MB-Dude

Does anyone know what the high/low R134a pressures should be for a W220? Need to recharge the system but can't find how much refrigerant to is nominal.
Cheers,
Jeff

tower

High / low pressures of R134a won't tell you anything unless you know the temperature surrounding you and use a thermometer at the outlet. If the system is evacuated, there should be a sticker in the engine compartment that tells you how much weight (mass) to add.

MB-Dude

Yes, I know. Auto AC is similar to building AC in operation in that the refrigerant pressure changes based upon ambient air temp. But if I can obtain the nominal high/low pressure at say, 75ºF (or any temp, really), then I can convert the pressures to my current Texas sweltering air temperature using AC pressure charts. What I don't have is any starting point. I also don’t have a vac pump. Otherwise, I would evacuate the system and refill. So I'm stuck with 'topping off'. That's where the pressures are vital for optimal R134a load.
Cheers,
Jeff

tower

To the best of my understanding, the ac systems in a residential building are usually of the fixed orifice tube system (cheap) vrs what is used in the mercedes is the expansion valve system. This system is a little bit less sensitive to over filling as the receiver/drier also acts as a reservoir. Because the sensing bulb attempts to keep the pressure in the evaporator constant over a range of total freon volume, you really cannot do what you want to do accurately without measuring the mass of freon total.

Look at this for a short explanation:

http://www.autoair.ie/types.html

If you have replaced a part and therefore have exposed the system to outside air, putting a small amount of refrigerant into the system and then allowing it to purge out will vacate most of the air leaving vapourous refrigerant only. Now add the proper amount by refrigerant by mass. Don't forget new oil (lubricant) because if the system leaked refrigerant, it also leaked oil.

I have no idea if this method is legal without the proper equipment to capture the purged gas so you are on your own.

MB-Dude

Thanks Tower for the link - interesting information. I haven't replaced a component; the A/C in my S55 is just not as cold as it should be. Subjective, I know, but my '96 SL600 puts out down-right frigid, artic air as compared to the '05 S55. I suspect the S55 system to be a little low. Thanks again. I'm going to either approach this one differently, or look for a good, used vac pump.
Cheers,
Jeff

kraut56

Heck - put in one can of R134 and see what it does; you don't need a vacuum pump for that (and you don't need to evacuate if the system still has some pressure (hint). The advice to fill in the specified weight from an almost depleted system is good. If the reduced cold output happened over the last year, this should be ok.

Remember: the newer S class automatic ac systems don't put out that "ice cold" feeling (can give you headaches), but try to achieve cool conditions in a more subtle way.

If you're not sure, there are threads here that give you the diagnostic display mode to read the compressor output pressure.

MB-Dude

Thanks for the suggestion but truth be told, I did this already. But my conscience got the better of me and that's when I began investigating 'getting it right'. Adding a 12oz can did improve the 'coolness' at idle, which was the only noticeable problem. But is one can enough? Is 12oz too much? LOL That's where I am now. Really more in learning mode than fix-it mode. Surprisingly, I can't find anything in WIS... which sort of concerns me a bit.
Cheers,
Jeff

kraut56

you didn't get my "hint"!
with the system practically empty (i.e. still some very low pressure), fill in the exact amount as quoted on the decal. Then your system should perform "on spec".

If it doesn't, you need to go into trouble shooting.

tower

kraut has it right.

put a thermometer at the exhaust vent. You want around 40 degrees at the vent. If its lower than 35 degrees say (it is somewhat subjective) you can add some more R134a. put the blower on low speed if you can with the compressor working. When you get to around 40 degrees F, adding a little more will not hurt, but your temperature should not change. Just don't add too much as then you could get a slug of liquid freon into the compressor and things will get expensive.

Don't forget the lubricating oil. Just a little as too much will do the same thing.

Good luck.

marktan

Not sure on our W220's but my 2002 C32 had a blow off valve on it's AC system. If it was overfilled and the pressure got too high with the compressor engaged it blew off (I discovered this when my pressure gage failed and I overfilled her). Scared the bejezus out of me when it happened T(**POP** Pfttttt...). the side of the engine bay smelled of AC compressor oil for a week. Not that you want that to happen, but I thought it was a pretty interesting safety to have in place compared to blowing the compressor or a hose apart. (I guess I'm wondering if the w220 M275 in particular has this feature).

MB-Dude

Thanks guys for the input. I put a 12oz can in and no strange noises, no compressor dumps, and colder air, especially at idle. The one can definitely helped. But what bothers this engineering mind of mine is that I can't find any reasonable method to 'top off' the AC system to a specific level/pressure. Kraut suggests draining the system to ambient pressure and loading the recommended weight, which is not bad strategy; albeit a bit wasteful. Tower recommends evacuating the system and refill - essentially the same as Kraut. Both valid. And Marktan is lucky with a blow-off valve. But if I took my W220 to a dealer and complained of marginal AC performance, how would they 'top off' the system? Drain the system and reload? Surprisingly, I can't find any information in the WIS for how to do this. Even the procedure for 'Checking the Freon Level' is really lame and vague. No worries, no problems. Just curious. Thanks again guys for the healthy discussion!
Cheers,
Jeff

kraut56

Yes, they drain (recuperate) the system, and recharge it with the exact amount.

MOST reputable ac shops do exactly the same (their equipment measures the drained amount, and measures the re-fill amount.

There is no real "top off" method; to measure superheat would take forever, and requires a lot of instrumentation.

tower

kraut +1