Why Is My SH75 Final Drive Stuck in High Range

[MikePhua]

Quick Answer
A final drive stuck in high range on a Sumitomo SH75 excavator often results from incorrect plumbing of the two-speed hydraulic signal line. The MAG-63 drive motor requires zero pressure for low speed and positive pressure for high. Misidentifying the 2-speed port or mismatching drive motor types can cause persistent high-range operation.
SH75 Excavator Background and Drive Motor Configuration
The Sumitomo SH75 is a compact excavator designed for urban and utility work, featuring a short tail swing and efficient hydraulic system. It typically uses MAG-63 series final drive motors, which include integrated two-speed and park brake functions. These motors are manufactured by Eaton or Nabtesco, depending on the variant, and are labeled with codes like MAG-63VP-770, where:

• 63 = motor displacement in cc/rev
  
• V = two-speed function
  
• P = integrated park brake
  
• 770 = torque output in kgf·m
  

The two-speed system allows the operator to switch between high-speed travel and low-speed torque modes. This is controlled by a small hydraulic signal line that toggles internal valve positions.
Symptoms of Incorrect Plumbing

• Machine travels in an arc when levers are pushed evenly, indicating one side is in high range and the other in low.
  
• Low torque on hills, requiring boom assistance to climb.
  
• Tracks appear synchronized only in high range, but diverge in low.
  
• Temporary low gear engagement after startup, reverting to high after a few seconds.
  

These symptoms suggest that the two-speed signal line is either misrouted or not functioning correctly. Disconnecting the signal line may briefly force the motor into low range, but it will default back to high if the internal valve is not properly actuated.
Understanding the Two-Speed Hydraulic Logic

• Zero pressure = low speed
  
• Positive pressure = high speed
  

The MAG-63 motor has two small ports near the main A and B pressure ports. One is for the brake release (requires pressure), and the other is for the two-speed signal. Confusing these ports or plumbing them incorrectly causes the motor to stay in high range.
Corrective Actions and Field Fix

• Identify the correct 2-speed port—typically the small plug between A and B ports.
  
• If the correct fitting is unavailable, a temporary solution is to drill and weld a test fitting into the plug.
  
• Confirm that the hydraulic signal line delivers zero pressure when low speed is desired.
  
• Ensure both drive motors are of the same type and plumbed symmetrically, with A and B ports reversed on opposite sides to maintain directional consistency.
  

Real-World Anecdote
In Oregon, an operator installed a replacement MAG-63 motor but found the machine veering and lacking torque. After trial and error, he discovered the 2-speed signal was connected to the brake port. By modifying the plug and rerouting the line, the motor returned to normal function. He noted that the original motor had a different internal configuration, and matching motor types was critical.
Recommendations for Future Repairs

• Always verify motor model and port layout before installation.
  
• Use manufacturer diagrams or teardown photos to confirm plumbing.
  
• Pressure test the signal line to ensure correct behavior.
  
• Replace both drive motors together if compatibility is uncertain.
  
• Label hydraulic lines during disassembly to prevent mix-ups.
  

Conclusion
A final drive stuck in high range on an SH75 excavator is usually a plumbing issue, not a mechanical failure. Understanding the hydraulic logic of the MAG-63 motor and correctly identifying the 2-speed port are essential for restoring proper function. With careful inspection and precise routing, operators can resolve travel inconsistencies and regain full control of their machine.