Diagnosing Shutdown Issues in a Sullair 185 Portable Air Compressor

[MikePhua]

The Sullair Legacy in Compressed Air Systems
Sullair, founded in Michigan City, Indiana in 1965, has long been a leader in rotary screw air compressor technology. Their 185 series portable compressors are widely used in construction, mining, and utility maintenance. With a reputation for ruggedness and simplicity, the 185 model—especially those built in the 1990s—features minimal electronics and a robust mechanical layout. Tens of thousands of units have been sold globally, and many remain in service decades later.
The 1996 Sullair 185 in question exemplifies this durability, but also highlights the diagnostic challenges that arise when older machines begin to shut down unexpectedly.
Terminology Annotation

• Rotary Screw Compressor: A type of air compressor that uses two interlocking helical screws to compress air continuously.
  
• Scavenger Filter: A filter that removes oil mist and contaminants from the air-oil mixture before discharge.
  
• Discharge Oil Temperature: The temperature of the lubricating oil as it exits the compressor element, critical for safe operation.
  

Symptoms and Initial Observations
The unit consistently shuts down after approximately seven minutes of operation. It restarts easily and runs briefly before shutting off again. The mechanical gauge shows a discharge oil temperature of 260°F, while an infrared temperature gun reads a maximum of 220°F on the air end. This discrepancy suggests either sensor error or localized overheating.
The compressor has undergone several maintenance steps:

• Replacement of scavenger and hydraulic filters
  
• Fluid change using ATF (approved for short-term use up to 300 hours)
  
• Fuel shutoff solenoid replacement
  
• Oil cooler cleaning
  
• Temporary bypassing of temperature sensors
  

Despite these efforts, the shutdown behavior persists.
Potential Causes and Diagnostic Pathways
Several factors could contribute to the shutdown:

• Faulty temperature sensor triggering false overheat signals
  
• Safety shutdown circuit activating due to sensor misread
  
• Fuel shutoff system receiving erroneous signals
  
• Internal wear in the compressor element causing heat buildup
  
• Electrical fault in the shutdown relay or timer
  

One technician suggested that worn bearings in the screw compressor could reduce tolerances, increasing friction and heat. While plausible, this would typically result in gradual performance degradation rather than a timed shutdown.
Terminology Annotation

• Safety Shutdown Circuit: A system designed to protect the compressor from damage by shutting it down when critical parameters exceed safe limits.
  
• Fuel Shutoff Solenoid: An electrically actuated valve that controls fuel flow to the engine, often triggered by safety systems.
  

Sensor Accuracy and Electrical Behavior
The fact that the compressor shuts down as if someone flipped a switch—rather than sputtering or stalling—suggests an intentional shutdown via the safety system. Even with the temperature sensor bypassed, the shutdown continues, indicating that another component may be overriding the bypass.
To isolate the issue:

• Use a test light on the injection pump to confirm power loss during shutdown
  
• Install a fuel pressure gauge to rule out starvation
  
• Reconnect all safety sensors and monitor which one triggers the shutdown
  
• Inspect wiring harnesses for corrosion or shorts
  
• Check for hidden timers or reference circuits that may be misconfigured
  

A technician noted that some compressors use a reference signal to start a timer; if the signal is absent, the timer expires and shuts down the unit. This could explain the consistent seven-minute runtime.
Recommendations for Field Technicians
To resolve the issue:

• Verify actual oil temperature using multiple methods
  
• Replace or recalibrate the discharge oil temperature sensor
  
• Inspect the shutdown relay and timer circuits
  
• Confirm fuel delivery and solenoid function with direct testing
  
• Reconnect all safety systems and isolate the trigger point
  
• Consider replacing the compressor element if internal wear is suspected
  

Preventive measures include:

• Regular cleaning of the oil cooler and airflow passages
  
• Using compressor-specific lubricants with correct viscosity
  
• Logging shutdown events and sensor readings for pattern analysis
  
• Inspecting electrical connections quarterly
  

Conclusion
The Sullair 185 air compressor is built for reliability, but age and wear can introduce complex shutdown behaviors. In this case, the consistent timing and clean restart suggest an electrical or sensor-driven fault rather than mechanical failure. By methodically testing each subsystem—fuel, temperature, safety circuits—technicians can pinpoint the cause and restore full operation. Whether powering jackhammers or sandblasters, compressed air must flow uninterrupted, and the key lies in understanding the machine’s logic as well as its mechanics.