1990 300SE,
Hello All,
My 300SE has been converted to 134A some time ago. As is typical on these 126 models the air system does not seem to cool very well.
In this car at 80 degrees outside temp. the pressure reading on the low side is 35 psi and on the high side it is 205 psi. This seems to be in range...please correct me if I am wrong.
Out of the vents however it blows about 80 degrees. If I switch the blower to low I do get a drop in temperature to about 75 degrees.
If I re-circulate the cabin air there is no difference....in temperature.
Can some one please advise?
Are the pressures correct?
What can I do to trouble shoot further?
Thanks

With 35 psi, you should see some noticable cooling. Perhaps the expansion valve is restricted.

With a blockage, I'd expect something funny with the pressures, and those numbers, if at 1500 rpm, look darn good.

Is your monovalve broken and defaulting to full heat, cancelling out the expected cold air?

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The Golden Rule

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

I have your exact car and have converted to R134a as well. One more question, are these pressures with the auxillary fan running? If so then they are about right on target, if not then you are low on charge or you need to rev the engine up to 1500-2000 rpm and give us a reading with the aux fan on. If the low side falls a bit more than that is good other wise, I turn the compressor off and run the fan and see what the vent temps are and you may well have a monovalve failure. You should be getting to 50 degree air with those pressures. And lower when driving.
Wayne Hyde

Thanks guys for the input....
Yes the auxiliary fans were running and I waited at least 15 minutes till the pressures stabilized.
The pressures were at 40 and 250 PSI …since it was 80 degrees out I thought the pressures were a little high.
I released some refrigerant to bring them down (35-200). There was no difference in cooling.
On my 91 350 SD
Again this vehicle was also converted at one time.
With similar pressures I can get 48-49 degrees out of the vent. And while driving it will go to 45 degrees. It does however take a real long time to cool down.
On my 99 300TD It blows a chilly 33 degrees in a matter of minutes……..
It will put frost on your twizzler! With similar pressures......
?????
OK...I turn the compressor off and run just the fan and see what the vent temps are. The monovalve Controls the hot water...... I supose i could bypass the heater hose to see if i get a temperture change.
I do notice that some times the air only blows out the defrost vent.
That seems to be a different issue with the flaps....

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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

If you want to have some fun, here's what you do.... Call Mercedes and tell them you have chronic a/c problems, and even when "fixed", it's not very cool. Their first line of defense will be to tell you they've never heard of this problem before, which is bulls*it, because I've called them several times about mine, but they say the same thing each time. It's fun to hear them lie and to play stupid. Their 2nd line of defense will be to ask if you've had your car "properly maintained". This is another way of saying they're shifting the blame to you instead of admitting their a/c system sucks.

I had asked around to see if anyone could take a quality a/c unit from a used car, say a Toyota Corolla or Honda Civic, and put it in a Mercedes, so we can get cool air for a change, as opposed to sitting in the SWELTERING heat due to the crummy Mercedes a/c system. I'm thinking it can't be that hard; there's plenty of room in the engine compartment of a MB. Rip out what you can of the crummy Mercedes a/c, and put in the a/c from a used quality car. That seems to be the only solution to get a/c that actually cools properly.

1991 300d with a/c that after a lot of work and money, DOES blow fairly cool.

My goodness, what a tirade.

I'm sure that some people have trouble with air conditioners in fifteen-year-old cars. It's to be expected. And it's also to be expected to not get the design capacity if you don't use the design refridgerant.

The idea of ripping the air conditioning system from a Honda or Toyota is the worst idea I've read since... well, since you posted it last time. There's a chronic problem, all right.

And about "after a lot of work and money," did canabalize another make of car?

Anthony,

I've been lucky, I guess. Recently converted our 300 SD to 134a, and have had good success. I was thorough in replacing all O-ring seals and the 'valve block' some time ago, while still under R-12, and the dryer bottle just recently after evacuating all the mineral oil, just before switching to 134a. So I'm getting nice, icy air from a 23-yr old car!

If you didn't get all that mineral oil out, it is known to sludge with the new PAG or ester oil and seriously diminish performance, or even damage your compressor.

As to your air-from-wrong-vents issue: the vent flaps are controlled, pneumatically, via electrical activation controlled by the center-console 'pushbutton switch'. These switch units are notorious for failing on their own, and can be cooked by a failure of an in-line component (in my case, the fan blower motor was working, but drawing more than the designed 18 amps max). You may also simply have a vacuum leak; very common on these machines.

Your failure can be as simple as a broken pneumatic connector (tiny rubber hose or plastic pipe; cheap parts, but a lot of hours) or could be the pushbutton switch (I got mine replaced for under $200, if I recall).

Best luck, Lou

Thank you Dr Lou,
Yes I have owned my 350SD and 300SE for a few years now. Both systems have been losing steam....er… ahhh….. cold air.. every year. I toped them both off Including my 99 E300TD All systems have about the same pressure readings. I don’t know who did ....or when the two older cars were converted.
I think I need to base line each car. Evacuate the system, clean / flush, replace o-rings etc and replace the receiver dryers. Then go from there. I don’t have a charging station to get an accurate fill. I should be able to do it by weight.
I did find a vacuum leak on the 300SE. Now the air blows out of the proper vents.
Ill look at it again this weekend ……Thanks All