I just got an 82 300SD and the A/C is a bit "cool" not cold. I haven't checked to see if the PO had converted it to 134a or if its still R12. I was told when I picked it up that the PO had "toppped" off the freon. I know that 134 does not compress as well and if you have done an upgrade, you sometimes don't get the best performance. This is especially true if the compressor is small to start with.

My question is, how good is MB A/C to start with? Not all factory A/C is very good. If its a small compressor, should I go with something like freeze12?

The car was designed in a cool, forested part of northern Europe. How good could it be?? Let a Texan or a Saudi design an auto A/C system and I think we'd see something!

The R4 is an old design compressor and was a workhorse for GM for many years..cooled a lot of big sedans down nicely in it's day....BUT....it's not a powerhouse by todays technology. Nevertheless the R4's performance is not the limiting factor with an R134a conversion on the w126. Generally it's thought that inefficiency in the heat transfer of the condensor is the bottleneck on our cars.

There's been a lot of peeing and moaning about alternative refrigerants on the forum recently. Wade through the debris (on search above), and decide.

I know what I'd do with your car (because I did it).

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

What would you do? Is there a drop in after market compressor?

Forget replacing the compressor (unless it's shot)...and, even then, an R4 is the only practical fit on our cars and it actually puts out greater volume than many other types.

First, I'd identify the refrigerant. If it's been converted correctly, they'll be tags and new fittings and you'll know. If there are not, you should have it identified (there are simple machines to do this that all A/C shops are supposed to have). Many conversions are half arsed and not labeled.

If it's R134a, I'd have it recovered, then I'd charge it with dye, a little ester oil (~2 oz.)and the correct ~weight and pressures of 134a, check aux. fan, clean condensor and evap(see archives) and "test" run it for a while as a leak check. Put a detector to it (if you, or a buddy, have one). If the whole system is a sieve and is just being recharged again and again, you want to know early on. Purists disagree, but this is my technique. If no leaks and it isn't a total dog at cooling, then recover, flush evap and condensor, new R/D, some people put a new TXV in (I disassemble, clean and adjust mine; don't you,.. unless....), correct oil charge, vacuum, and correct ~weight and pressures of R134a. The reason for the flush, even though it's cooling, is you don't know amount or types of oil in the system. Some conversions leave the mineral oil IN and it comes back to haunt you now, or in later repairs. If you knew for dead sure the history, do it more simply.

If it's a total dog, with your relative humidity, or shows leaks at condensor, compressor or evap(rare on 126) I'd think parallel flow condensor and/or reconvert to R12 and obviously some new parts to replace leakers.


If it's R12 now, I'd recover, check aux fan, clean condensor and evap, charge with dye, a little mineral oil and an enviromentally safe "test" refrigerant. Run it for a while,repair/replace for leaks, flush, new R/D, correct oil charge,vacuum, and correct weight of R12. Again, the whole flush dance is because of unknown history on oil.

If it's a hodge-podge refrigerant or refrigerants.....post again.

My test run technique is not how a pro shop does it, but, on a older, running system , I feel it's safe and catches a lot more leaks than just a leak detector or R22 and nitrogen with a sniffer. I've got two electronic leak detectors (one a Yokogawa) and I like them and use them, but I really want more on an old, old, unknown system like yours or mine, five years ago. On a known system, in good repair, I do it differently.

A side note on adding a little oil to the system sight unseen for the test.
Of the five A/C techs I am pretty well aquainted with, all say: when oil is an issue with a compressor failure, it is always because there is not enough; not because of too much. You can hydr(au)-lock a compressor with too much oil, but they've never done it (or so they say ). You can kill a LOT of compressors slowly with too little oil or very, very, very few, quickly, with too much. Too much oil, or mineral oil left in a R134a conversion, typically causes loss of cooling.I wouldn't have the whole flush thing as part of this if I didn't take the amount of oil seriously, but I'm practical and not a worrier, so.......FWIW.

In my car, (this is not a tall tale, honest ) I have frozen the evaporator at 100+ºF in Mohave, CA. I adjusted my ETR switch a little too low, I guess. I do have a parallel flow condensor and I was running some tests with a "test" refrigerant at the time. The leak I was chasing was through the potting into the electric wires on the pigtail from the aux fan switch on the receiver/dryer. Try to find that one!!

Anyway, this is just how I do it. I have a LOT of A/C tools, so it's a fun hobby. No one get all in a bother over "test" refrigerants or what not. I'm sure there are other, better ways, but.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I don't know the history and can ID the fittings. I was leaning toward a complete evac and new freon/oil. I'm familiar with the Saab setup but suspect that MB may have a different arrangement. e.g. I don't know what TVX is or an ETR. It may be something I know with a different name.....I'll have to go over the documentation.

I have a vacuum evaporator and a set of professional gauges with hoses and fittings for 134a and R12. I'll check out what I have and get back to you. I am glad to see that your suggestions confirm the way I was already leaning.

ETR = evaporator temperature regulator, a thermostatic switch on our vehicles; a sensor on later w126.

TXV = expansion valve, old Chrysler style H block on ours.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Expansion like a schreider valve?

schrader valve is a tire valve. A/C systems use versions of them at the access ports.

expansion valve is where the refrigerant is vaporized and controlled, at the evaporator.

Picture is A/C schrader

Attached Thumbnails

 

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

If you want cool air on an MB, get an old one from a wrecked Honda or Toyota. See if you can find a small one man shop guy who likes to tinker, make it work for you. Rip out the MB stuff. Just cut the lines and take it out. Then install the Toyota stuff to the lines you have left. Just a thought.

I know on Saab OEM compressors (I had to put one in) the overpressure valve is listed as a shrader valve.

Nice thought - that's very helpful. Are you volunteering to help this guy or something? While you're at it, if you want a reliable engine on an MB, get an old slant-6 from a wrecked Dodge (those engines run forever). See if you can find a small one man shop guy who likes to tinker, make it work for you. Rip out the MB stuff. Just cut the fuel lines, vacuum lines, trans lines, electrical lines, etc. and take it out. Then install the Dodge stuff to the lines you have left. Just a thought...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Diesel Giant has a DIY guide to AC on 126's.
I'm not an AC expert ... learning by need not choice, but this link has better pics than my manuals and is written for the DIYer.