Resolving A Adjustment Issues on Kobelco SK150 Mark III Excavators

[MikePhua]

The SK150 Mark III and Its Role in Kobelco’s Lineage
The Kobelco SK150 LC Mark III excavator was part of Kobelco’s third-generation hydraulic excavator series, launched in the mid-1990s to early 2000s. Kobelco, a division of Kobe Steel Ltd., has been a pioneer in hydraulic excavator design since the 1960s. The Mark III series introduced refinements in electronic engine control, hydraulic efficiency, and operator ergonomics. The SK150 LC, with its long carriage and approximately 33,000 lb operating weight, was designed for mid-size earthmoving, utility trenching, and site prep work.
Equipped with a four-cylinder Isuzu diesel engine and advanced hydraulic circuitry, the SK150 Mark III featured a stepping motor-controlled fuel system and an onboard CPU that managed throttle response, auto-idle, and engine speed adjustments. These systems were part of Kobelco’s effort to improve fuel efficiency and reduce operator fatigue.
Understanding the A Adjustment Procedure
The “A adjustment” refers to a calibration routine for the stepping motor, which controls the fuel rack position and engine RPM in response to operator input and load demand. This adjustment ensures that the motor responds accurately to throttle commands and integrates properly with the CPU’s auto-acceleration logic.
Terminology:

• Stepping Motor: A precision actuator that moves in discrete steps to control fuel delivery.
  
• Auto Accel: A feature that automatically adjusts engine RPM based on hydraulic demand.
  
• CPU Test Mode: A diagnostic mode allowing technicians to access calibration and sensor data.
  

The A adjustment typically involves:

• Entering the CPU’s test mode via the onboard interface
  
• Running a sweep of the stepping motor to confirm full range movement
  
• Disabling auto accel to isolate motor response
  
• Re-enabling work mode to finalize calibration
  

Common Problems During Calibration
One recurring issue is the inability to disable auto accel while the CPU is in test mode. This prevents the technician from completing the final sweep of the stepping motor, which is essential for proper calibration. The root cause often lies in how the CPU interprets switch inputs during diagnostic operations.
Observed symptoms include:

• Auto accel switch functions normally in work mode but is unresponsive in test mode
  
• CPU fails to recognize switch state changes during calibration
  
• Calibration halts before final sweep, leaving motor partially adjusted
  

In one documented case, a technician replaced a failed stepping motor but was unable to complete the A adjustment due to this exact issue. After removing the CPU and sending it to a repair facility, a burnt internal component was discovered and replaced. Upon reinstallation, the technician could enter test mode and begin calibration—but the auto accel switch still failed to respond during the final step.
Electrical and Software Considerations
The SK150’s CPU relies on discrete voltage signals from switches and sensors. In test mode, some inputs are ignored or rerouted to prevent accidental activation of hydraulic functions. This design, while safe, can interfere with calibration routines.
Recommendations:

• Verify switch continuity with a multimeter before entering test mode
  
• Check for voltage drop across the auto accel switch terminals
  
• Inspect CPU harness for corrosion or pin damage
  
• Confirm that the CPU firmware version supports full A adjustment access
  

Terminology:

• Voltage Drop: A reduction in voltage across a circuit, often caused by resistance or poor connections.
  
• Firmware: Embedded software controlling hardware functions within the CPU.
  

In a similar case involving a Kobelco SK200 Mark III, a technician discovered that the auto accel switch shared a ground path with the travel alarm circuit. A short in the alarm wiring caused intermittent switch failures during calibration. Rerouting the ground resolved the issue.
Stepping Motor Replacement and Precautions
When replacing the stepping motor, technicians should:

• Use OEM or high-quality aftermarket units rated for the correct voltage and torque
  
• Align the motor shaft with the fuel rack before installation
  
• Avoid forcing the motor into position, which can damage internal gears
  
• Perform a full A adjustment immediately after installation
  

Failure to calibrate the motor can result in:

• Erratic engine RPM
  
• Poor throttle response
  
• Increased fuel consumption
  
• CPU fault codes
  

In 2020, a fleet operator in British Columbia reported that improperly calibrated stepping motors on three SK150s led to over 15% fuel inefficiency and frequent stalling under load. After performing proper A adjustments, fuel consumption normalized and engine performance improved.
Workarounds and Field Solutions
If the auto accel switch cannot be disabled in test mode, technicians may consider:

• Temporarily bypassing the switch with a jumper wire to simulate “off” state
  
• Re-entering work mode, disabling auto accel, then returning to test mode
  
• Using an external diagnostic tool to force switch state recognition
  

Caution: Any bypass should be done with full understanding of the circuit and only during calibration. Permanent modifications can compromise safety systems.
Terminology:

• Jumper Wire: A temporary electrical connection used to simulate switch or sensor inputs.
  
• Bypass: A method of rerouting or overriding a circuit for testing purposes.
  

Conclusion
The Kobelco SK150 Mark III remains a capable and respected excavator, but its electronically controlled systems require careful handling during maintenance. The A adjustment procedure is critical for stepping motor performance, and issues with auto accel switch behavior in test mode can complicate calibration. By understanding the electrical logic, verifying switch integrity, and using safe diagnostic practices, technicians can restore full functionality and ensure the machine operates at peak efficiency. As these machines age, such knowledge becomes essential for keeping them productive and reliable in the field.