Here is part 2 of the discussion with Trevor Matthews about the causes and prevention of air conditioning and refrigeration compressor failure. Slugging occurs when the compressor attempts to compress oil or liquid refrigerant. A telltale sign of slugging is valve plate damage. On a semi-hermetic compressor, you can remove the screws on the head to access the valve plate. Wrist pin wear occurs during slugging the wrist pin is between the rod and the piston; you should test the wrist pin to see if it makes a knocking sound before you dismiss all other possibilities and replace the valve plate. Overheating occurs when there is a system-related issue. Compression ratio is an indicator of overheating, but few technicians check it often enough. A requirement for external cooling and dirty condenser coils can cause overheating. Overheating also causes oil issues; when a compressor gets too hot, oil breaks down and loses its ability to lubricate the bearings. Oil loss is a tricky cause for failure; it is hard to notice without a sight glass. Short-cycling can lead to oil loss over time, and bearings will begin to wear when there isn't enough oil to lubricate them. Contamination usually occurs when moisture corrodes the copper plating and introduces acid to the system. Acid and sludge are the most common contaminants. Proper reaming practices also keep copper out of the system and reduce the risk of acid contamination. Trevor also discusses: Slugging in scroll compressors Sight glasses and oil measurement System load and suction pressure Maintaining design compression ratio "Blow by" Oil separators Replacing line dryers Components to troubleshoot and inspect Verifying System Operation Sheet from Emerson http://hvacrschool.com/Emerson Verify Diagnosing Compressor Failures from Emerson http://hvacrschool.com/CompFailures