1990 300SEL a/c problem
 

If working on cars drove you crazy I'd been there a long time ago!! I purchased this car knowing the air and radio did not work. The compressor clutch was locked up. I have replaced the following in the a/c system. New dryer, txv, and reman compressor. Flushed the system. Charged the system today. I am using R134. Static pressure 100 lbs. Low pressure at idle 45 lbs. High pressure at idle 250 lbs. Air temp from center vents 78 degrees. I have heard that these a/c systems arn't great but I was hoping for a temp of 60 or below. Low pressure at 2000 rpm's 20 to 25 lbs. High pressure at 2000 rpm's 225 to 250 lbs. I know there is no water going through the heater core because I cut the rubber hose and plugged both ends. Is there an air flap that would cause the air to bypass the a/c core? I have had this car a year and would soon like to start driving it. Any help would be appreciated. :dizzy2:

I was getting the same temperature out of my 420 SEL till I found the evaporator to be clogged full of pollen and Lord knows what other crap. This highly reduced the air flow. Now I get a temperature of 42 F out of the center vents and my son telling me he's to cold and to turn it off!

Hope This Helps!

Thanks William,
How did you clean it? Do you have to remove it from the car? If so I see a major dash removal!!

I fitted a new one with an already connected expansion valve. This is a dash out operation and requires some time and patients. To clean it you would have to blow it out with air in the opposite direction. But for a hundred and forty bucks off of e-Bay for a brand new one. Well... you can see which is the logical direction. Unfortunately these vehicle came before the pollen filter and definitely show the reason why the filter is needed.

On a car that was originally R12 - shall we assume the car has been converted?

The car was R12 but had no freon when I purchased it. I decided to use R134a because all of my other cars have been changed over. I flushed the system and put in the correct amount of PAG oil. I changed all of my other cars when they leaked out with no problems.

What is the temp of the suction line (low pressure) where the service valve is ? Does it feel cold ? I am not sure that there should be such a large variation in suction pressure when you go from idle to 2000 rpm. 78 deg means the cooling is very weak. Is the belt nice and tight ? Was this a new expansion valve ?

Yes the txv is new. This compressor is a reman. The line is cool to the touch but because of the air temp coming out I can't tell if I have too much freon or not enough. I can feel air blowing out through the grill under the wipers. I am not sure that the frash air door is not staying open all the time. This is my first MB so I am not sure what to expect from the AC system. I do not want to replace the evaporator unless it is necessary.

suggest that you remove the glovebox to see fresh air door.it needs to be pushed forward to close.i made a pce of two by four with a v cut in one side and was able to jam the door closed.try this first and see what diff it makes.to get to the coil face you will need to remove the blower motor and reach down through the duct with a brush[it can be done].

Huh !! Did you say that the air was blowing OUT of the vent under the wiper ? I have never checked this but I would think that the air would be sucked in rather than blown out. Did you rewire the motor recently, could the blower motor be running backwards ? If this were the case, a use a small piece of newspaper paper on any of your vents, the paper would get sucked in rather than blown away. Someone can correct me, but you should hardly get any outward flowing air thru the vents if the blower was running backwards.

correct but i think he meant the defrost vents.

How much freon did you put in?

Thanks for all the ideas. It will be this weekend before I can check out things. I will check the fresh air door first. When I press the recirculate button I can not tell a difference. Air is coming from under the plastic grill on the outside of the car below the wipers. The blower motor has not been rewired and is rotationg in the right direction because air is coming from the vents inside. I don't know that it has anything to do with the a/c system. I will explore more and see if it is coming out with the blower off. Would it be possible to spray coil cleaner on the evaporator, with the blower motor removed, and then flush it off with water? I do not have a way to measure how much freon I put in because I am using a 30lb canister. On all my other cars I can pull a vacuum, charge the system to between 40 and 45lbs, increase the rpms to see if the low pressure switch cuts out. I add freon a little at a time so when I increase the engine speed and hold it the draw down on the low side is @ 5 lbs above the low pressure cut out. I know that is a funny way to charge a system but it has worked fine for me. My GMC van low pressure at idle is 45lbs and will pull down to 30lbs when the idle is increased but cools great. GMC, GM, Fords have acumalators which also get very cold and sweat when the freon level is correct because they are on the suction side. I think the core is clogged up in this MB because there is no water coming from the drain on the drivers side. There are no freon leaks in the system. The last time I ran the MB air system it was about 80 outside temp. When I pressed the EC button the compressor cut off like it is supposed to do and the air temp coming from the vents was about 85. When I pressed the normal a/c button the compressor came on and the temp dropped to about 78 but I could not here a change in the air flow such as the fresh air door closing. Thanks again for all your input!!

I replaced an evaporator in a '90 124 this year. To my surprise, the evap was not dirty, except where the oil had leaked.

This is a no-filter car.

I guess the condensate rinses the dirt off and out the drains.

The heater core was not dirty either, and its fins are extremely fine.

there is only one problem with your rational for charging gm cars.there you are dealing with a flooded coil system and not a tx valve.by the way a blower turning the wrong way will still move some air thru grills in car but at a much reduced volume.check with paper and see if what you feel at grills is suction and not pressure.