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  #16  
Old 07-16-2003, 12:02 PM
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Ive seen both ideas published about whether you need barrier hoses for R134a. You do need it for R22. The industry info says that if your hoses were used with R12. You don't have to replace them. The mineral oil seeps into the hose and creates a barrier that will not allow the R134a to leak through the hose. So you don't have to replace the hoses as long as they were not leaking.
If your compressor has had the black death

http://www.ackits.com/index.cfm?fuseaction=Black%20Death

You will have to COMPLETELY flush the AC system, replace the R/D and the expansion valve which will be blocked due to the black death. If going to all this work put in inline filters between the compressor and the condenser as well as between the R/D and the expansion valve. This will prevent any particles that you may have missed in your flushing to cause blockage or damage again.

Dave

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  #17  
Old 07-16-2003, 02:12 PM
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The a/c hose question is quite simple.

If you put NEW hoses on the car, then yes they must be Barrier(134a) hoses. IF you convert a system from R12, the mineral oil is already absorbed into the hoses forming a barrier. So, you don't have to replace them.

If you look carefully at the SAE test data supports this. I think early on then they were designing R134a systems, the need for barrier hoses became obvious and thus it was assumed to be needed for retro fits. Testing did not show this to be true.



Michael
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  #18  
Old 07-16-2003, 10:17 PM
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29" is OK, but it won't remove water from the silica gel dessicant in the dryer. If you can only get 29" vac, I'd put in a new dryer. As I said, I have the luxury of a serious vac pump (one of the uses is as a rough pump for mass spectrometers) that will pull 30" of vac on an AC system in a matter of seconds. If you have a spare $1200 or so, they are readily available....

Sadly, I no longer have access to a very senstive leak detector we had at work for use on the GC/Mass spec -- it could see about 10 uL per hour of Freon. Again, for a mere $3500, they are readily available.....

You must use one h... of a lot of air with a vernturi unit -- I'd get an old compressor out of a dead fridge that still ran and put some fittings on it. A little slow, but they work fine.

Peter
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  #19  
Old 07-16-2003, 11:50 PM
Greg Martin
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Thanks for the cogent responses re the hoses. I googled around a bit and found other references supporting the idea that old r12 hoses aren't as leaky w r134 as was originally thought. So I won't look to replacing hoses unless something else tends to indicate I should.

At this point, I'm looking at getting some additional equipment. I don't have a shop compressor of the kind of capacity I'd need for the venturi devices. I'm considering getting one, which might be useful - but they take a fair amount of space, which is an issue. Also looking at vacuum pumps such as the robinaire and ritchie. Seem to be fairly good prices on eBay. But even so, for me it would be a one use, expensive tool. That also takes space to store. Also, a uv leak testing kit.

It seems like I do have some leaks, and the compressor clutch is not engaging. The compressor may be ok - the likelyhood of two points of failure is less than one. So next step is to check the clutch coil electrically. If I can verify it's bad, replace the clutch or clutch coil; if not, I guess I have to pull the compressor.

"Free" labor, and parts at about half the cost a shop would charge me gives me the opportunity to make the mistake of replacing components unnnecissarily, so that the job is done right.

I guess this saga will continue at a later date.....
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  #20  
Old 07-17-2003, 07:55 AM
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"but it won't remove water from the silica gel dessicant in the dryer. ......................., I'd put in a new dryer. "---PSFred

That is a true statement.... and I believe MORE correct with the middle clause taken out... because from what I have read NOTHING is able to take out the moisture from the Silica Gel....this may be due to it being UNDER oil.... and is the reason you never hear books saying " if you have a good enough vacuum" you don't need to change the Reciever/Dryer....
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  #21  
Old 07-17-2003, 10:10 AM
LarryBible
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I theorize that once the water molecules are "attached" to the silica gel, the gel provides a "heat sink" that prevents the water molecules from warming up enough to boil away.

As far as buying a compressor to run a pneumatic vacuum pump, I personally feel that this is really serious false economy. You can make a really decent vac pump for $15 out of an old refrigerator compressor. You can buy a small Robinaire used for $100. Either of these will do a MUCH better job than a venturi pump.

My $0.02,
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  #22  
Old 07-17-2003, 10:19 AM
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Yes, the combination of the shape of the reciever/dryer and the oil being in it would certainly provide ' heat sink ' function. But in addition , a vacuum will not boil oil, so I think that is why the R/D needs to be replaced.. since it is a vessel for the oil with little oil surface area exposed to the effects of the vacuum.
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  #23  
Old 07-17-2003, 01:51 PM
Greg Martin
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I gather from the Mastercool book that it is possible to replace the gel in some receiver/dryers, either when it's saturated or in the event of changing type of refrigerant and oil. I haven't seen any suggestion that this be done with the MB receiver/dryer, however; just replacement of the whole thing.
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  #24  
Old 07-17-2003, 05:49 PM
LarryBible
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The only r/d that I know of in which you can EASILY replace the gel, is the GM VIR.

Others require cutting apart and rewelding.

Have a great day,
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  #25  
Old 07-17-2003, 10:35 PM
MVK MVK is offline
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This is what EPA has to say

Hi Greg:
This is what EPA has to say about the retrofit and related issue.
http://www.epa.gov/ozone/title6/609/technicians/retrguid.html#oem

MVK
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  #26  
Old 07-17-2003, 10:42 PM
MVK MVK is offline
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R-12 systems use an XH-5 desiccant, while R-134a systems use either XH-7 or XH-9 desiccant.

MVK
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  #27  
Old 07-17-2003, 10:49 PM
MVK MVK is offline
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A new R/D costs 30-40 bucks, I rather spend that much than to cut refill silica bags and reweld. and God forbid you broke the bag inside, it will be really a disaster. Last year I had a R/D for my wifes Honda Accord, when it arrived in mail, it was already spilling some white poder from there and also some welding BEADS. Can you imagine me installinfg that ??? I had to get the replacement.

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  #28  
Old 07-17-2003, 11:31 PM
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If the water goes into the silica gel, it will come out, oil or not, provided the vapor pressure is low enough. I suppose I could check it with some of the indicator stuff at work....

Heat does the same thing, so if you are working on a hot system (i.e after driving the car and getting the rad and R/D hot), the vac pump will remove the water PROVIDED you have low enough pressure. As I said, you aren't going to get anywhere near the vac I can with this pump -- it's an oil bath pump will pump down ot 100 miliTorr, don't know the exact equivalent in inches of mercury or water, but will bet is is near zero pressure. It will certainly remove water from silica gel in a dessicator, anyway (if the dessicator, made of glass, holds up). The original use is to pull the air and water off a turbopump or diffusion pump in high vac systems for mass spectormeters (where charged particles must be able to fly a half meter or so without hitting any air or water molecules). It's over kill for an AC system, but hey, why not?

I would certainly never cut open a R/D to change the dessicant!

Peter
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  #29  
Old 07-17-2003, 11:44 PM
MVK MVK is offline
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I remember back in my Pharmacy college days we had an experiment on evaluation of varius dessicant materials and then drying them.
One step involved reclaiming the dessicant by keeping it over night in hot air electric oven at certain temp( I dont remember exact value it was in 1981). Then reusing them nex day and rechecking the moisture absorbing capacity. So as the temp increases and vacuum is applied, the evaporation rate increases.

Never knew my phamaceutical sciences will help me in MB forum. LOL

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  #30  
Old 07-18-2003, 07:45 AM
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Silica Gel was used in the craft industry for things like drying flowers.... I have at least a half gallon jar of it in my garage.... it may have had some color change indicator added to it...it was blue or pink when dry and the other color after cooking in the oven for several hours....

However, while the descriptions of the nature of the beast already mentioned may be true for some set of conditions I personally do not believe one can take the necessary amount of water out of the dessicant/oil while it is immersed in the Reciever/Dryer.

My feelings come from the design of the R/D and the fact that this is a closed system when using the vacuum. The oil , as it circulates through the R/D uses gravity going in and the oil is forced through the dessicant and into through the oil bleed hole in order to continue its travel through the system.

This ' gravity drop' provides the same effect as opening a siphon tube at the top of the curve to the atmosphere... it causes a stop in the forced flow of liquid while not impeding the flow of gas...

I believe because of the small amount of surface area exposed to the vacuum , and the fact that no movement is taking place due to the AC compressor not engaged, while it MIGHT be POSSIBLE, to get the moisture out...BUT IT MIGHT TAKE SIX MONTHS ON THE VACUUM MACHINE....

I note also that the MB AC manual when talking about servicing the pro flush/service machine makes no mention of being able to service the dessicant used in it.....and it is made for easy changing... which would allow insertion in an oven if that were a legitimate way to conserve the dessicant for re use.

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