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This is copied as is from an e-mail correspondance courtesy fellow Saaber, Steve Sass (with his permission)- I asked him about the "venturi pump" to start...
Brad,
I bought the $100 dollar pump for two reasons. First, I don't have a compressor and that venturi vac really works a compressor (4.2 cfm at 90 psi). So I got tired of borrowing peoples compressors. Second my pump pulls a very deep vacuum so I can do A/C work on colder, wet days. So it gives me piece of mind that all the water is going to boil off even if it isn't 80 and dry outside. First cash is a great place to get professional quality tools at the DIY price.
On the conversion procedure. I had a place take my R12 for 15 bucks. You may be able to sell it and get it done for free. Haggle. You sound good and cheap to me. Don't worry about the compressor running, the pressure switch will shut it off when there is a low charge.
Here is my recommendation on parts and procedure. Buy a new r134a expansion valve and receiver drier from sobstory.com. I get all my parts there and I have checked just about every place on the net for the best price and 80-90% of the time they have it. They will even buy dealer items for you with their 10% discount if you provide the part number. I bought a flush gun from acsource.com and used paint thinner to flush. This flush gun is handy and I have also converted to a fuel injection cleaner. At this point you should have the receiver drier, expansion valve and compressor removed from the car. Back and forward flush the system with the paint thinner. Use about 1.5 gallons or until the thinner comes out clean. Do not flush compressor, expansion valve or receiver drier. You may want to have some extra hoses laying around so you can capture the paint thinner easily. Run compressed air through the system until no mist comes out. I have heard that some people use Ditzler's Laquer Thinner for refinancing their leather because of the short flash time, or quick evaporation. That would be great, but I just grabbed the cheapest thinner at wal-mart.
Now replace all the o-rings in the system. And per you idea I also recommend nylog lubricant.
Next drain the oil out of the compressor oil tank. Rotate the shaft to get all the oil out. Oil should also run out the connections. I put about one oz of ester in the connections and spun the compressor to lube it up, keep spinning to let it run out. Replace the o-ring on the oil plug and put 2 oz of oil in the compressor. Tighten plug.
Next install the compressor and expansion valve. Be sure that the temp probe on the expansion valve is secured tightly to the neck of the exit from the evaporator. The black insulating tape should be reapplied so that it is well insulated. If this got wrecked, you can buy some at a refrigerant shop. This is very important.
Next pull a vacuum of at least 28.5 in/Hg, I would say 29 in/Hg minimum. Pull the vacuum through both service ports down to the desired level and continue running the pump for about 20-25 min. Then close your gauges and shut off the pump. The system should hold a vacuum for hours like this. If not you have a leak. Let the system sit like this for a hour or so. This will allow the water to boil under the vacuum. Then turn the pump back on and open the gauges and evacuate for around 20-30 minutes. Your system should be dry as the desert.
While the system is under vacuum inject 7.5 oz of oil into the system. I turned the ac on and after a little bit the pressure switch allow the ac to run. Then add you sealer, booster and r134a to the low side only!!!! I filled it to 31.5 oz. per Tom Townsend's recommendations but after running on a 95 F day the pressures where way to high and the cooling performance dipped. So I removed a little refrigerant until the pressures where 32 psi low side and 225 psi high side (low side comes from the evaporator and high side runs to condenser). So I would recommend charging to these pressures. Test the pressures with the engine hot, radiator fans cycling, windows down, all but center vents closed and the temp gauge in the center vent. The temp should be in the 50's out of the vent and the pressures should be around 30/225.
Also I would recommend replacing the pollen filter or cabin filter.
Here is what I used as far as A/C tools:
gauges (these are a must)
thermometer
vacuum pump
flush gun
air compressor (I flush with one of those cigarette lighter compressors and ran air through
the system at the local tire filling station)
Buy that stuff at ACsource.com, first cash or borrow.
Parts and Supplies:
Cabin filter ($22 bucks at sobstory.com)
receiver drier ($35)
expansion valve ($18)
compressor ($80 from a junk yard, found it searching through car-parts.com, mine was seized, I just bought an 89 9000CD for
$1000)
O-rings ($7 got these at o-reilly auto parts)
2 Gallons of paint thinner ($4 at Wal-Mart)
3x2 oz cans of ester oil charge (that's 2 oz of oil and 2 oz of r134a per can, also at Wal-Mart about $3 per can)
2x12 oz cans of r134a (about $10 at Wal-Mart)
1x3 oz can of maxi cool (about $3 at Wal-Mart, contains about equal parts of oil and refrigerant, also has a little performance booster added in, I don't know if it works great, but I since I originally charged to weight spec I needed that extra 1.5 oz of r134a and the 1.5 oz of ester oil)
Nylog (couple bucks at AC-Source.com)
Well those are my recommendations.
If you have any questions don't hesitate to ask.
Steve Sass
ssass@iastate.edu
PS
The o-rings are at every joint except the two fittings that run to the
compressor. Replace every one. They will be dried, and flat. They will also
deteriorate faster once you convert to r134a do to residuals left on the
orings. They are all pretty easy except the upper connection to the condenser.
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