The heart of any air conditioner is the compressor. This pump sucks in refrigerant vapor and compresses it into a liquid. The oil-laden refrigerant cools and lubricates the compressor. It is protected from operating with low refrigerant levels by a switch that turns the compressor off when the refrigerant pressure falls below a set value. Once the compressor stops, the refrigerant quickly expands and pressure builds. The low pressure cut-off switch is placated, and the cycle starts again.
The thermostat also shuts off the compressor, but only after the selected temperature is achieved. This normal cycling process closely resembles symptoms created by slight refrigerant shortages. Selected temperatures are nearly attained, and compressor run times may seem normal. This condition may be revealed by high heat loads. A compressor that cycles off before lowering the temperature sufficiently may be lacking a small portion of its refrigerant charge. This symptom may escape notice if heat loads are moderate.
Excessively low refrigerant levels result in distinctly short compressor running times. This symptom is readily apparent from an appreciable loss of cooling, accompanied by the noise created by the compressor. This action is clearly perceptible in automotive air conditioners. The air conditioner clutch clicks when it engages the compressor. The compressor quickly draws down refrigerant pressure, and the low pressure switch disengages the clutch. The loud clicking sounds are only seconds apart when the system charge is seriously low.
Most air conditioners are designed to halt compressor operation when little or no refrigerant is in the system. Safeguards are in place to protect the compressor if the low pressure switch fails or is bypassed. The compressor gets exceedingly hot when forced to run without the cooling and lubricating afforded by proper amounts of refrigerant. The heat melts a conductive material, interrupting power to the compressor. This type of device does not reset and will prevent compressor operation until it is replaced.