagnostic issues, and contacting them with the machine’s serial number and kit ID can expedite troubleshooting.
Conclusion
HKX pilot valve systems offer reliable control for excavator attachments, but their electrical components require careful installation and maintenance. Diodes and resistors in relay circuits play a critical role in protecting the machine’s electronics, and bypassing them can lead to long-term damage. With proper diagnostics, protective wiring practices, and an understanding of circuit behavior, operators can ensure safe and effective operation—even in the harshest conditions. In the world of hydraulic control, a single diode can make the difference between precision and chaos.