Diagnosing Heat-Related Drive Failure in the Bomag 120 AD-3 Roller

[MikePhua]

The Bomag 120 AD-3 and Its Historical Context
The Bomag 120 AD-3 is a compact tandem vibratory roller introduced in the early 1990s by Bomag GmbH, a German manufacturer founded in 1957 and renowned for its roadbuilding and compaction equipment. The 120 AD-3 was designed for small to medium-scale asphalt and granular compaction tasks, especially in urban environments and repair work. With an operating weight of approximately 2,700 kg and drum width of 1,200 mm, it became a popular choice for municipalities and contractors across Europe and North America. Bomag’s rollers have consistently led the market in vibration technology and hydraulic efficiency, with the 120 AD series selling tens of thousands of units globally by the early 2000s.
Core Specifications and System Overview

• Engine: Typically equipped with a Kubota or Deutz diesel engine rated around 30–35 horsepower
  
• Drive System: Hydrostatic drum drive with variable displacement pumps
  
• Vibration System: Dual amplitude vibratory drums with frequency around 60 Hz
  
• Cooling System: Radiator with electric fan, often modified in older units
  

The hydrostatic drive system uses pressurized hydraulic fluid to power the drum motors. This system is sensitive to fluid viscosity, temperature, and seal integrity.
Symptoms of Heat-Induced Lockup
A recurring issue in aging Bomag 120 AD-3 units is drive lockup after extended operation, typically occurring after 15–30 minutes of use. The machine starts and runs normally, vibrates effectively, and moves without issue—until it suddenly refuses to drive. Once cooled, it resumes normal function.
This behavior suggests:

• Hydraulic fluid thinning due to heat, reducing pressure and allowing internal bypass
  
• Seal degradation in drive motors or pumps, leading to pressure loss
  
• Overheating of hydraulic components, possibly due to fan failure or restricted airflow
  
• Thermal expansion affecting valve tolerances or solenoid function
  

In one case, a contractor in Ireland purchased a 1995 unit at auction and discovered the issue only after field deployment. The previous owner had wired the radiator fan to run continuously, indicating prior overheating concerns.
Troubleshooting and Diagnostic Steps
To isolate the fault:

• Check hydraulic fluid type and condition. Use ISO 46 or ISO 68 fluid depending on climate. Replace if discolored or foamy.
  
• Inspect hydraulic filters for clogging, which can restrict flow and increase heat.
  
• Test drive pressure using inline gauges at the pump and motor ports. Compare cold vs. hot readings.
  
• Examine motor seals and case drain flow. Excessive leakage indicates internal bypass.
  
• Verify fan operation and radiator cleanliness. A constant-running fan may mask deeper cooling inefficiencies.
  

If pressure drops significantly when hot, the issue likely lies in the pump or motor tolerances. Rebuilding or replacing these components may be necessary.
Preventive Measures and Long-Term Solutions
To extend the life of the drive system:

• Use synthetic hydraulic fluid with better thermal stability
  
• Install auxiliary cooling fans or upgrade radiator capacity
  
• Replace aged seals and gaskets during routine service
  
• Add temperature sensors to monitor fluid heat in real time
  
• Avoid prolonged idling with vibration engaged, which heats fluid without airflow
  

Operator Anecdotes and Shared Experience
A technician in Oregon reported similar symptoms on a Bomag 100 AD roller. The machine would lock up mid-shift, forcing crews to wait for cooldown. After replacing the hydraulic pump and switching to synthetic fluid, the issue disappeared. Another operator in Poland noted that drum movement resumed instantly after spraying the hydraulic lines with cold water—an improvised but telling diagnostic method.
Bomag’s Support and Serial Number Breaks
Bomag equipment often features serial number breaks that affect parts compatibility and service procedures. When diagnosing older units, technicians should reference the exact serial number to ensure correct component identification. This is especially important for hydraulic pumps, motors, and control valves, which may vary across production years.
Conclusion
The Bomag 120 AD-3 remains a robust and capable roller, but age-related hydraulic issues—especially heat-induced drive lockup—require careful diagnosis and proactive maintenance. By understanding the interplay between fluid viscosity, seal integrity, and cooling efficiency, operators can restore reliable performance and extend the machine’s service life. With proper upgrades and attention to thermal behavior, even a 1995 unit can continue compacting with confidence.