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Re: A/C Compressor Upgrade

Posted: Mon Oct 08, 2018 7:57 am
by 528i-1981
My AC system has had the refrigerant recovered, and I removed the compressor (Bosch 0140 903 506) which seems okay. After a flush (assuming no surprises) and some new hoses, I'm planning on using ester oil to maintain R12/R134a refrigerant compatibility. Any reason not to?

The compressor has a label instructing a 350 ml (approx. 12 oz) charge for the compressor. I assume this is the amount for the entire (empty, evacuated and flushed) circuit? It seems like a lot of oil.

Lastly, the online Tech Manual says to use a charging can to get oil into the crankcase. Any reason I can't just pour the oil in from a bottle? I'm assuming that would be fine.

Thanks gents, and happy Columbus Day.

Re: A/C Compressor Upgrade

Posted: Sat Oct 13, 2018 5:46 pm
by 528i-1981
For posterity, I'm posting research results here. For the Bosch swashplate (or "axial") compressor, the fill capacity of 350 ml appears to be correct for the entire system, flushed. If only the compressor is removed and drained for whatever reason, the reservoir drain bolts on the compressor can be opened, and the crankcase filled by pouring in oil and rotating the compressor until the reservoir is full (this is from the BMW service manual). I have read guidance to not leave a large volume of oil in the crankcase of the compressor, as damage can occur when starting.

Ester oil does not appear to offer the same performance as mineral oil. It is compatible with both 134a and R12 but is a compromise intended for retrofitting. Besides offering worse wear protection, ester oils can apparently degrade into their hygroscopic chemical components which include an acid. Sounds bad. While there are some ester oils with certain phosphate additives that achieve good performance, it is unclear what the effect of those additives could be in an old system and the longevity of these oils.

Lastly, oil can be poured into an open system, it needn't be injected from a can or gun. It can be poured into the drier (shortly before sealing the system), hoses, condenser and evaporator to help distribute the oil charge, and of course oil can be drawn into the system under vacuum.