W126 Air Conditioning - PeachParts Mercedes-Benz Forum

drosen · #1 05-21-2009, 06:28 PM

On the 1990 300 SEL:

I had the system converted to the new gas.
It has been blowing cool for the last 2 weeks and I don't notice any change so I think there is no leak. However, it is not COLD.

The shop was mad at me because I made them re-do some other work, so they were not interested in re-checking their work on the air conditioning.

I noticed in auto parts stores a rather inexpensive item: A small cylinder of the 134a, complete with a valve and a gauge on top, and hoses. A nice little all in one sort of product.

Since the system was just evacuated and refilled, should I be able to check the gas pressure and fill with a little more gas (if needed) myself?

230/8 · #2 05-21-2009, 06:53 PM

You certainly can service the system yourself. But before you do, check those system pressures to ensure they are correct. There are tables for the various ambient temperature/pressure relationships so be sure to confirm yours are correct for your conditions. R134a does not tolerate over-charging the system.

When you convert from R12 to R134a you only use about 80% the amount of 134 vs R12. (If your car required 2.2# of R12, a bit less than three 13oz. cans, it would only need two of the 13 oz. cans of R134a to be fully charged.) Your shop may have put in too much 134, and this would cause the unit to not cool well, or even not cool at all. All these systems depend on transforming a liquid to a gas (the cooling phase in the evaporator) and then back again to liquid with the compressor and condensor. IF there is too much 134 in the system you cannot generate any cooling because the low side pressures remain too high for the evaporator to do its job.

You may find the path to cold air is by way of removing some R134a from the system. My 126 uses R134a and it throws out COLD air.

230/8

drosen · #3 05-21-2009, 08:20 PM

Thank you very much for the info.
There is no standard number of pounds to put in, it depends on the outside temperature? One more of the millions of things I didn't know.

How do I know how much pressure (gas) it should be? Is that in a service manual? I don't know how much it was supposed to be with the old Freon, much less what 80% is supposed to be.

Am I understanding correctly that the shop may have looked up the amount of gas for the original system and put that much in, rather than reducing it by 20%?

E150GT · #4 05-21-2009, 09:26 PM

there should be a label under the hood that shows the charge amount. Where do you live? In a w126, r134 may not be good enough to cool you down. I kept my car r12, and it still struggles when its hot and in stop and go traffic, but i am in south texas.

tyl604 · #5 05-21-2009, 11:01 PM

I have always heard that the systems built for freon do not really cool well on the new gas. Has something to do with the size of the evaporator or condensor - not sure - but the rumor was that they will never cool as well as they did with freon. My '81 300SD still has freon and cools like a meat locker.

drosen · #6 05-21-2009, 11:17 PM

I also heard and found that 134a does not do the job like freon from other cars I used to work on, but around here there are just too many ripoff mechanics and I need to protect myself even if that means sacrificing some cold air. I just want to minimize the inconvenience. Needing the old freon is just one more excuse for them to go crazy on me.

It's sounding like I need to go to some other shop who might know what they are doing and just have them check it out.

Will the commercial machines they use automatically reduce the pressure by the 20% mentioned by 230/8 in the 2nd post, or is that something I need to tell them?.

Hit Man X · #7 05-21-2009, 11:41 PM

Go back to R12

Find a better mechanic if you can't do the labor yourself.

drosen · #8 05-22-2009, 12:27 AM

At the rate I'm going, I will be bankrupt before finding the few mechanics that are worthy, and will have supported all the many that are not.

I'll keep trying.

Hit Man X · #9 05-22-2009, 12:29 AM

These cars aren't worth keeping unless you can do the work and enjoy doing it, to be totally honest. NOT stating your location doesn't help, we could recommend people and other board members for the help if we knew where you were.

Don't be scared off at the R12 business, just look on ebay, craigslist, etc for it. You'll be out $10-30/lb at the most (16oz, not the 12oz you find at stores). W126 need 2.9lb to be full.

You can get your EPA license online (OPENBOOK) for about $40.

230/8 · #10 05-22-2009, 10:37 AM

Quote:

Originally Posted by drosen

Thank you very much for the info.
There is no standard number of pounds to put in, it depends on the outside temperature? One more of the millions of things I didn't know.

How do I know how much pressure (gas) it should be? Is that in a service manual? I don't know how much it was supposed to be with the old Freon, much less what 80% is supposed to be.

Am I understanding correctly that the shop may have looked up the amount of gas for the original system and put that much in, rather than reducing it by 20%?

The pressure that you observe is influenced by the ambient temperature when you make the observation. Your system takes 1,300 grams (2.9# of R12), as referenced in another post. Your mechanic should have installed about 2.3# of R134a in order to have an amount of R134a that is sufficient to cool but not too high to cause excessive head pressures.

With this amount in your system there are pressure tables that give the values for high and low side pressure that should be typically found for the system at the ambient temperature in your garage at the time you make the pressure check.

As others have said, R12 is what your car was designed for, and it will be most efficient with R12 in the system. R12 requires a license to buy and if you do not have the license and the expertise and tools to do the work you are left to the mercy of another mechanic to do the work. And, you will be required to pay his shop rate for the work and what ever mark-up he chooses to apply to the R12.

If your system is converted to R134a, then it will be less efficient, but it will still work fairly well as do both of my MBs which have been converted and blow really cold air. I have no complaints with either of them. If you live in an extremely hot or humid place, like Florida or Texas or another southern state, you will not enjoy the same level of cooling you would get if you live in a milder climate. But you will be able to service the system with a product you can buy at O'Reilly's or Walmart for a few dollars a can.

I do not know what your shop did when converting your car, but if you have had trouble with their work in the past, then you may want to check their work on the conversion effort. If you are getting poor cooling it may be because you have too much R134a in the system or too little.You gotta check it before you add more.

It sounds like you are a novice, so I cannot recommend doing this work yourself. Ask friend for a reference to a good AC specialty shop and let them do it for you. Working on a pressurized AC system can be dangerous even when you know what you are doing.

Last thing...much of what you find on EBay is not clean R12. It is a substitute that can be made of R134a and/or other chemicals some of which can be very volatile. So be wary of this source unless you know the vendor is actually selling clean R12.