Here is a simple question, with hopefully a very simple answer re an AC system:

Is the Pressure in the evaporator, higher with the compressor ON, or with the compressor OFF?

The reason I am asking is that I have a leaking compressor, and possibly a leaking evaporator as well, and I am trying to determine which of the two components has a larger leak. I noticed that if the compressor is ON, I loose the refrigerant within 5-6 days. With the compressor, off the refrigerant lasts at least 3-4 Months, So I am trying to determine if I can get away by just replacing the compressor, without replacing the evaporator as well. At least for now

I am thinking that if the pressure in the evaporator is LOWER with the compressor ON, it will then leak even Less, so I maybe able to get away without replacing it, at least for awhile.

Thanks

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'95 E300 Diesel, 264,000 Miles. [Sold it]

The pressure in the evaporator is lower when operating. Usually about 100psi when off and around 30-40 when running.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thats what I was hoping to hear. So based on that it means that the evaporator will leak even less when the compressor is ON. So I can safely assume that if I only replace the compressor for now, and leave the evaporator be, than I should be OK with the evaporator for awhile.

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'95 E300 Diesel, 264,000 Miles. [Sold it]

You lose refridgerant so fast with the compressor running that you'll probably go more than 4 months after changing just that. As hard as the evaporator is to get to, compared to the compressor, I'd do just the compressor first. An extra fill is nothing compared to an evaporator R&R.

The evaporator is supposed to be at around 40 degrees temp at design conditions. That is 37 psi in the evap and 157 psi in the condensor or high side R-12 at design conditions of 95 degree ambient. You car uses an expansion device instead of a fixed metering device. When the car is off the pressure in the evap will rise slightly closing the expansion valve and sealing the evaporator to a degree. That is why you loose less refrigerant when the car is off.

Here is some A/C help for your car.

http://dieselgiant.com/repairyourac.htm

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

... just some added refrigeration info..
There are two sides to a refrig loop.. low side/high side...
When compressor is running , the seperation points of the two sides are at the compressor [ internally , between suction/pressure ] and whatever type metering device is used [ Expansion valve, cap tube. POA valve, etc]
Anything from the compressor pressure to the metering valve is referred to as the high side and anything between the metering component and suction port back at compressor is the low side.
All pressure are equalized when the unit is not being used , but when the compressor is on, the high side will be more pressure than a dormant unit and the low side will be less.
Many hard to find high side leaks can only be found when the high side is pressurized b/c the are more likely to leak at the higher pressures of a running unit, whereas an evap leak is more likely to leak sitting dormant as the equalized pressure of a charged unit is higher in the evap [ low side] than when unit is running.
When looking for an evap leak, some techs will roll up the windows , close the doors and let the car sit overnight [ charged w/refrigerant]. The next morning they will stick a sniffer up into the evap drain holes .. the refrig. is heavier than air , so it tends to settle at the lowest point and can be detected.
Same technique is used by commercial refrig techs....they always snif the floor of a unit that is suspect of evap leaks.

Hi Arthur, and Diesel Giant

Thanks for both of your explanations. Your responses also confirm that my evaporator leak is not nearly as bad as my compressor leak. Also based on Arthur's explanation, and assuming that I do change only the compressor, than the longer that the compressor is on, the less refrigerant I should expect to loose through the evaporator. Under the circumstances I guess that this is a good thing.


Thanks

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Hi pberku, another thing to consider is the climate at your location. During the winter, while your car isn't being used, the pressure in the evaporator is much lower than it is during the warmer months. Install a gauge on the low side and see what it says after the car has been sitting overnight in -5C weather.

I was thinking about what you said, but than realized that in the winter the evaporator would actually be in higher ambient temperature not lower.

The reason I am saying this is that the evaporator is within the car's cabin, so in the winter, when the heater is on, the temperature of the cabin is usually higher than it is in the summer, when hopefully, with the AC running it is lower. Additionally, the evaporator is mounted right behind the hot heater. core. This further increases its temperature in the winter.

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'95 E300 Diesel, 264,000 Miles. [Sold it]

While the evaporator and heater core are next to each other physically, they are separated by the blend-air flap(s). The point I was trying to make was that the pressure of the A/C system, when not in use, varies with ambient temperature. The lower the temperature, the lower the pressure. Sorry for the mis-understanding.