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Old 04-02-2003, 08:55 PM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
I'm perfectly satisfied with my 450SL that was converted to 134a from R12 6 years ago. Since that car uses a compressor with it's own crankcase oil converting oil only required pouring out the compressor and filling back up with 134 compatible oil (no flush needed). I've only needed to add about a can of 134a so far.

I repeat myself by saying that I live in a dry and rarely over 90F climate; This is not Texas. I know this would not be acceptable by many of us americans who crave what I would consider extreme cooling at home, work and in the car. This is none of my business but the point is I'm staying with 134a and I have no interest in changing the condensor.

This whole flushing thing has me concerned and a bit confused. I guess I will belly up and order the FJC flush kit for $36.05 and a quart of FJC flush solvent for $13.57. I expect to make a mess both inside and outside of the car. The expansion valve will be removed so even with buckets and rags I'll probably have a mess. I will have 8 hoses disconnected (2 @ compressor, 2 @ R/D and 4 @ at the exp. valve). Should I blow flush into each of the 8 hoses or should I just focus on flushing the condensor and evaporator. Will follow up by blowing lots of air through the system.

As for evacuation I was happy to find the Robinair venturi style one for $39.98. It's the one the previous poster (Rick) suggested I look at. BTW, all prices are from The Tool Warehouse. The spec says 29.7" Hg of vacuum. If standard atmospheric pressure is 29.92 I would have an absolute pressure of .22"Hg which converted in millionth of a meter is 5,600microns of Hg. (Did I calculate that right?). I guess most of you are telling me that that is not enough. Funny enough both places you mentioned, e.g. sells this style vacuum pump and believes in them.

So my final sticking point is the pump. I will not buy a $200 one so I'm left with building my own (I wonder how many microns the old fridge pump is capable of) or using the Robinair venturi type. After all if I flush first and remove most the oil I would hope that I wouldn't be so loaded up with moisture in the old oil.

Thanks for all your kind words of encouragement and advise.
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Old 04-02-2003, 09:42 PM
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Join Date: Feb 2002
Location: Florida
Posts: 52
I used an old pump from a chest style freezer. The gauge read 32hg while applying a vacuum. Used it many times with no failures.
Unless you just absoutlely have to have the a/c done right now, take your time and get the tools you will need to do it right. Once you got them you will be suprised at how many times you will use them! And at 45-85$ an hour labor they will pay for themselves in the long run.

just my .02$

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Old 04-02-2003, 09:45 PM
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Join Date: Apr 2000
Posts: 459
I hope no one is stressing you out! I think we are all just trying to be as helpful and thorough as possible. That aside, if you want to flush, flush the lines and condensor as well as possible. Flushing the evaporator can make a real mess, and I chose to just blow it out from one pipe while I held a rag on the other pipe. I stopped blowing when the rag no longer showed oil residue. Its small, so you only have trace amounts of oil.

Your other otion besides buying a vacuum pumps is simply to pay a shop to evacuate and charge the system once you have it all back together. I'm being objective: yes everyone sells them, but I have never heard anyone say they work well. And again, it is extremely important to get all the moisture out of the system, and that little thing just won't cut it.

Lastly, if you do pull vacuum yourself, do it on a hot day in direct sunlight, and drive the car first to get the engine compartment good and hot.

I hope it all goes well.

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Old 04-03-2003, 07:31 AM
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You're getting lots of good advice here. It sounds like Moab operates as I do. I will buy tools and equipment to do a job when I could pay to have it done for less money. After I'm done I then have the equipment for the next time.

There will be MUCH oil in the evaporator, it really should be flushed. Since the lines are right next to each other after removing the expansion valve, the rag suggestion will work pretty good. I've done this several times before on 123 cars and have never made an objectionable mess.

If you choose to not follow our advice here and use the venturi pump anyway, make SURE you have enough compressor to keep up with it. It takes LOTS of compressor to keep up with it for the length of time that you really need to evacuate. As mentioned before, do this in the warmest part of the day with components as warm as possible. Don't try it in 60 degree or colder weather.

Finally the a/c systems in the 123 cars are some of the worst candidates for 134 conversion. Your SL is a different animal all together and has a much better chance of successful 134 conversion. The capacity of the 123 a/c is adequate with 12, but with the 20% loss of capacity when going to 134 it will be marginal. The only car I know of that is a worse candidate for conversion than yours is the 124 MB, it is virtually hopeless for conversion.

I can tell you are dead set on 134 and that's okay. We are only trying to share our experience(s) with you and help in any way we can.

Best of luck with the project.
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Old 04-04-2003, 10:32 PM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
I very much appreciate all of your help and patience with my AC questions. I have worked on just about every part of a car and have avoided bringing it to a shop except for tires, alignment and AC. With all of your help I feel ready to do the AC, but I'm still learning and acquiring the tools needed. I am still looking for a fridge compressor but wonder how it will hold up for 2+ hours (or overnight) if there is no refrigerant circulating through it as it is designed to do in a fridge. Doesn't the compressor need lubrication/cooling? I have not dug into a fridge yet but I would guess I will also need a/the motor to drive the compressor. Or is the compressor and motor integrated into one on more modern units?

I don't mean to beat a dead horse to death but what is the real problem with the Robinair venturi pump? Would it be one of these reasons:
1. Uses too much shop air (spec says 4.2CFM at 75psi)? My 3hp 30gal compressor is rated at 9.6cfm at 90psi. I know it's not good to run so hard but for me it'd be once in a while. Hopefully much less than once every few years.
2. Not a deep enough vacuum? The Robinair claims 29.7" Hg vacuum at sea level which is deeper than other brand venturi pumps i've looked at. LarryBible said in a post from May 2002 titled "Ac Compressor Into Vacuum Pump ?" about his now gone home made vac pump, "It pulled a very good vacuum, about 27 or 28 inches." and that was enough? Moab_dan in this post string said his freezer vac pump goes to 32Hg gauge. This would seem unlikely (impossible?) to me if 29.92" is 0 absolute at sea level (ignoring barometric fluctuations which are less than +/- .6"Hg I believe). I doubt he's a couple of miles underground either. It's impossible to go deeper than zero!
3. Your gut feelings are that venturi pumps are no good and you can't trust their spec. But an electric pump from a working fridge or freezer is good/better?

So how deep is deep enough. Can a fridge pump really go down in the double digit micron region. Somehow I really really doubt it.

Larry and Dan, please don't think i'm picking on you by quoting you. I value and learn from your advise as well as from most of the other posters. Thanks for helping me out!

Last edited by erubin; 04-04-2003 at 10:58 PM.
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Old 04-05-2003, 12:00 AM
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Join Date: Sep 2000
Location: Shalimar, FL
Posts: 79
I bought a venturi pump off ebay for $20, my 5hp compressor set at 80psi keeps up with it no problem, run it for an hour, gauges reading 28". One thing I would add is if your system uses an R4 compressor, NEVER buy a rebuilt, my first one lasted 15 months, second one lasted 18 months, then started making horrible knocking noises. next one will be new.

Bob H.
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Old 04-05-2003, 03:42 AM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
lucky for me Bob. i got a new compressor. It "only" cost $200. It's a Four Seasons. Hopefully it's a good choice. Will let you know although I really hope you don't have to change yours again!
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Old 04-05-2003, 05:29 AM
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Join Date: Jan 2003
Location: Littlestown PA ( 6 miles south of Gettysburg)
Posts: 2,209
R4 compressor

I have a local "U Pull" auto Junk yard that will let me pull any A/C compressor for $10. What should I look for to have a good chance of getting a good one? Do they throw a line of oil on the underside of the hood like the old A6 did? Is the R4 still a current GM product? Are all R4 compressors interchangeable?
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Old 04-05-2003, 10:43 AM
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Don't worry, I know you're not "picking on us." In fact you're bringing to light some good discussion.

If the venturi pump will indeed pull 27.8" then it will indeed work. I think the reason I wouldn't like using one is the fact that I like to evacuate overnight when I can. I can't imagine running my compressor all night long.

You quoted something I said in 2002 about an a/c compressor converted to a vacuum pump. I must have misstated. It was an ol refrigerator vacuum pump. It came out of a refrigerator that my Dad bought new in about 1954. The reason it works without circulation is that there is oil in the bottom of it. It would probably still be working except I stored it open in my shop which gets very humid. It finally gave up.

Yes 32" is theoretically impossible. Most gauges are marginally accurate on the vacuum side.

Good luck with your project,
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Old 04-05-2003, 03:23 PM
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Location: Tampa, FL
Posts: 3,079
I've typed three replies to this thread over the last two days and thrown them all away in the interest of avoiding a flame war. Since Larry has softened his position somewhat, I'll jump back in.

My compressor is a 4.5HP rated at 6.2CFM at 90PSI with a 20 gallon tank. I just connected it to the pump, and the pump to an empty gauge set - no car involved. The compressor running continuously was able to keep the input pressure at about 85PSI. The gauge registered 28" of vacuum. I think there must be some disconnect between Robainair's claim of 75PSI input pressure and 29.7" max vacuum. With this result, I'm skeptical about your compressor's ability to keep up. Maybe the CFM rating is more important than the 3HP rating, but regardless it will probably run continuously.

Maybe you should look into renting a vacuum pump and then buy the venturi only if rental efforts are unsuccessful.
Rick Miley
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Former MB: 86 190E 2.3, 87 300E, 80 240D
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Old 04-05-2003, 07:34 PM
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I think my ideas are at least partially clear. Unless I had a huge 10HP, 80 gallon compressor or something like that. I believe that I would want to change the air compressor oil after such constant running of my compressor.

You can get a wet vane pump from Robinaire or somebody for about $200 and have a really good piece of equipment that will serve a DIYer for a lifetime.

My $0.02,
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Old 04-06-2003, 11:38 AM
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Join Date: Dec 2002
Location: east of Atlanta, north of Macon Ga
Posts: 185
LarryBible, you mentioned earlier about taking a test on the web to get certification to legally purchase R-12. Where is this site and more importantly where can you purchase R-12 ? I casually asked at one of the local parts houses recently and they told me it's not available for sale.
1991 300SE (my ride, 279,000 miles, looks almost new
1954 Cadillac (21 yo son's car, he bought when age 15)
1972 SeaBird 19 ft runabout (old but solid, slant six, Volvo sterndrive perfect condition, undergoing complete overhaul and refit)
1998 Toyota Rav4 (my sons daily driver when he is in the Continental US, PROUDLY serving in US Navy)
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Old 04-07-2003, 07:06 AM
Posts: n/a

To begin with I think you need to try another auto supply. You probably can't get any at the stores such as Auto Zone, you will need to go to an Auto Supply that caters to the independent garage trade. If that doesn't work, go to an automotive a/c supply.

Secondly, there are several sites you can go to for this. I would start at The price varies, if I remember correctly, I found one for $14.99, but it might have been $19.99. You need to print the study material and read it. Then keep it handy and go back and take the test with the material at hand. It is an open book test. It's actually appears to be kind of a loophole thing, but it is totally legal.

Good luck,
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