This is my first request for help from the experts! I previously owned a 240D with a wonderfully simple manual CC system. I just bought an 85 300D. The owners manual is very vague. The CC system hot/cold air works great according to the temp selected and even seems to change the vents correctly. The fan speed is fine on the low and high buttons. However, when in "automatic" fan setting the fan speed will continously rachet up through the speeds to about speed 4 or 5 and stay there. It doesn't drop back down. I didn't see any other thread with the exact problem. Any ideas and thank you in advance.

The auto speed setting is looking at the temperature sensor on the dash to figure if it is too hot of cold. It will set the blower motor speed based on the temperature spread, actual vs desired.
Since it operates thru the 1-6 stages, as you have explained, it soulds like the temperture sensor tube is bad. The sensor is getting a bad indication due to the air not being pulled over the sensor correctly.
Do a search on temp sensor vent tube.
Or pull the glove box liner and see if the foam tube that goes from the sensor ( located in the middle of the dash, above the 2 center vents) to the hard plastic section is bad. The hard plastic tube goes to the blower motor area to such the air over the temp sensor. It turns into dust after a couple of years. more than likely this is the case.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

OK. Glove box is out and what I see is a 2 piece section of RUBBER hose joined together and it connects the fan area you spoke about to the temp sensor. The last 6-8 inches closest to the sensor is wrapped with the foam pipe insulation and covers where the rubber hose joins. It looks recent. Is 6-8 inches of pipe insulation enough? Are you saying that it is not supposed to be a rubber hose but a foam tube? Also, it it supposed to suck fairly hard or just barely, mine barely moves the tissue paper. Thanks.

That foam tube is all. No hard tube underneath the foam. Remove the foam tube and make sure it has not collapsed inside.
The air movement is very small. your tissure movement sounds about right. remember is is related to the fan speed.
If the foam is fine and the bottom of the tube is connected correctly then we may have to move on toother things.
Make sure your defroster vent is closed when asking for full cold and the fan is in auto. The temp sensor fan can pull the cold air thru the temp sensor if the defroster is open, I have this problem right now with my SL.
First I would suggest playing with the unit. You have to be patient with the temp dial contoller. Move it just a little and then wait for a responce. If I move my temp dial off the "max" detent and move it to the white section. It's a little to much for the controller and I find the fan goes to the lowest setting. Then it may kick off the compressor.
As I said I find the temps dial to be a little sensitive.

Now if it does not seem right. Then we may have to check a couple of things.

The temp sensor. Based on temperatures it should show certain resistance.
The dial may also be at fault.
the ACC control unit are known for needing resoldering. My SL just needed this.

http://www.peachparts.com/shopforum/showthread.php?t=86871

Scroll down to find the resolder post.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

I really appreciate the help. In furthur explanation, the foam is used only to insulate the rubber hose on the sensor end. In other words there is a new rubber hose connected from the sensor's white barbed elbow fitting all the way to the fan. The foam is only used for insulation on the sensor end. The lady I bought the car from said she had just had the A/C completely redone. Also, do you have an opinion of the index point on the temp wheel? Is the set temp in the middle of the slot, on top, or on bottom. There is no mark on the wooden slot to align the celsius wheel numbers. My thanks continue.

The foam is to connect the hard plastic tube on the passengers side to the temperature sensor tube, the small L shape. You have about 4 inches of "only foam" providing the connection

http://www.peachparts.com/shopforum/showthread.php?t=70866&highlight=temp+sensor+tube+foam+dave

scroll down. I use a hard white tube since I had the dash out.

The center between the top and the bottom is the "indexed" point. When you roll the dial to "max" the Max is half way between the top and bottom. It is in the indexed position.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Well the hose with the new foam is in place and still the auto is not right. It was hot today coming home from work so I had the CC in the normal mode for the A/C. The cabin got cool but the fan stays on 4 or 5 always. When I got home I even took the length of foam hose that was left over and held one end at the middle cold air vent and flexed the other up and completely covered the little grill with it. I hypothosized that if THAT didn't cause the fan to drop down in speed then nothing would! (and it didn't) I am about to the point of saying that I have low speed, high speed, and about speed 4 or 5 when on auto and let it go at that rather than tear into the dash. I supposed all I would be missing would be the "automatic" mode and speeds 2 and 3. You have been a great help, any more ideas a college teacher with 5 thumbs on each hand could tackle?
Stan

__________________
85 300D 107K present
82 240D 190K present- next-to-slowest car I ever owned
81 240D 1995-2000- slowest car I ever owned
79 350 SE Euro 1996-2001- fastest car I ever owned

Does anyone know what the CC system is supposed to do with the fan speed if the temp wheel is not set at the MIN/MAX detents and the fan is set for either HI or LOW? Does the fan quit and restart at the set speed when the temp is reached or called for and keep the HEAT/AIR supply constant or does the fan continously run and the HEAT/AIR flow start and stop?

__________________
85 300D 107K present
82 240D 190K present- next-to-slowest car I ever owned
81 240D 1995-2000- slowest car I ever owned
79 350 SE Euro 1996-2001- fastest car I ever owned

The fan should run continuously when it is in auto. The fan speed adjusts itself based on the difference between the temp setting and the actual temp. It will normally start on a high speed and slow down as the actual temp gets closer to the setting, but it should not shut off. The heater valve (monovalve) also automatically opens and closes.

But what about when the fan is set for LO or HI, does it stop when the set temp is reached or does it continue to run and the heat/cool flow start and stop? That's the part I don't understand. I am try to find a way to make this problematic CC system as manual as possible andd still leave it original and not spend a fortune (see problems above). Thanks
Stan

__________________
85 300D 107K present
82 240D 190K present- next-to-slowest car I ever owned
81 240D 1995-2000- slowest car I ever owned
79 350 SE Euro 1996-2001- fastest car I ever owned

The fan will continue to run in LO or HI indefinitely. The AC and heat will adjust the air temp based on the setting, but the fan will just keep running. BTW, if the control unit is acting up you can by a rebuilt unit for less than $250.

I appreciate all the help. Hopefully I can be of assistence one day.

__________________
85 300D 107K present
82 240D 190K present- next-to-slowest car I ever owned
81 240D 1995-2000- slowest car I ever owned
79 350 SE Euro 1996-2001- fastest car I ever owned