Deep Technical Chapters · Compressor & Pressure Path
Compressor Diagnosis
Is the compressor actually the fault? Prove supply, control and load before condemning the compressor.
The compressor is the most expensive part to replace and the most commonly misdiagnosed. Before you condemn one, prove it isn't a supply, control, or load problem.
Running but not pumping
Equal suction and discharge pressures with low amps means the compressor is running but not pumping — failed valves or mechanical failure. Confirm with a pump-down or winding test before replacing.
Noisy compressor
Noise is evidence it has been running. Investigate mechanical noise, vibration, mounting, pipework, floodback, suction and discharge conditions, high compression ratio, oil, valves and operating conditions — in that order of likelihood, not habit.
Won't start
Follow supply → control → contactor → capacitors → windings before condemning the compressor. See The Diagnostic Method.
Diagnosis at a glance
Symptoms of failure
- •Equal pressures, low amps (not pumping)
- •Knocking or slugging (floodback or oil)
- •Won't start (supply/control first)
- •Trips on overload (load or electrical)
Measurements to take
- •Suction and discharge pressure
- •Compressor amps vs FLA/LRA
- •Winding resistance and insulation to ground
- •Capacitor values
- •Discharge temperature
What can mislead you
- •Condemning a compressor that is actually starved by a restriction
- •Calling a noisy compressor "worn out" when it's floodback
How to prove the fault
- •Equal pressures + low amps + good electrics = mechanical/valve failure
Common mistakes & callbacks
- •Replacing the compressor without proving supply and control
- •Ignoring floodback as the cause of noise
