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How cold is your AC?

2K views 11 replies 4 participants last post by  Graminal95 
#1 ·
I'm trying to figure out how cold my AC should be and was wondering what other have for Evap temp values, or vent temp with a thermometer.

I was having trouble with the poor cooling and wildly fluctuating high side pressure. I replaced the TXV and drier, vacuumed the system and put in just under 36 ounces (I know a its a little overfilled) or fresh 134a.

The evap, and both heater core temps are all in the low to mid 50's F. The high side pressure is back down to 8 on the highway and up to 15 in traffic. No matter what I do I cannot get lower then 49F evap temp. The compressor is not shutting off and the low side pressure is 38-42psi, and nothing seems to effect it.

I just moved to DC for the summer and really want cold AC.
 
#3 · (Edited)
What is a wild fluctuating high side pressure?
These cars have a variable displacement compressor so the normal high side can be from 6 to 22 bar depending on the conditions.

Do your condenser fans come on at 14 bar?

My car 36*F discharg temp @ center vent 85*F OAT 100 humidity, A/C set to 72*F in auto mode fan speed 4.

FYI the compressor only shuts off to protect its self or if you push the EC button, otherwise it runs all the time.
 
#4 ·
The highside pressure was going from 7 to 28+ (75F outside temp) and then back down to 7 within a min or so, before I replaced the expansion valve and drier. Now it is very stable at 8 to 10 on the highway depending on outside temp.

I've cleaned the condenser with soap and water, and fixed the aux fans so they run properly. The ccu does turn them on but only after the high side pressure is over ~15bar. The viscous fan is working well and keeping the engine just below 80C most of the time. I have cleaned the duo-valve and the heater core temps are only 2 to 4 degrees higher then the evap temp, so I can assume little to no coolant is getting by the valve.

36F is really what I was hoping to see.

I think it maybe time for a compressor according to the MB spec sheet I just found for the gasser version of the w210.
 
#6 ·
Graminal
You were low on freon before you do the repair. As you say, it goes up to 28 bars, then the compressor shut off for good, so it goes back down to 7 bars. You compressor is good, do not waste money, keep an eye on the freon, it you have a leak before, that may still there.
 
#7 ·
When I picked the car up after having sat for 6 or 7 months the AC was low on refrigerant. The evap was leaking ever o slightly, I put in a can w/dye and watched as the pressure would fluctuate all over the place. After a few days of running it and looking for the leak, I found a little dye on the evap drain at the heater box. I used some red angle leak stop, which worked since after evacuating the system it held a vacuum. She was fulled up with the right amount of r134a and works okay.

The MB manual says that if the temp is high, the low side is ~1bar high and the high side reads normal, that the compressor needs to be replaced :crybaby2:

I can hear the expansion valve cycling at 1hz, and wonder if I'm still a tad low on refrigerant. I might try adding a little for the commute home tomorrow after work.
 
#8 · (Edited)
Compressor pumping high to 28 and shutting off means overpressure.
That can be due TOO MUCH refrigerant or restriction in the system.
Draining couple of bars off is easy start to figure it out.

But if I read it right - this is the past before repair. Now you go to 10 bars max, what means too low.
 
#12 ·
Just to follow up on my AC troubles. The aux fans died on me and with last weeks temps in the 100+ I was using the AC and just letting it shut off when the high side reached 30bar. This worked ok until the clutch started slipping and melted the rubber holding it to the shaft of the compressor. I decided to bite the big one and purchased a new compressor for techchoice. Install was fast, maybe 1.5 hours.

I first tried using a hydrocarbon based refrigerant since it works so well in my 240D as R12 replacement. The cooling was marginal at best, so I decided to try good old 134a. Vacuumed the system down and filled it up with 36oz. Now I get 37F evap temp on the highway in 88 degree weather.

It looks like the low side was to high on the other comp, and should have been around 25-30 at 1000rpm. Upon inspection of the old unit, the pressure it adjustable externally and I could have had cold AC from the get go if I had known.
 
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