THE COMPRESSOR – WHAT GOES WRONG?

 Like a combustion engine, the compressor needs lubrication. Instead of possessing a sump, the compressor is oiled and cooled by a special PAG (Polyalkylene Glycol) lubricant that is carried by the refrigerant. This is why running the compressor with low gas pressures can starve it of oil. On compressors without separate clutches, certain internal parts rotate with the engine running; having no gas in the system means that no oil can lubricate those parts.

Should you discover that all of the refrigerant has escaped, you can't presume that the system is devoid of oil. The quantity left depends on the leak’s location and there's no means of checking how much oil remains. While the consequences of not adding sufficient lubricating oil are obvious, over-filling the system can cause an internal hydraulic ‘lock’, which not only overloads the compressor but risks overheating its clutch through excessive slippage.

Should you be unclear about how much oil remains after you've replaced a damaged component, request that a professional air-conditioning technician flushes the system with refrigerant to remove the remaining oil, before recharging the system and adding the recommended quantity of fresh lubricant. These are further justifications of why DIY re-gassing is unviable.

As well as the wrong amounts, incorrect-specification lubricant can cause severe harm. Certain garages use universal PAO (Poly Aphaolefins Oil), instead of the more specific synthetic PAG types, which tend to feature different viscosities. While PAO oils tend not to absorb moisture, unlike PAG types, the two oils don't mix thoroughly; they separate, which can result in poor compressor lubrication. It can also lead to paraffin forming, which can block the flow of both the refrigerant and the oil, leading to seizure of the compressor. DENSO told CM that it rejects a quarter of warranty claims, because it has found that the incorrect oil was added. Other reasons include inadequate flushing and using non-recommended chemicals, such as low-priced leak detection dyes and flushing chemicals, plus the presence of counterfeit refrigerants.

When a compressor fails internally, metal fragments can be carried by the refrigerant and damage the entire airconditioning system, causing further damage. So, replacing the compressor alone could result in the replacement part failing soon afterwards. This explains why most new compressor suppliers will not honour their guarantees, unless you prove that other parts were replaced at the same time. These include the receiver-dryer and the orifice tube. You must also prove that the whole circuit was flushed to remove old oil and lingering fragments. Therefore, discuss these options with your garage and consider if the compressor failure was the result of an underlying problem, or not.