Some aftermarket Trinary switches also contain a high pressure blowout plug. Basically a lead filled orifice that is the final fail-safe in the event of severe overpressure. They can fail occasionally over time though. Take a close look at the switch and see if that's what you have. Should be a nice clean hole if it had a blowout plug. Generic trinary switches are available at most mobile A/C places. AZKits.com has them if you don't have any locally. I've used theirs before without issue.

Keith Hughes
'86 Westy GenV (Marvin)

 
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 Date:    Thu, 29 Sep 2016 20:30:43 -0500
 From:    John Rodgers <jrodgers113@GMAIL.COM>
 Subject: Re: '91 Carat AC - again!
 
 Thanks guys.  Now this is beginning to make sense. It
 really helps to
 understand to know when to quit.
 
 John
 
 On Sep 29, 2016 19:58, "Dennis Haynes" <d23haynes57@hotmail.com>
 wrote:
 
 > That trinary switch does a number of functions. It is
 both a safety limit
 > and it operates the radiator fan intermediate speed
 based on pressure on
 > the high (condenser) side of the system.
 >
 > The first function is to ensure there is some pressure
 in the system to
 > allow the compressor to turn on. Since it is on the
 high side once the
 > compressor turns on this switch will continue to be
 satisfied even if onl=
 y
 > air is in there. At around 275 PSI it should put the
 radiator fan into th=
 at
 > intermediate speed. This is done operating the relay (I
 think space 5) in
 > the fuse box to apply power through the other side of
 the fan resistor to
 > bring up the speed. At some higher pressure it should
 turn the compressor
 > off.
 >
 > The fact that you blew the back off the switch
 indicates that you have a
 > blockage in the system and that the safety circuit
 didn't do its job. You
 > probably also compromised other parts of the system. My
 bet if the system
 > was just cleaned and flushed and whatever is that there
 was too much oil
 > that couldn=E2=80=99t pass through the port in the
 Receiver dryer and the=
  system
 > went into hydraulic lock up. The compressor is probably
 toasted also. Loo=
 k
 > for bubbles in the hoses, cracks in the condenser and
 distortion or the
 > blow port on the dryer failing.
 >
 > Summer is over so you some time to recuperate. It is
 time you get a pro t=
 o
 > do this complete and pay your dues. Someone can get
 hurt here.
 >
 > Dennis