81 300sd a/c problem

i vac'd the empty system to -30, put in 2.5 cans freeze 12 with engine running and pump hotwired. compressor wont run w/o hotwire. tried jumpering pressure switch first. switched 3 different dash
button/wheel control assemblys, still wont run w/o hot wire. at idle, i had 38 on low side, 170 on high, 80 degree outside air, car always runs at 80 centigrade. aux fan works automatically. at idle cabin temp cxame down to 60 degrees. at 1200 rpm low 30/high 190 then temp came down to 50 degrees. still try plugging back in pressure switch wire and no compressor. at 2800 rpm 60low/90 high temp in cabin still 50 degrees. after turning eng off low side 80/high100. i can see flow like crazy in the sight glass, although can't say it looks like bubbles. i bought this car as is so don't have a history. if i got the compressor to run on its own be a big start... thanks for any help.. -gary
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Sounds like you don't have enough R-12 in the system if you can still see bubbles in the sight glass. A full system normally gives no indication of bubbles, thus you can't see anything moving in the sight glass. However, 38 psi on low side should certainly be enough to at least operate the system. I like to hold the can while viewing the sight glass and stop adding coolant when the bubbles stop. This may well get you more than 38 psi on the low side but that's usually okay anyway.
You said you hot wire jumped the pressure switch. Is this actually what you did rather than put a jumper between the two connections on the pressure switch plug? This is a two-prong switch connector which you should hot-wire first rather than the compressor anyway. Hot wiring the pressure switch itself does nothing.
I'm guessing the problem is the pressure switch if the compressor runs when you jumper the switch plug. Problem is you normally have to take the switch out to check or replace it. There is a way to check the circuit if you have another switch laying around and an air compressor. Take the uninstalled switch, plug the connector onto the two terminals, start your car and turn the AC unit on. Take an air nozzle with a rubber tip and press it tightly against the air orifice on the bottom of the switch where it screws into the drier. If the compressor comes on when you apply air pressure your existing switch is bad. If it doesn't, and you are sure you're getting pressure to the switch, then the problem is somewhere else in the system.
If that is the case your dash control may well be the source although you say the fan works properly.
How much R-12 did you say you had? Changing the pressure switch means draining the entire system and starting over.
To all you environmentalist kooks, please don't tire me with those claims about that alleged hole in the ozone caused by cfc, etc. That's a proven cyclic event that takes place every 50 years or so.
Good luck Gary.
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thanks for your reply, i'll try your t/s suggestions. what i meant by jumpering the switch was like pulling the leads a bit but not disconnecting them. then then take like a dime or something and touch both posts. same as jumping the leads right? still no compressor run. only when a hot lead from batt to dual wire lead makes comp run. here's my question. is it basically the dash controls and the pressure switch that control the compressor? are there other things in the circut i can check. i appreciate your help, want to travel to south dakota to see my brother early july.... -gary Ernesto wrote:

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btw, that is a great idea to check the pressure switch circut before filling with freon... cann save some bucks if pressure switch is bad... -gary zdilly/gary wrote:

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