Diagnosing Air System Overpressure: Governor vs. Dryer in Heavy Trucks

[MikePhua]

Introduction to Air System Components
In heavy-duty trucks, the air brake system relies on a network of components to regulate pressure, remove moisture, and ensure safe operation. Two critical parts—the air governor and the air dryer—play distinct roles. When a truck builds excessive air pressure, pinpointing the faulty component requires understanding their functions and interactions.
Terminology Clarification

• Air Governor: A pressure-sensitive valve that controls when the compressor loads and unloads, maintaining system pressure within a safe range.
  
• Air Dryer: A moisture-removal unit that filters water and oil from compressed air before it enters the brake system.
  
• Unloader Valve: A valve inside the compressor that opens to stop compression when the governor signals unloading.
  
• Cut-Out Pressure: The maximum pressure at which the governor signals the compressor to unload.
  
• Cut-In Pressure: The minimum pressure at which the governor signals the compressor to resume loading.
  

Understanding the Problem: Excessive Air Pressure
When a truck builds too much air pressure—exceeding the normal cut-out threshold (typically around 120–130 psi)—the issue is almost always linked to the air governor or the compressor’s unloader valves, not the dryer. The dryer does not regulate pressure; it simply conditions the air.
Diagnostic Strategy

• Monitor Pressure Behavior
  If pressure continues to rise past the cut-out point, the governor may not be signaling the compressor to unload.
  
• Listen for Unloading Sounds
  A functioning governor triggers a distinct hiss or purge sound as the compressor unloads. Absence of this sound suggests governor failure or stuck unloader valves.
  
• Inspect Governor Lines
  Check for cracked or blocked air lines between the governor and compressor. A damaged line can prevent proper signal transmission.
  
• Test Governor Function
  Remove the governor and inspect its internal diaphragm and spring. Replace if worn or contaminated.
  
• Bypass Dryer Temporarily
  To rule out the dryer, bypass it and observe pressure behavior. If overpressure persists, the dryer is not the cause.
  

Field Anecdote: The South Carolina Compressor Case
A technician in South Carolina encountered a truck that consistently built pressure beyond 150 psi. Initial suspicion fell on the air dryer, but further inspection revealed a faulty governor diaphragm. Replacing the governor restored normal pressure cycling. The dryer, though aged, was still functioning as intended.
Industry Insight: Governor Failures in Aging Fleets
In 2022, a fleet maintenance report from Alberta noted that over 60% of air system failures in trucks older than 10 years were traced to governor malfunction. Environmental factors—heat, vibration, and moisture—contribute to diaphragm degradation and spring fatigue. Regular inspection and replacement every 3–5 years is recommended.
Best Practices for Air System Maintenance

• Replace air governor every 3–5 years or at signs of erratic pressure behavior
  
• Drain air tanks daily to prevent moisture buildup
  
• Inspect compressor unloader valves during routine service
  
• Use OEM or high-quality aftermarket governors for reliability
  
• Keep spare governors and dryer cartridges in fleet inventory
  

Conclusion
When diagnosing excessive air pressure in a heavy truck, the air governor is the primary suspect—not the dryer. Understanding the governor’s role in pressure regulation and its interaction with the compressor’s unloader valves is key to resolving the issue. With proper diagnostics and preventive maintenance, operators can ensure safe and efficient air system performance—keeping the wheels rolling and the brakes ready.