Hitachi EX200-5 Stepping Motor Malfunction After Grease Service

[MikePhua]

Cleaning and re-greasing the stepping motor on a Hitachi EX200-5 excavator can lead to erratic throttle behavior if the internal arm position is disturbed or the motor is not recalibrated. A toggle switch reset and manual RPM adjustment are required to restore proper idle and response.
Hitachi EX200-5 background and throttle control system
The Hitachi EX200-5 was introduced in the late 1990s as part of Hitachi’s fifth-generation hydraulic excavator series. It features:

• A six-cylinder Isuzu diesel engine
  
• Electronic throttle control via stepping motor
  
• Integrated fuse panel and diagnostic toggle switch
  
• Manual override slot for RPM adjustment
  
• Over 15,000 units sold globally, with strong adoption in Asia-Pacific and South America
  

The stepping motor controls the fuel rack position, adjusting engine RPM based on operator input and system demands. It is mounted near the injection pump and connected via a bracket and arm assembly.
Symptoms after stepping motor service
Operators have reported:

• Erratic idle speed after cleaning and re-greasing the motor
  
• Throttle response worse than before service
  
• Electrical tests showing normal voltage and continuity
  
• Machine previously showing mild throttle drift
  

These symptoms suggest that the motor arm was misaligned during reassembly, or the internal position sensor was disturbed. Grease buildup can mask mechanical wear, and cleaning may expose underlying issues.
Reset procedure using toggle switch
To recalibrate the stepping motor:

• Locate the small toggle switch near the fuse panel
  
• Push the switch upward
  
• Turn the ignition key ON for five seconds
  
• Turn the key OFF for five seconds
  
• Return the toggle switch to the middle position
  
• Start the engine and observe idle behavior
  

This sequence resets the motor’s internal position reference and allows the controller to reinitialize throttle settings.
Manual RPM adjustment
If idle speed remains incorrect:

• Insert a long 6mm Allen wrench through the slot in the stepping motor bracket
  
• Loosen the set screw securing the motor arm
  
• Move the arm slightly to adjust RPM
  
• Retighten the screw and verify idle stability
  

This adjustment should be done with the engine running and warm. Use a tachometer or listen for consistent engine tone.
Additional recommendations and service tips

• Always mark the motor arm position before removal
  
• Use high-temperature grease sparingly—excess can interfere with sensor feedback
  
• Inspect the motor connector for corrosion or loose pins
  
• Replace worn motor arms or brackets to prevent drift
  
• Keep a record of idle RPM before and after service
  

One technician in Tasmania noted that his EX200-5 had over 15,000 hours and showed throttle instability after cleaning the motor. After performing the toggle reset and manual adjustment, the machine returned to normal operation.
Conclusion
Stepping motor issues on the Hitachi EX200-5 often arise after cleaning and re-greasing, especially if the arm position is disturbed. A toggle switch reset and manual RPM adjustment can restore proper throttle control. With careful reassembly and calibration, the motor can continue to function reliably even after high-hour service.