AC STUMPER IN 85 W123 - Please Advise... - PeachParts Mercedes-Benz Forum

FRANKNBENZ · #1 08-17-2007, 03:07 PM

Patient is Gray Ghost, a 1985 W123 300D Federal, converted by the OO (Original Owner) to R134a. Bought it from him about 40K ago, AC blows OK, but certainly not great.

Periodic maintenance called for a coolant flush and replacement. One of the steps during the flush cycle is to run the heater on the high temp (and high blower?) setting for ten minutes. Spun the climate contol whizwheel to the red stop MAX and indeed eventually got hot air...

Here's the question: whle waiting for the above cycle time, I checked around under the hood and noticed the AC clutch engaged. That shouldn't be happening when the heat's on, right? Please advise possible issues to check.

Thanks in advance. This forum is great.

#2 08-17-2007, 03:11 PM

Yes, the AC should be on in all modes except economy, this is to dehumidify the air.

FRANKNBENZ · #3 08-17-2007, 04:40 PM

Thanks, Craig,

Sounds wierd, though. Wintertime air is almost by definition dry (relative humidity). One would logically think that cold cabin air, when heated, would be drier still.

But maybe I am getting my physics backwards.

Cervan · #4 08-17-2007, 04:56 PM

hot air, is dry air. cool air is moist.

#5 08-17-2007, 04:59 PM

You are correct about the physics, in very cold weather the AC will shut off based on low evaporator temperature. When the AC is useful is when it's 40 or 50F and raining and you have four people in the car with the heat on. That's when you really need to dehumidify the air. If you don't want/need the AC, just select the ECON setting.

#6 08-17-2007, 05:03 PM

Quote:

Originally Posted by Cervan

hot air, is dry air. cool air is moist.

Sort of; hot air can contain more total moisture than cool air, but hot air will have a lower relative humidity than cool air for a given amount of moisture content.

dmorrison · #7 08-17-2007, 05:16 PM

OK time for a weather class here. (N C State class of 1976, BS Meteorology )

Hot air is hot, cold air is cold.

Moist air is moist and dry air is dry. Temperature only determines how much water the square foot of air can hold. Cold air can hold less than hot air.

So cold air can be dry, think Arizona in the winter, or damp, think of the Maine coast in December.

Hot air can be dry, think of death valley, or damp, New Orleans in August.

The Mercedes AC system turns on the compressor when any button other than the "off" and "EC" buttons are chosen. They want to dry the air to prevent condensation on the windshield.

Dave

#8 08-17-2007, 05:26 PM

Quote:

Originally Posted by dmorrison

OK time for a weather class here. (N C State class of 1976, BS Meteorology )

Hot air is hot, cold air is cold.

Moist air is moist and dry air is dry. Temperature only determines how much water the square foot of air can hold. Cold air can hold less than hot air.

So cold air can be dry, think Arizona in the winter, or damp, think of the Maine coast in December.

Hot air can be dry, think of death valley, or damp, New Orleans in August.

The Mercedes AC system turns on the compressor when any button other than the "off" and "EC" button are chosen. They want to dry the air to prevent condensation on the windshield.

Dave

You are correct (MS - fluid dynamics/thermodynamics)