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Old 06-02-2005, 08:15 PM
pberku's Avatar
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What is a normal AC outlet temperature?

With the AC running, what should be a the normal center vent outlet temperature at any given ambient air temperature? Is there a rule of thumb that says it should be 'X' degrees below ambient?

The above assumes that the AC has been running for at least 10 Min, and that it is R134 based.


'95 E300 Diesel, 264,000 Miles. [Sold it]
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Old 06-02-2005, 08:21 PM
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You'll likely receive as many opinions on this as there are on the "what oil to use" subject.

I've seen 134 cars blow ice cold air and I mean ice cold. These 134 systems were in Asian cars.

For me, if you're not getting at least 45 degress F after it's cooled down, you're not going to stay cool long.

I have a 17 yr. old Accord running R-12 that blows 38 degree F in the low 90's. Not bad.

So somewhere between 40F - 45F would be ok in my book.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 06-02-2005, 08:33 PM
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Here are a couple of graphs showing what the various temperature and pressure specs. are for a Porsche 993. I've never seen equivalent data for an MB, although it probably exists.
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What is a normal AC outlet temperature?-ac1.jpg   What is a normal AC outlet temperature?-ac2.jpg  
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Old 06-02-2005, 10:28 PM
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how do you test the temp? hold a thermometer in front of the vents?
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Old 06-02-2005, 10:36 PM
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I agree, 45 degrees F seems about average. If it gets MUCh colder than that you run the risk of the water freezing on the evaporator coil.
Use a thermometer resting right into the vent, most techs use a digital thermometer designed for this job, long probe so it can't fall into the vent.
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Old 06-03-2005, 10:41 AM
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Nominally, you should see a 30-40 degree drop from ambient. But there are many, many variables. Humidity - dry air cools more than humid; the a/c system expends much of it's capacity dehumidifying moist air. (The old latent vs. sensible heat thing.) Compressor output, which is related to engine speed. Air flow over the condensor, related to vehicle speed (and in Texas, wind speed and direction ).

I agree with the previous posters - unless it settles in at 45 degrees or less, it won't be really comfortable. Fifty is tolerable, but much above that and it starts to have a clammy/humid feeling to the air.

- JimY
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Old 06-03-2005, 11:00 AM
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A properly operating 124 Climate Control system will run at 38 degrees at the center vents once the temperatures and pressures are stabilized and before the cabin is cooled off such that the system begins to cut back on cooling to regulate temp as commanded at the temp wheel.

I keep a dial temp in my center vent all the time and monitor it constantly. The MB documentation offers a section on the test procedure for the system. It indicates 38 degrees as well.

Have a great day,
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Old 06-04-2005, 07:35 AM
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On the 124's with center vent temperature at 38 degrees how high could the ambient temperature be for the system to be able to keep the vent temp. at 38 degree. This is with engine at about 2000 rpms and not taking humidity into consideration.
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Old 06-04-2005, 07:06 PM
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Once the pressures are stabilized and the car is moving, not in stop and go traffic, it will maintain 38 degrees in 108 degree ambient temp as long as the recirc mode is working properly.

108 degrees ambient is the highest temp I have been in since I started keeping a thermometer in the vent. This is an R12 system in top shape that has obviously not been converted.

If your 124 a/c system won't do this, I would first check the pressures and if all is well then, I would start checking to see if the fresh air door is staying open.

Good luck,
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Old 06-04-2005, 07:33 PM
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My car was converted before I bought it about 6 years ago to R134a. Last year when the car was not cooling well I put in new, compressor, expansion valve, and reciever/dryer, refilled with R134a. The old compressor was leaking oil from just about everywhere, or so it seemed. I kept a thermometer in the center vent for a while but don't remember how hot it was when it got as cold as 38.

I'll have to check when we get some really hot weather.

With my R134a I may not get it to 38 when it's 108 degrees ambient.
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