This is a great site.
I purchased a Bonneville LE 1988 for the A/C compressor since the one on my car expoded late August 2002.
I paid my mechanic to transfer the new (old) one over to my 90SE and convert the system over to R134a in May 2003 at a cost of $475.00 which included the conversion kit and vacuum but no flush. The A/C was cold but not extremely cold. I was satisfied since it was not really that warm out yet. But in June it got really humid so I went back to the a/c guy and he said that the orifice filter was blocked and that I needed a new one and that I should also change the short high pressure hose as well, needless to say i wasn't about to spend another $175 for the long high pressure hose, $75 for the short high pressure hose and $50 for the installation. So I just left . At that point I didn't have any a/c as the mechanic evacuated the r134 to remove the hose.
What I did was ask another a/c person and they told me that if the original compressor exploded i would have to flush the system, change the accumulator, change the orifice tube and evacuate and then refill with r134. My guy did not do this.
He just evacuated and vacuumed and refilled with r134 after changing the compressor.
I then went to a scrap yard and purchased a used accumulator and orifice tube.
I installed it myslef and then went to a new a/c guy. He said I didn't need a flush, a vacuum would do. he then evacuated or vacuum for 20 minutes and refilled 2.98 lbs of r134a exactly what it said on the accumulator but for r12. he said this was okay that it is never really that hot here and that the pressures would not go up that high because of that. The a/c was really cold. It'* still woking great today. However, 2 weeks after I had it done a strange thing happened. After work on a hot and humid afternoon the temp outside was 30 cel. After my errand I clicked the AUTO button and got hot air.
No a/c. When I got home and had supper, I turned on the a/c by pushing AUTO and got cool air again, I set the AUTO to 16 and checked the accumulator and high pressure line it was cold and sweating. Hooray a/c again. i've used it all summer and not one time did I not have A/C (knock on wood). does anyone know what could cause this?
Sorry for the long post.

First thought is that was too much R-134a - generally you want about 70% as much as R-12.

Second thought is that a compressor was changed out with you supplying the compressor for $475 (cdn ?) and it was neither flushed nor oriface (a $2 item) tube and dryer replaced ? That just is not right. I replace the dryer any time a system is evacuated. Of course living in Florida and Texas makes a/c a necessity.

Some of the kits claim to be able to just changeout now without flush but *any* time there has been a known compessor failure you should flush.

For some inteesting pictures of oriface tubes see:
http://www.ackits.com/index.cfm?fuse...=Black%20Death

That said if the hot-cold servo binds (rod to diverter is on top of a/c programmer behind glovebox with ATC) neither the a/c nor the heat will work properly yet is just hard enough to get to that is rarely checked. If the pressures look right and or on MAX HEat and not getting much, check the diverter.

If you push "AUTO" and nothing happens the first thing to check is the a/c clutch. If it is not engaging then the next stop for me is the a/c compressor relay followed by the pressure switches - you really need a schematic.

Hopefully the 90 Reatta/Riviera Service Manual will be on line soon (waiting approval from GM) - the HVAC section and most of the drivetrain is the same as for a Bonneville.