Anybody know A/C REAL well?

[BobcatSteve]

I'm trying to fix an Astrovan for my cousin with dual evaporators. He bought a rebuilt compressor on a Sunday, and by the way, he was unemployed for 6 mos, so he's pretty low on cash. The system apparently had a full charge, because he started taking hoses off before I got there, and blew an O-ring, had a van full of mist when I got there. I put the new compressor in, added 10 oz of PAG (compressor dry), vacuumed for over 2 hours. Tag said 3 lbs. I had a constant 25 on the low side as soon as it started coming up, but before I got to 2lbs high side kept spiking close to 300. I stopped at about 2.5 and checked things out. Had about 41 degrees at vent, decided it might be OK. Revved engine under hood, and high side went about 350, decided that was too high. Turned it off and recovered 6 oz, started it again and low side immediately went into a vacuum, high to 400. Lasted a few seconds and compressor made a growling sound. Jerked the plug out of it, shut it down.
Things I shoulda done-
Didn't replace drier-forgot to tell him to get one. No frost on it, but the most likely culprit in my mind.
Bad pressure switch-had to jumper, the reason there's numbers like 400 above.
My other question-what the heck's the little switch that plugs into the back of the compressor?
Thanks,
Steve.

[BOOZER]

Any time a compressor fails the system should be flushed, most good a/c suppliers wont warranty a replacement comp without buying a drier. The main
problem I think you have is the orfice tube, its what
gives you a high side & a low side. The orfice also
has a filter that is probably loaded with the failed comp metal, you always pull it at least to just check
it or replace as maintance.Its located in the liquid line at the condencer or at the fr evap.

[Chipper2]

Out of curiosity what caused the compressor failure? My '98 with rear air lost refrigerant and we found out that the manifold had rubbed against the block. I had to have entire manifold replaced. My compressor is OK but there was some shavings circulated through the system. Not sure how long it will last.

[RABarrett]

Something to remember here is that the compressor does two things. It COMPRESSES or pushes freon toward the condensor and ultimately toward the orifice tube. The purpose is to create a pressure drop, which creates the vapor used to absorb heat from the interior. As the heat warms the freon, it evaporates causing a tremendous heat absorption. This vapor then is PULLED toward the compressor, where it then pressurizes and the process begins again. The problem is that there are two places where the freon changes state; the condensor and the evaporator. When the compressor let go, it dumped its fragments into three places, the accumulator and the orifice tube. The resulting restriction caused the comprerssed freon to build up against the orifice causing high pressures on the high side, and on the low side a restriction, causing a diminished supply at the low side of the compressor. The resulting drop in volume caused the vacuum on the low side and the high high side pressures. You will need to replace the orifice tube, probably the expansion valve on the rear evaporator, the accumulator, and then try again. You were lucky that the compressor did not ingest the shrapnel from the first compressor's failure. P.S. The compressor failure was due to age; it is typical of these units. Ray