Learn how to replace your Volvo air pump.
If your 850 volvo air pump has failed, there is an alternative to buying an expensive new or used Volvo air pump: you can buy a rebuilt or used VW air pump and convert it for use in your Volvo.
Learn how to replace it with a much less expensive VW part and save yourself from looking at a Check Engine light constantly: failed air pumps trigger the CE light.
The VW pump will work right and you can save about $100-250! I bought my pump on eBay and it cost me only $36.85, including the postage. Plus, it had been rebuilt. The official Volvo part (1270558) costs around $295 new and around $150 used. Working used Volvo pumps are somewhat difficult to find probably because they fail in service due to corrosion.
Note: before replacing the air pump, I removed the old SAS valve (Volvo part 9125623) and replaced it with a new one. I have read that the underlying cause of a failed pump is failure of the SAS valve to close, allowing water from the exhaust stream to condense in the pump, causing corrosion and eventually motor seizure.
There are likely other sources besides VW air pumps, but the one that I used is VW part 021 959 253C. The VW parts 021 959 253B and 037 959 253A are similar and would probably work. The 021 959 253C that I bought was made by Pierburg, just like the original Volvo air pump. I think the 021 959 253B and 037 959 253A are made by Pierburg also.
Whichever pump you buy, it is a good idea to test it electrically before beginning any conversion work. I connected my rebuilt pump up to the car battery and verified that it worked. If you do this, follow standard safety practice: make sure the pump is firmly held (because it will kick when started up and could fall) and make the second connection to chassis ground and not the battery to avoid creating a spark at the negative battery terminal, which could cause hydrogen gas at the battery to ignite!