"Spiderman" > wrote in message >...

> I have a 1989 Chevy S10 Blazer that blows out warm A/C. It's
> low on R12 so the compressor just cycles on/off. I'm gonna top
> it off with R12. Is a gauge necessary or can I just play it
> "by air".

Try the forums at www.aircondition.com, a place dedicated to car A/C.

People who know better than me are going to hate what I'm about to
say, but if nothing else is wrong with the A/C you can get the R-12
charge about right if you monitor the inlet and outlet pipe
temperatures of the evaporator while the engine runs at fast idle
(must be fast, and run the engine at least 10 minutes before charging)
and you slowly (emphasis on "slowly") put in R-12 until the outlet is
almost as cold as the inlet. Ideally they should be equal, but it's
safer to undercharge than overcharge.

> Where can I find the normal low-side, high-side pressure ranges?

Any car repair manual that covers GM vehicles since they went to
orfice tubes?