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The problem is on newer cars (I remember this being a 94 I think?) there usually isn't a high pressure port. There are the low pressure ports at the firewall. Maybe on some older 700s there is a high side port up there too, but I really haven't noticed that either. Mostly the only ports on the 700/900 exist at or on the dryer (all low side).
Because most of the later 900 and most of teh R134 equipped Volvos don't have a high side port, you have to charge by weight into an empty system. Otherwise (unless there is a high side available) you won't ever be able to know if it is working properly. A slight overcharge can cause poor cooling, as can an undercharge.
With R12 you don't have to be nearly as careful with refrigerant charge as with R134a. The temp/pressure curve of R134 makes it extremely sensitive to proper charge. It is almost a reason to have to re-learn a lot of things when working on R134a systems, vs. R12 systems. The 12 systems are much more forgiving.
As A. Roll said, the fact that your mech dismisses the board, is hardly surprising. If you were to look from that perspective, onto most automotive technical boards, you would dismiss a lot of the information immediately. And the thing is, even on this board (which is very good) a very large amount of A/C advice here is purely and simply, wrong. A lot of it is based on "it worked for me and hasn't broken.... yet" kind of posts, which are fine as far as anecdotal advice, but they really don't relate to the practical, service-side of the equation. So most mechanics would doubt the internet as a source, because a lot of what they hear through customers is what they read, and sometimes either didn't understand, or just heard wrong to begin with. It isn't necesarily an indication of a problem.
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a Brickboard.com Expat
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