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Old 06-15-2007, 01:43 PM
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Join Date: Apr 2007
Posts: 4
What the best A/C temp at the vents

I have a 98 E300. What should the temp at the vents be compared to outside ambient air temp. When set in auto mode and a digital temp probe in the vent it goes from 60' down to 48' then back up and keeps going up and down...

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Old 06-15-2007, 02:47 PM
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Join Date: Dec 2002
Location: Visalia, CA
Posts: 379
For starters, don't use auto mode. For test purposes you'll need to be in control.

Then adjust airflow to the dash vents, set the blower on a med setting, say 2 out of 4, and hold the engine RPM between 1000 and 2000. Higher blower speeds feel good to humans, but the fast moving air isn't able to shed it's heat as well. Thermometers can't respond to the "wind chill" effect the way human skin does. You'll be able to take a fairly good temp reading that way. You can roll all the windows up and use recirc to get a slightly lower reading, but it's not really neccessary.

Now the downside. There is no "correct" temp. It depends on way too many variables to be summed up with a simple number. Ambient temp and humidity are two huge variables. But right down to the color of your car, there are so many things that can affect the A/C temp.

What sort of outside temps are you dealing with, measured in front of the running car, not The Weather Channel's best guess, but an actual reading (preferably with the same thermometer)? That and a guess at the humidity (unless you actually have the proper equipment to measure it) would help to gather some guesses as to whether your system is working well.

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Old 06-15-2007, 11:02 PM
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Join Date: Feb 2002
Location: St. Louis, MO
Posts: 340
On my '97 E320...

with the AC system set to 72 degrees and auto mode on both sides, the center vent temperature will be 50 degrees lower than the outside ambient if that is 95 degrees or above. Car has a charcoal imterior.
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Old 06-16-2007, 09:23 AM
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Join Date: May 2007
Location: Arizona
Posts: 757
Here's a page from the 124 service manual. It looks like the coldest (at least for my car) should be 5 C +/- 1 from the center vent. At 95 F, I have 40 F air coming from the center vents, which is right about 5 C. In stop and go traffic, the temp goes up to about 48 F. There's a run-around chart in the repair manual that comports with the 5 C figure.
Attached Files
File Type: pdf ColdestAtVent.pdf (19.8 KB, 86 views)

1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles
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