Charging A/C system.. - PeachParts Mercedes-Benz Forum

baudenfj · #1 08-19-2009, 12:48 PM

Dear Forum:

- I have replaced the evaporator/compressor/dryer on my 1995 wagon E320. Yes, I filled the correct amount of oil into the system - See long thread A/C oil...

- Now the system was evacuated for 8 hours.

- How do I fill the system...

Here is how I planned to do it...

- I got 3 cans of R134a total of 1020 grams close enough to the 1000 grams (1 Kg - 2.2 lbs) needed.

- Connect a can fill device (hose with valve) to the low pressure side and let all three cans in... without breaking the vacuum.

- Turn the compressor a couple of times

- Start the engine and engage the compressor

- Measure the temperature at the outlet (5 deg C0

This is pretty much how I read the manual....

Any objections or tips appreciated...

Thanks...

mramay · #2 08-19-2009, 01:10 PM

"1020" grams is not close to "100" grams. Did you mean to state "102.0"??

Is this an R134a system or R12 system?

Most folks I know put a set of gauges on the high and low AC fittings and fill to a pressure, but I guess the quantity process would work.

gmercoleza · #3 08-19-2009, 01:41 PM

Actually charging by weight is the preferred method. Charging by pressure is not as precise, but is sometimes necessary if you can't start with an empty system.

CWW · #4 08-19-2009, 02:03 PM

Quote:

Originally Posted by gmercoleza

Actually charging by weight is the preferred method. Charging by pressure is not as precise, but is sometimes necessary if you can't start with an empty system.

+1

Charging by weight is definitely what you what to do. Unfortunately, nobody commonly does this, at least at the shops I visit. I am having this debate with one right now, in fact, on my SL600.

I think the reasons are:

#1 It's just easier to slap a set of pressure gauges on and charge until they get where they're supposed to be, this is faster and less work, so it's just what people do.

and

#2 Many (especially older) systems have some kind of efficiency problem or another, like a clogged condenser, clogged evap, sticky expansion valve, gunked up R/D, etc., and so won't take a full charge without sending the pressure gauge shooting way up. The tech's "solution" is usually then to charge by pressure, rather than by weight, assuming it will still work as it should.

The proper thing to do is to find out the correct amount of refrigerant your vehicle should carry, charge it with that amount, and then see what happens. If the operating pressures are out of whack after you're certain you've filled the system with the correct amount of refrigerant, then you have some other problem that needs to be addressed before system performance will be normal. But simply reducing the amount of refrigerant in order to get the pressures into spec is only putting a temporary band-aid on a lurking problem.

Hence why my car stops cooling almost entirely after about 60 seconds of sitting still, even with a brand new fan clutch and the electric fans working as they should. Not to mention new expansion valve, receiver/drier, and compressor. I bet the problem all along is a plugged condenser. That should have been the first place I started, damnit! But I also bet that, if I measured by weight, the refrigerant charge is probably very low, given that even with this obvious problem going on, the pressure gauges read out within specs. The only way the tech could have NOT caught this problem is by charging using the pressure gauges, instead of charging by weight and then using the gauges as an after-the-fact checkup.

And that's the problem with not charging by weight, the system pressures can be artificially inflated by other problems that are reducing efficiency. So you *think* it's right, but it's not, and then you're still wondering why it's not cooling properly.

The bottom line is, the spec pressures and temperature curve are only valid measurements if you are already certain that you have the proper refrigerant fill. So if you charge ONLY by pressure (which most people do) then you're missing the other half of the information you need to make sure everything is correct.

mramay · #5 08-19-2009, 04:03 PM

What an excellent answer!! Succinct and knowledgeable.

baudenfj · #6 08-19-2009, 04:52 PM

Dear Forum...

yes, I will charge exactly by weight! This is actually what the manual suggests...

Can anybody confirm that you charge the system WITHOUT it running! This makes the most sense for me...

I have witnessed somebody topping up the system while it was running based on the pressure readings. I guess this is what most shops would do.

Cheers

Matt L · #7 08-19-2009, 07:19 PM

You can charge it without the compressor running, but you will have to heat the refrigerant in the cans to get it to move. A heated charging cylinder does this job very nicely, but so will a bowl of warm water (170F or less). You'll have to keep reheating the water as the refrigerant sucks out the heat. A hair dryer might also come in very handy.

gmercoleza · #8 08-19-2009, 09:53 PM

You can charge without running the compressor, but why? Just charge with the compressor running - it will seem to "suck" the refrigerant in much more quickly. You can start out charging both high and low sides with the system off, but make absolutely sure that you shut off your high side valve BEFORE starting the car, or you'll be eating some shrapnel!

brewtoo · #9 08-21-2009, 12:06 AM

As a practical matter you will have to put in as much refrigerants as it takes to bring the pressure up enough for the compressor to start and then you will add the rest with the compressor running. No big deal.

gmercoleza · #10 08-21-2009, 10:29 PM

Like I said, start off by charging through both high/low sides with the system off; this will build the needed pressure to trip the low pressure switch and enable the compressor to start. Again, once you decide you're going to start it, make sure to close the high side valve.

baudenfj · #11 08-22-2009, 12:16 AM

Thanks Forum and your help....

I have not filled my system yet.

I had a small little vacuum pump which I used and it has reached a pressure of 25 inch of Hg or about 10^-2 mbar. I now have a bigger pump which should go down to 10^-3 mbar.

I could get a research grade pump which will get me down as far down as 10^-6 mbar. However, the limitation is going to be the piping and the connections etc...

What is the pressure one would recommend to reach before filling the system. I pumped on the system for more than 24 hours ...

I can not imagine that the backgroung pressure can be that critical for the system performance...I can get it down to whatever seems feasilbe... but I can not imaging that it would go below 10^-5 mbar due to the rubber hoses and seals...

Any insight woudl be appreciated....

Matt L · #12 08-22-2009, 12:30 AM

I've only seen the measurements in microns (micrometers Hg) and the required figure depends on the temperature. You must have less pressure than the boiling point of any water in the system.

1000 microns is probably OK in warm weather, but it's good to get lower. When I vacuumed my car, I got it to stay below 400 microns after about four hours (that's with the pump running).

brewtoo · #13 08-22-2009, 01:00 AM

You want to see near 30 inches.

leathermang · #14 08-22-2009, 06:29 AM

I think usually below 500 microns is suggested...
the lower the better...
the LONGER at LOWER is needed because trying to boil off that water through those tiny tubes is difficult... overnight with a good vacuum pump is nice....

#15 08-22-2009, 08:29 AM

In my experience, I have found very few a/c pro's who are even aware of the measurement of vacuum in terms of microns. If you have a vacuum meter that will give you microns, it would be very good to see 100 microns.

If the truth could be known, I fully expect that most a/c systems rarely get evacuated much below 1,000 microns. If for no other reason than the fact that many people don't change the oil in their vacuum pumps often enough.

I have given lots of thought to coughing up the money for such a vacuum meter, but other needs keep cropping up diverting the funds.