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  #1  
Old 04-21-2004, 06:09 PM
blackmercedes's Avatar
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Location: St. Albert, Alberta, Canada
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Do MB AC systems magically lose R134A?

On another board, there is a group of W202 owners having AC problems.

After finding their systems without any capability, they take them into their tech and the system is found low on R134A. After "topping off" the system once again cools.

I am sure that MB systems do not require "topping off" and are in fact sealed systems. I think that a loss of coolant is indicative of a leak, and the leak should be found and corrected.

Am I off base?
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  #2  
Old 04-21-2004, 06:31 PM
Ali Al-Chalabi's Avatar
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I believe you are correct. My car is 7 1/2 years old and to the best of my knowledge the AC has never been touched.

Although, now that you mention it, one odd thing I have wondered about is that in the maintenance manual it lists as one of the maintenance items every 15,000 miles to check AC refrigerant level. I wonder if anyone can give me a definitve answer as to whether this is really necessary and what process they are using to determine the refigerant level? Use the climate control to watch refigerant pressure, my system appears to be behaving normally.
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  #3  
Old 04-21-2004, 09:51 PM
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All cars lose refrigerant at some rate. There are many variables - the materials used in the a/c system, how the various parts are joined/sealed, the type of o-ring used, the coupling technology, the choice of refrigerant, the rate of diffusion through the hoses, the expansion valve. Too many things to be able to make broad statements.

Ever heard of Ford's spring lock connectors used on a/c systems in the 80s & 90s? You could tell how much they leaked by the oily mess on every joint in the system.

In my experience not too many systems make it past ~7 years before needing some kind of service - be it major or minor. Part of it likely depends on the number of hours of use per year, but part is probably just the normal leak rate.

Even if the system leaks only one ounce of refrigerant per year - a very tight system - after 10 years it will lose enough cooling capacity to be noticable. Toss in one slightly degraded o-ring (out of the dozen or so in a typical system) and you're looking at perhaps six ounces per year. Wait three-four years and you have a car that barely cools.

R-134a currently runs about 8 bucks a pound in the U.S (retail at McParts). Most MB models take a kilogram, call it 20 bucks for a full charge. So long as the system doesn't leak more than a pound per year, a ten buck topup in the spring will keep it cool all summer long. Apart from the environmental impact, the economics aren't too bad.

There are occasional exceptions. My wife's 1993 300E has never had the a/c system touched. The car is just coming up on 100K miles and it cools great in Texas heat. I think Larry's 300E still has the original R-12 charge - truly amazing for any car, but especially one with a swiss-cheese evaporator. Perhaps he'll toss some comments into this thread.

- JimY
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  #4  
Old 04-22-2004, 06:57 AM
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R134A is a wild material. The molecules of it are much smaller than R12. The stuff will find every little place to leak out it can. For cars that have been converted, it is even worse. The stuff runs right out thru R12 hoses. That is why R134A systems use "barrier hose". Also, the o'rings are different. Topping off the system with more R134A is fairly acceptable, but you should consider two things: If the system was R134A from the factory, or even if not, you should see if you can find the leak-probably with a dye charge. If it is leaking R134A, it is also leaking oil! Eventually it will run out of oil and lock up. At least add a little oil with the R134A, or better yet have all the leaks fixed and have it properly charged with gas and oil.
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  #5  
Old 04-22-2004, 08:09 AM
LarryBible
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These are some very good responses in this thread. If all the threads on mshop were this good, about 2 days worth of them would make a text book.

Yes most systems will lose refrigerant at a tiny rate, some a little more than others, but you should never assume that this is what has caused a low charge. Check for leaks before recharging, or add UV dye while recharging.

Have a great day,
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  #6  
Old 04-22-2004, 09:49 PM
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R134A is "leakier" than R12, but you shouldn't loose enough in less than five years or more to cause cooling problems. Causes more problems on converted systems than on OEM, though.

If you have to top up more often, you have a leak and will need to find it.

Dye helps (listen to Larry!), so does an extremely sensitive leak detector (GoMac thermal conductivity type).
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