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Old 12-08-2001, 01:01 PM
Dirty Ern
Posts: n/a
Is the A/C compressor always on?

I just went through replacing the climate control in my 81 300SD. While fooling around with the old one trying to get it to work, I noticed that the first three buttons would turn the compressor on even when the dial was set at max heat. So now with the new (from a wrecking yard, but works) installed, I find that the compressor still comes on with the first three buttons. Is that how it works or is something funny going on?

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Old 12-08-2001, 03:01 PM
Registered User
Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
The only button that has NO A/C compressor operation is the EC (economoy) mode.
All others will run the compressor to dehumidify the air.
Donnie Drummonds
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Old 12-08-2001, 08:59 PM
Dirty Ern
Posts: n/a
Thanks Donnie,
I kinda knew that some automakers ran the compressor for that reason during defrost, but I didn't know about it during other heat modes. Will the compressor run all the time, or does it only run long enough to to reach a predetermined evap temp? Maybe I got a little antsy and didn't run long enough for it to cycle. What happens when the ambient is really cold? Does it still come on? I pulled a wire on the drier so it wouldn't run till I found out, figured it wasn't helping fuel mileage much if it wasn't needed.

Another question....I noticed that the supply vent in the tail end of the console blows cold air all the time. Really cold when the compressor is connected. What controls that?
Thanks again.

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Old 12-11-2001, 12:51 AM
Dirty Ern
Posts: n/a
Anybody have a CD rom for the W126's and if so, does it explain the sequence of operation of the a/c compressor during the heat cycle? Incase there is a difference, this is a 81 300SD.

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Old 12-11-2001, 01:51 AM
Registered User
Join Date: Jan 2001
Location: Tucson, Arizona
Posts: 758
Unhappy Same cool rear seat

Greetings Ernie,

I have a '84 300SD and was getting ready to ask about the rear seat vent as well. This item was not mentioned in my owners manual, or I overlooked it six times while reviewing it. I thought heat to the back seat when it's cold outside would be a nice idea but it never gets warm going to the rear. You can however almost shut the entire flow of air to the rear with the sliding lever located on the vent housing. Anyone out there that knows more about this vent please chime in. The sequence of air flow depending on which button you push on the dash I can send you with pics if you PM me your email.

"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan
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Old 12-11-2001, 08:19 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
On the 1981 300SD there is an evaporator temp regulator that controls the A/C compressor on-off. Should cycle at 36-40deg evaporator temperature.
As far as the rear vent operation goes, try search as that has been "hashed-over" many times! This vent is ONLY for A/C or fresh air. UNDER the seats are heater ducts that allow heat to the rear seats.
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Old 12-11-2001, 09:27 PM
Dirty Ern
Posts: n/a
Thanks Doc,
Has to have been the coolest trip to the Doc ever!!!!

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