Diagnosing Power Steering Failure on the Michigan 75A Loader

[MikePhua]

The Michigan 75A and Its Mechanical Heritage
The Michigan 75A wheel loader was produced during the mid-20th century by Clark Equipment Company, a manufacturer known for pioneering heavy-duty loaders and forklifts. The 75A was part of a generation of machines that emphasized mechanical simplicity and brute strength over electronics. With a naturally aspirated diesel engine, planetary axles, and a hydraulic steering booster, the 75A was widely used in quarries, logging yards, and municipal fleets across North America.
Though production ceased decades ago, many units remain in service or restoration. Their mechanical systems—especially the power steering—require careful attention, as age and prior repairs often introduce misalignments, reversed connections, and degraded components.
Terminology Annotation

• Steering Booster: A hydraulic assist system that amplifies steering input using pressurized fluid.
  
• Spool Valve: A sliding valve element that directs hydraulic flow based on mechanical input.
  
• Ball Cup: A socket component that centers and stabilizes the ball joint in a valve sleeve.
  
• Check Valve: A one-way valve that prevents reverse flow in hydraulic circuits.
  

Symptoms of Reversed or Failed Steering Assist
Operators may encounter:

• Steering wheel fights back during turns.
  
• Hydraulic resistance increases instead of assisting.
  
• No change after reversing hose connections.
  
• Engine bogs or belts squeal when steering is attempted.
  

These symptoms suggest either reversed hydraulic logic, internal valve misassembly, or a failed check valve. In one case, a loader exhibited aggressive counterforce during steering—fluid was being directed against the operator’s input due to reversed spool orientation and a missing ball cup in the booster sleeve.
Step-by-Step Inspection Strategy
Begin with the mechanical linkage:

• Remove the steering booster and inspect the spring-loaded sleeve.
  
• Verify the ball joint is seated correctly with a functional ball cup.
  
• Clean and grease the sleeve to ensure smooth movement.
  

Next, inspect the control valve:

• Disassemble and confirm spool orientation—some valves can be installed upside down.
  
• Check for debris or scoring on the spool surfaces.
  
• Reassemble with attention to flow direction markings, if present.
  

Then, trace hydraulic lines:

• Confirm hose routing from pump to valve and valve to cylinder.
  
• Reverse connections only after verifying valve orientation.
  
• Inspect the pump output—should deliver consistent flow under load.
  

Hydraulic Pump and Cylinder Integrity
If the valve and hoses are correct but steering remains unresponsive, test the pump and cylinder:

• Check pump pressure with a gauge—should exceed 1,000 psi under load.
  
• Listen for engine RPM drop or belt squeal when steering is engaged.
  
• Inspect cylinder piston seals—worn packing allows internal bypass.
  
• Remove cylinder and test with external pressure source if needed.
  

A technician in Wyoming discovered that a rebuilt steering cylinder had no internal seals installed—fluid passed freely without generating force. Repacking the cylinder restored normal steering.
Check Valve and Flow Restriction
A stuck check valve can block return flow or trap pressure, causing erratic steering behavior. These valves are often buried in the valve block or booster housing.
To test:

• Remove and inspect check valve for debris or corrosion.
  
• Replace spring and seat if worn or pitted.
  
• Flush the hydraulic system to remove contaminants.
  

In one restoration project, a loader’s steering improved dramatically after replacing a corroded check valve that had seized in the closed position.
Recommendations for Restoration and Field Repair

• Label all hydraulic lines before disassembly to avoid reversed connections.
  
• Photograph valve orientation and linkage positions during teardown.
  
• Use high-quality seals and ball cups during reassembly.
  
• Replace hydraulic fluid and filters every 500 hours or annually.
  
• Keep spare check valves, booster sleeves, and spool seals in stock.
  

Closing Reflections
The Michigan 75A’s power steering system may be primitive by modern standards, but its reliability depends on precise mechanical alignment and clean hydraulic flow. When steering fights the operator instead of assisting, the issue often lies in reversed logic or missing components. With methodical inspection and careful reassembly, even a stubborn booster can be brought back to life—restoring control, confidence, and the legacy of a machine built to move mountains.