CLimate Control Unit?
 

I purchased an 83 300SD about a month ago and it has 140k miles on it. The problem is the when I turn the rotary dial all the way down to cold position, I still get warm to hot air coming from the center vents. After about 15 minutes on the cold setting it will start blowing cold air. If I move the dial slightly to a warmer setting, I start getting hot air again

Secondly, the blower only works intermittently. It will work when I first start the car up and last for 2-3 minutes. It then shuts off and will come on sporadically for about the same time limit. I've hot-wired the blower and it works fine. I also checked the blower regulator, and when the blower works I saw voltage at the #8 pin and when the blower wasn't working I didn't see any voltage at the same pin. So I've ruled out the blower motor and the blower regulator.

Moving the rotary dial will change where the air blows from, and when the fan isn't working I can here the monovalve cycling and the small inline pump kick on when I hit the defrost button or I turn the dial all the way to hot.

Does anyone think that these problems can be remedied with a new climate control unit?

This is a common problem. The most recent I have seen was posted by someone else on this board after I replaced my CCU. On mine, the new CCU did fix the problem. The other post indicated that a worn key switch could be causing the problem. Try jiggling the key when the blower cuts out to see if that has any effect.

I forgot to mention that I did try the key trick, but I wasn't that lucky.

There is a small fan motor under the dash by the passenger dorr. It is used to draw cabin air past a temperature sensor. If the fan is not working, you will have unpredictable fan/temperature operation.

As rockinwagon notes the problem is probably the CCU. If you are adventurous you can repair it yourself. It may take a couple of tries though.
First thing to do is remove it. Not hard to get it out. Several threads explain that. Then get it apart to where the printed circuit board is exposed. Then take it further to where the relays are exposed under the circuit board.
You will need a good soldering iron and a magnifying lense. Get a temp contolled iron if at all possible.
Examine the circuit board closely. You will very likely find some cold solder joints throughout it, but it can take only bad solder joint to cause it to fail.
Check the edges where there is a line of solder along the vertical short boards. They get fractures there.
Then turn it over and use a very thin strip of very fine sand paper or a very thin point file and clean up the contact points on the relays. They get cooked with the voltage running throught them.
After everthing is resoldered, and before reassebling the covers, take it back to your car and plug the tow wiring harnesses with the CCu hanging out where you can see it. Start the car and run it through the paces to see if it works now. You will be able to see the relays open and close, I used a small probe to push them around, found out one was still in need of attention.
That is how I got mine to work again, the caution is that once you attack the CCU with the soldering iron it is no good as a core should you decide to send it out to a rebuilder.

test fully before replacing CCU
 

I would identify the right wires out of the CCU and fake the signals to the monovalve and then compressor and see if they respond as they should.

I could well be the CCU but equally the valve not shutting down or the condenser relay even if CCU ok. It would be expensive to replace the wrong one.

Keith

Condenser relay?

This is the easiest and cheapest thing to check. Your symptoms sound alot like mine before I replaced the tube between the temp sensor and fan. Getting air flow over the sensor is critical.

???
 

I haven't found my particular problem after seaching the forum. The "flapper door" on my '91 300SE wont stay on vent with the A/C running. There is plenty of cold air but it comes out of the defrost instead of the dash. I can "play" with it and get the air to come out the vents but as soon as I have to push down on the gas it perhaps is changing the vaccuum and out the drfrost it comes again. Any ideas of what could be the problem, from what I have read I don't think the monovale is at fault because I can control hot and cold.
Thanks Everyone

Sounds like you have a leak in one of your vacuum pods.

Where exactly is this unit and does my 1981 300SD have one?
I have similar random temperature operations. :)

The small fan is located in the panel at the right side of the glove box.

JBC, in addition to all that's been said, if you're getting hot air out of the center vents then your diverter flap vacuum pod is bad. Mind you I found that to be a blessing in one of my SDs as long as I had control of air temp. I'm thinking of adding a switch to the SDL to I can open the diverter flap at will to allow heated air through the center vents. My right hand and ear should enjoy every comfort my left hand and ear enjoy :)

Bruce B, if ACC settings change when you lose engine vacuum, check the yellow 3-way check valve mounted on the forward bulkhead ahead of the brake master cylinder. You shouldn't be able to draw air from front to back. Check also the line that attaches to one of the back side ports and disappears behind the fuse box. The line goes to a vacuum reservoir ahead of the driver door hinge. It should hold vacuum but it's a good size container so you'll be pumping the MityVac for a long time before you get a solid reading. If the recirc flaps hold but the center vent flap doesn't hold under the same conditions then the center vent pod is shot. It's the most difficult to fix or replace too :(

To clarify Michael's last post, you have to remove the glove box liner. There is no access to the ACC aspirator motor, as it's called, with the glove box liner in place.

Sixto
95 S420
87 300SDL

Thank you :)

samo
 

i have 83 300sd and same problem i fixed mine with a replacement climate
control works perfectly... got off ebay for 30 bucks... :P