Diagnosing Hydraulic Steering Issues on the BOMAG AW 90 Pneumatic Roller

[MikePhua]

The BOMAG AW 90 and Its Role in Road Compaction
The BOMAG AW 90 is a pneumatic-tired roller designed for asphalt finishing and soil compaction. Manufactured by BOMAG GmbH, a German company founded in 1957, the AW 90 was part of a series of mid-sized rollers used extensively in municipal roadwork, airport paving, and highway shoulder compaction. With its multi-wheel configuration and adjustable ballast system, the AW 90 delivers uniform pressure across the surface, making it ideal for sealing and kneading asphalt layers.
Unlike steel drum rollers, pneumatic rollers like the AW 90 use rubber tires to achieve a flexible compaction footprint. This design reduces surface tearing and improves density in asphalt mixes. The AW 90 also features hydraulic steering, allowing precise maneuvering in tight work zones and around curbs or obstacles.
Terminology Clarification

• Hydraulic steering: A system where fluid pressure is used to actuate steering cylinders, replacing mechanical linkages.
  
• Orbitrol valve: A hydraulic steering control unit that meters fluid to the steering cylinders based on operator input.
  
• Priority valve: A hydraulic component that ensures steering receives fluid before other functions.
  
• Steering cylinder: A double-acting hydraulic cylinder that moves the axle or frame to change direction.
  

Symptoms of Steering Failure
Operators encountering steering issues on the AW 90 often report:

• No response when turning the steering wheel
  
• Delayed or jerky steering movement
  
• Steering only in one direction
  
• Excessive effort required to steer
  
• Audible pump noise or fluid cavitation during steering attempts
  

In one case, a road crew in Alberta found their AW 90 unable to steer left after a cold morning start. The issue was traced to a stuck spool in the orbitrol valve, which had seized due to moisture and corrosion.
Hydraulic System Overview
The AW 90’s steering system draws fluid from a shared hydraulic reservoir and uses a gear pump to supply pressure. The orbitrol valve directs flow to the left or right steering cylinder based on wheel rotation. A priority valve ensures that steering always receives fluid before auxiliary functions like ballast adjustment or brake actuation.
Key components include:

• Hydraulic reservoir with filter and breather
  
• Gear-type hydraulic pump
  
• Orbitrol steering valve
  
• Priority valve
  
• Steering cylinders and hoses
  
• Return lines and check valves
  

Troubleshooting and Diagnostic Steps
To isolate steering problems:

• Check hydraulic fluid level and condition. Milky or dark fluid indicates contamination.
  
• Inspect hoses for leaks, kinks, or abrasion.
  
• Test pump output pressure with a gauge. Normal range is 2,000–2,500 psi.
  
• Remove and inspect the orbitrol valve for internal wear or sticking spools.
  
• Verify priority valve operation by checking flow to steering when other functions are active.
  
• Inspect steering cylinder seals for leakage or bypassing.
  

A technician in Georgia diagnosed a steering delay by pressure-testing the priority valve. He discovered that the valve was partially blocked by debris from a deteriorated hose lining. Replacing the hose and cleaning the valve restored full steering response.
Common Failure Points and Solutions

• Orbitrol valve: Internal wear or contamination can cause erratic steering. Rebuild kits are available with seals, springs, and spools.
  
• Priority valve: Dirt or metal shavings can block flow. Disassemble and clean thoroughly.
  
• Pump: Worn gears or cavitation can reduce pressure. Replace or rebuild if output drops below spec.
  
• Cylinders: Leaking seals or bent rods can cause uneven steering. Repack or replace as needed.
  
• Hoses: Internal delamination can shed particles into the system. Replace with high-pressure rated hose.
  

A contractor in Texas rebuilt his AW 90 steering system using aftermarket orbitrol components and added a magnetic inline filter to catch future debris. The roller returned to service with improved responsiveness and reduced pump noise.
Preventive Maintenance Recommendations

• Change hydraulic fluid every 500 hours or annually
  
• Replace filters every 250 hours
  
• Inspect hoses quarterly for wear or swelling
  
• Grease steering pivot points monthly
  
• Flush system after major component replacement
  
• Store machine indoors or cover orbitrol valve during off-season
  

A fleet manager in Ontario implemented a fluid sampling program and discovered early signs of water contamination in several AW 90 units. Switching to sealed reservoirs and desiccant breathers reduced moisture ingress and extended component life.
Conclusion
Hydraulic steering issues on the BOMAG AW 90 are often caused by contamination, wear, or neglected maintenance. Understanding the system’s layout and prioritizing diagnostics around the orbitrol valve and priority flow paths can lead to quick resolution. With proper care, the AW 90 remains a reliable and precise compaction tool, capable of navigating complex job sites and delivering consistent surface results. Whether restoring steering function or preventing future failures, attention to hydraulic health is key to keeping this classic roller rolling strong.