recharging a/c...how much r134 do I need, and what are the low/high side pressure? - PeachParts Mercedes-Benz Forum

darkostoj · #1 05-25-2010, 09:24 AM

just filling up my old mans 98 e300 today. What are the recommended high/low pressures and how many cans does it usually take to fill up?

Graplr · #2 05-25-2010, 09:32 AM

It should say on the tag under the hood how much freon it takes. I don't know off hand as I'm not familiar with that model.

vstech · #3 05-25-2010, 09:59 AM

are you filling an evacuated system, or adding to a leaking system?

darkostoj · #4 05-25-2010, 10:02 AM

not evacuated since i dont have a pump, I'm just going to add it to a leaked out system.

vstech · #5 05-25-2010, 10:11 AM

well, in that case you have to know how much is still in there. it's best to evacuate, and then put in what is called for on the sticker.
is there pressure still in the system?
the sticker will tell you the maximum amount you should put in. start with 1/2 that, then see how it runs. add more if it's not right, but don't go over what's called for on the sticker.

darkostoj · #6 05-25-2010, 10:13 AM

i dont have a pump so i cant evacuate it, and the system is empty. I was going to fill it to match the reccomended low/high pressures. Anybody know what those are?

vstech · #7 05-25-2010, 10:15 AM

you mean it has no pressure? if that's the case, you should fill it with nitrogen and find the leak. then replace the dryer and evacuate the system and charge it up via the sticker.
pressures are meaningless for the most part. a running system can be 20-60 low, and 115-225 high and be correct. ambient out door temp, indoor humidity, and temps all factor into the pressures.
the systems in cars use a variable speed compressor, a variable speed ifm and an expansion valve for control of freon. temps of the lines, load on the coils all are important factors for charging a running system on the fly.
you'd need several contact thermometers to get the charge right. bear in mind the a/c system is designed to work at 75mph... that's a lot of airflow across the car and the coils you cannot reproduce in your driveway... do you have a wind tunnel handy?
I keep earning my "the man" title bestowed on me by LarryBible!!!

#8 05-25-2010, 10:27 AM

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As he says, it is a daunting task to charge one of these systems accurately by pressure readings alone. Even if you succeed, you obviously have a leak that you will have to deal with, probably soon.

Recovering the remaining refrigerant, fixing the leak, evacuating and charging by weight is BY FAR the preferred approach here.

My $0.02,

vstech · #9 05-25-2010, 10:30 AM

lets not forget that any freon that leaked out took some oil with it on it's way.
you should add the correct amount of oil as well. if you are taking out the dryer, you're going to need at least 4oz of oil in the new one. it's best to pour out the oil that remains in the dryer and measure how much was in there.

#10 05-25-2010, 11:02 AM

When I'm replacing a dryer, I turn the old one upside down, drill a hole in it the bottom, then chuck it in a vise over a measuring cup. I let it drain while I'm doing other things then come back and see how much oil came out. Then I know how much to put in the new one. If there was a leak, I add my gut feel amount to the amount that came out.

Except for the cases where you are flushing the entire system, oil quantity is nothing more than a guessing game.

4bnbad · #11 03-16-2012, 08:28 PM

MBZ tech, Mate on a 300 TE what discharge temp can I expect to achieve on a very good functioning system. I'm getting 58 F out center vents on high on 86 degree day.
But this not gonna work when it warms up to 125 degrees in the middle of summer.
It is really hot in AZ. I mean Hell like thx, 4bnbad

vstech · #12 03-16-2012, 08:56 PM

what year vehicle?

SOOOO many things affect vent temps...

engatwork · #13 03-16-2012, 10:16 PM

Quote:

on a 300 TE

If it was r12 from the factory and is now r134 then you are not going to be impressed unless the condenser was updated.

vstech · #14 03-16-2012, 10:22 PM

Quote:

Originally Posted by 4bnbad

MBZ tech, Mate on a 300 TE what discharge temp can I expect to achieve on a very good functioning system. I'm getting 58 F out center vents on high on 86 degree day.
But this not gonna work when it warms up to 125 degrees in the middle of summer.
It is really hot in AZ. I mean Hell like thx, 4bnbad

at 86F ambient, any a/c system should be putting out 40F vent temps easily... if not, you have a bad frost switch, or vac pod mixing outside air... low freon charge, non working condenser fans, dirty condenser coil and or dirty evap coil... etc etc etc...