nitrogen pressure test on my A/C? - PeachParts Mercedes-Benz Forum

whyameye · #1 05-23-2012, 07:34 PM

I'm going to fix my A/C (new compressor and friends) this weekend and convert this system back from R134 to R12. I have the standard R12 set of 2 gauges (high and low side) with hoses to evacuate and pressurize the system. Given the hazardous nature of R12 and the cost, I'd like to do a nitrogen pressure test to confirm there are no leaks before I put any R12 in. I am stuck imagining how to pull this off though:

1. I can rent a nitrogen tank with nitrogen from a local welding shop but the rental doesn't include a regulator. Nitrogen regulators are expensive and it's hard for me to justify that expense for my one-time use. Can I skip the regulator and instead ever-so-gently crack open the nitrogen tank valve then use my gauges to know when to close it?

2. I have a friend with a CO2 regulator. I'm also finding that home-brew beer stores are selling CO2 Regulator to Nitrogen Tank Adapters. Will his regulator with one of these adapters work for my purposes?

2. With or without a regulator (CO2 or nitrogen), I'm not sure how to hook up the tank to the R12 hose that goes to my gauges. What sort of adapters do I need and how do I find them?

This is an 85 300D. Thanks in advance for any help/advice.

vstech · #2 05-23-2012, 07:56 PM

co2 equalizes at around 1500 psi, nitrogen is generally shipped in 2000psi tanks, if the adapter is EXTREMELY well made, it could work, but I'd not chance it. most metals explode at 600 psi, so... it's quite a large margin of error there.

you could simply use the co2. it's fine as a dry gas to check for leaks. I use it all the time.
heck, Lowes sells a small can, and adapters to hook a co2 line up for nail gun use... that'd work.
most have a 1/4' pipe fitting, so all you'd need is a 1/4" pipe to 1/4" male flare, and just hook your charging line from the manifold to it. presto! done.
pressurize to 200psi, and start looking for bubbles.

whyameye · #3 05-23-2012, 09:30 PM

So basically any CO2 canister would work? There's no moisture in a Lowe's can of CO2? If so, that sounds like the way to go. I figured moisture was the reason not to use compressed air. I also figured that moisture can be minimized but perhaps not completely eliminated with a vacuum. Are all these assumptions correct? Thanks!

BoiseBenz · #4 05-23-2012, 09:59 PM

Yes, cracking the valve will work; I test at 400 PSI, but YMMV. Don't you have a friend with a welder? DO NOT USE OXYGEN OR THE CAR WILL EXPLODE. Argon for TIG, argon/CO2 for MIG. Those regulators are typically 1/4" NPT, while your AC gauge set is probably 1/8" NPT. Piece together an adapter by using the welding regulator's tank outlet, and the AC gauge set's hose adapter.

cfh · #5 05-24-2012, 01:08 AM

I bought a used nitrogen tank on Craigslist; did not cost much at all; and then I got a regulator on flea-bay. The regulator has 2 or 3 compression-fitting male 1/4" connectors and I use one of those for the middle hose on my R12 gauges (which I also bought used on flea-bay). The threads seem to be hard to tighten and there is probably some kind of mismatch going on but the system does seal up -- with the application of a little torque. You can get a nitrogen tank re-charged for not much $$$.

I also bought a used Inficon Tek-Mate "sniffer" which was kind of pricey but well worth the money; it is super-sensitive and will find your leaks (there will be leaks) much easier than any other method. Looking for bubbles was absolutely useless, for me. I also bought a used R12 recovery machine locally on Craig's List - a huge contraption - I have not yet tried to recover freon but it has a nice vacuum pump on it and the price was not much more than a stand-alone vacuum pump and I like the idea of having the capability of doing a recovery. It takes up way too much space in my garage, though. If you don't want to find such a machine you will at least need to get (or borrow) a vacuum pump, to evacuate the system AFTER you have pressure-tested it for leaks with nitrogen or other inert gas.

I went through this whole drill last summer because I had two cars -- a coupe and a 240d -- that needed a/c -- one is my daily driver and one I was selling. Got them both working great. I spent some bucks on all this crap but it was still much less than farming out the work on even one of my cars, much less both of them. I really like having the nitrogen for leak-testing; it's nice to know that a system is sealed up properly before I actually charge it with exxxpensive and ozone- busting freon.

I'm no expert but I read up quite a bit on this and I'm convinced it's best to stay with real R12 rather than switching to anything else. Be sure to get a new dryer to put in any time you open the system (i.e., after you have chased down all the leaks, but before you evacuate the system for re-charging). On my coupe I even pulled the a/c cabinet to clean the evaporator (it needed it, as any 30+ year-old car would); I'd recommend that too. D. Morrison, one of my neighbors out here in the DFW area, who I hope to meet some day, has an excellent write-up on how to do this. Also get a new expansion valve as long as you're going to all this trouble. It can be a ***** to loosen the fittings on the expansion valve; the trick is to use striking blows rather than torque -- it's really easy to twist those copper pipes and then you're done for.

I used Nylog (a/k/a "snot") on all the o-rings; Google Nylog and get some; they make different versions for R12 and 134. Flush the system, too; and BE SURE to use the right oil for the R4 compressor - PAG 150 -- I used a lighter weight oil and now my compressor is making noise so I am probably going to have an opportunity to test my recovery unit soon.

It's nice to have real a/c, functioning as it was meant to, especially after you've gone without it. Learning to do the job right, and having the right stuff to do it with, can be very satisfying, at least for us odd-balls who are into DYI on this ancient German engineering that still seems so obviously better than anything offered nowadays (e.g., the types who are over-represented on this forum!! )

whyameye · #6 05-24-2012, 08:40 AM

cfh, it's interesting to read your experience and advice and I appreciate you taking the time to share it.

I just lost a bid on an R12 reclaimer on Purple Wave and was almost relieved because of the space it would have taken up in my garage, but I do see the value in a unit like that. I already looked at Nylog but became unconvinced it really did anything. What convinced you that stuff works?

For a vacuum pump I bought that $20ish venturi vacuum pump from Harbor Freight. Hopefully that will do the job. I'm borrowing a nice big compressor from a friend to power the HF pump and flush/dry the system before I put in the vacuum.

I'm also convinced about using real R12. I was going to skip cleaning the evaporator just because it looked like a big job to me. New expansion valve and dryer are already purchased for the job.

I'm concerned about what you say regarding the difficulty of removing the expansion valve. Did you use a rubber mallet?

It's already in my plans to flush the system. In terms of oil, I was planning to use the mineral oil that Four Seasons sells. I had the impression that PAG was harsher on these old systems?

In terms of torquing up the various connections on the system, do I basically make everything as tight as possible? Also I'm under the impression it's best to soak the O rings in the mineral oil (for an hour? two hours?) before using?

Graplr · #7 05-24-2012, 12:29 PM

I've been told those venturi vacuum pumps won't pull deep enough vacuum unless you have a very powerful air compressor. You should bring it back to HF and get one of their actual vacuum pumps for about $50 more. Or you may be able to find a parts store that will rent one to you. I was told awhile back some Autozones have them (I never found one so I just bought my own).

Also, I've been told by techs that work on AC everyday that instead of using actual Nylog, they just put the ester oil (or what ever type you are using) on the orings and connections.

I've had both R12 and R134a in my old 84SD. I also had a parallel flow condenser so that may have been a big factor but there was no difference for my between the 2. Also, keep in mind I live in MN, not a place where it is regularly 100F, so that may play a factor as well. My 79 300TD blows nice and cool with R134a and a york compressor. I do have some R12 sitting on the shelf if I ever need to fix the AC system but I'm not going to mess with it as it is working perfectly for my needs right now. My son just told me (from the back seat yesterday) that he was too cold with the AC on.

funola · #8 05-24-2012, 12:35 PM

Quote:

Originally Posted by vstech

.................................................................... most metals explode at 600 psi,

I've not heard this one before. Please clarify/ explain.

whyameye · #9 05-24-2012, 06:22 PM

@Grapir good call on Autozone. I had read before that they rented pumps but didn't believe it. This time I called them and they are renting me one for free (minus the deposit).

In terms of pressurizing with nitrogen, I jury-rigged a system which uses CO2 cartridges that are intended to inflate bicycle tires. I already have about a dozen of these cartridges. Whether or not this solution will work is dependent on the volume inside the AC system. If it is a larger volume than a full-size bicycle tire but a smaller volume than about 6-8 full-size bicycle tires then this might work. I don't have any sense of the volume inside the AC though.

@vstech I looked into CO2 canisters but by the time I buy the canister and regulator I'm pretty close to my $100 price tag again so I might as well go with buying a nitrogen regulator.

vstech · #10 05-24-2012, 08:59 PM

Quote:

Originally Posted by funola

I've not heard this one before. Please clarify/ explain.

most fittings are rated for a certain pressure... 600 is much higher than most standard fittings, "rigging" is not a good idea. most standard fittings are rated at 150psi... 2000is WAAAAAAY over that, and it's dangerous to attempt a rig job.

smiffy6four · #11 05-24-2012, 09:01 PM

Grab an old fridge motor and make your own vacuum pump. Free at your local dump

cfh · #12 05-24-2012, 10:37 PM

re: avoiding twisting the copper pipes when removing the expansion valve -- no I did not use a rubber mallet; I used a nice big hammer with sharp, striking blows; it is not the force applied but rather the shock of the blows that loosens the joint rather than twisting the pipe.

Also I need to make a correction on the oil viscosity -- as I mentioned I made a mistake and have to re-do my system with different oil -- my compressor is screeching and howling horribly. In the last couple of days I have been looking into this further and learned that the original r12 systems used a 525 viscosity ester oil. I just ordered some 500 viscosity that was made for R12; gonna use that stuff this time.

I concur with the idea of adapting an old refrigerator compressor as a vacuum pump; I haven't done that but I've heard it works and the cost is minimal. Get a proper set of gauges too.

vstech · #13 05-25-2012, 08:11 AM

most pawn shops have used vacuum pumps for around 50.00 the main VP I use is over 20 years old, and still pulls down to 150 microns on new a/c installs... (most cars never see 1500microns... so... yeah, good to go)

funola · #14 05-25-2012, 11:16 AM

Quote:

Originally Posted by vstech

most fittings are rated for a certain pressure... 600 is much higher than most standard fittings, "rigging" is not a good idea. most standard fittings are rated at 150psi... 2000is WAAAAAAY over that, and it's dangerous to attempt a rig job.

OK, so you are talking about pressure rating of fittings! What confused me was "most metals explode at 600 psi" lol. Yes, it is important to use the correct rated fitting for the job. 600 psi is rather low for "most fittings". I'd think they'd be higher than that.

vstech · #15 05-25-2012, 01:09 PM

Quote:

Originally Posted by funola

OK, so you are talking about pressure rating of fittings! What confused me was "most metals explode at 600 psi" lol. Yes, it is important to use the correct rated fitting for the job. 600 psi is rather low for "most fittings". I'd think they'd be higher than that.

you'd be surprised how low "most fittings" are...

"most fittings" are rated for water line or gas line use... 150 max usually...

HIGH PRESSURE fittings are usually only rated for 600, and HYDRAULIC fittings come rated for thousands, but sometimes it 1000, sometimes it's 4000... ya gots to check, cause shards of cast brass passing through your body tend to hurt ya!