I am seeing some interesting "hunting" in automatic temperature control and am not sure if this is proper operation, or if I still have some CCU issues.

It was cool on the weekend so I was driving with the CCU set on EC (economy) and fan on the AUTO setting. Some heat was called for by the thumbwheel setting I selected and I noticed that the heat would come on OK, then as the cabin got too hot the fan speed would INCREASE by a couple of notches before the air started to cool off. Then as the cabin cooled off the fan would be slowed again, then warm air remixed and the whole cycle repeated.

The outcome is an annoying burst of excess heat for 30 to 90 seconds at high fan speed. The same behaviour is not evident at the cooling end of the cycle. Neither do the swings seem to be as bad AC mode as opposed to EC mode.

With the low fan speed selected the system seems to control cabin temp much more accurately with fewer swings in temperature.

I should also note that I have at least one flap problem as the warm air comes in the dash vents, as I believe my footwell flap is jammed or the pod is inoperative.

That is not "correct" behavior, but I don't know what the problem is. Mine never seems to go back to high fan speeds once the correct temperature is achieved, but I never use ECON mode. When I adjust the temperature, the speed may increase until the correct temperature is reached, then it slows down again.

Check the condition of the sensor tubing above the visor mounting panel. Also, do you hear the sensor fan running with the key in #2 and eng. off?

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Sam

84 300SD 350K+ miles ( Blue Belle )

Sensor is new and didn't solve the problem!

Tube was in good shape at the visor end.

Where is this here fan I should be hearing - behind the glovebox? How audible should it be? I hear clicks and pops from the "guts" of the ventilation system when I switch on, but I am not sure about the fan...

Sam, where's the sensor fan on the early W-126? I've never found one.........??

Sensor fan sits on top of the duct that runs to the passenger front door.Take out glove box and look down and to the right. You can't miss it.

JOhn

Is this on your '85?

Yes,
On my 85 380se and on my 86 300sdl. They are mounted in the same place.


John

They are not in the same place on the '84SD. It does not have the sampling motor in that location.

Brian and team

I have the full shop electrical and climate manuals at home. I could check them this evening and see if the same fan exists on the 83. To my knowledge the systems were pretty consistent on the 83 to 85 300sd's.

There is (as I recall) a plastic tube as well as a two wire lead that plugs into the sensor between the visors.

So the theory is that the other end of this tube is at this little fan?

Of course this does not explain why the problem is worse in one CCU mode than another but it will keep my mind off the fact that I replaced one completely fried CCU with one that works, kind of, but not perfectly.

My drivers floor flap was always in the closed position, common I think, I pulled the trim just above pedals, and propped open the door, open all the time, suits me.

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83 SD

84 CD

I found it in the manual. The early W-126 doesn't use a separate sampling motor..........the main blower housing has a port that draws air into the unit.

If the '85 has a sampling motor, it's unique to that model year and it's not annotated in the manual.

I wonder if it has anything to do with build date. My 85 380se has a build date of May 1985, which is really late in the year, and my 86 300sdl has a build date of August 1985.

John

To be honest, I've never actually put my finger on it, but I do know that I can hear it running on mine in the #II key position. From what I can tell, the sound seems to be coming from behind the glove box. Now I don't know if this is "standard" on my car or not as the climate control had a few "mods" done before I bought it. In fact the air temp sensor was hanging out from the headliner when I bought it.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Are you sure that it's the aspirator fan and not the main blower fan?