I'm confused
 

My center vent will only blow cold air with the a/c on, but when i use the heater, it shuts off, hot air only blows on the side vents...Is this normal? I'm now confused...so which is which?

The problem is one or more of three things.

1. The vacuum pods (green / pink plastic with lever) has a leak.
2. The vacuum hose going from the switchover valve to the pod, leaks or is disconnected.
3. The switchover valve has a problem.

Don

I believe this is correct for a 300D with ACC. The center vents shut off when the heat is on and open for AC. I don't know why it was designed that way, but that is how mine (with a rebuilt ACC unit) works.

And it's a good thing too. I absolutely HATE hot air coming from the center vents.

Craig is right. The center vents are for cold air only. It sounds like yours are working properly.

I would check

'85 300d Acc
 

Ok, I will try once again.

I also own an ’85 300D 123.133 chassis
From my experiences at the back counter and personal experience here at home……

Your system is a Type III ACC (automatic climate control). I would first check the following:

I take it the center vents operated properly at one time

The foam hose coming off the inside air temp sensor. It attaches to the sensor, then attaches to a plastic hose coming off the blower motor. Over time, this hose will deteriorate, thus the ACC does not have a clue what the inside temperature is, and cannot properly adjust the desired temperature and possibly proper venting.
If it needs to be replaced, use a piece of foam copper pipe insulation. Usually it comes in three foot lengths, all you need is a foot. Wrap it with black electrical tape, this will ensure longer life.

Pushbutton unit may need resoldering. Rather than explain the procedure, I have included a link to an article written by George Murphy for the STAR magazine several years ago. There are a few strange characters in the article, pay no attention, they are references to pictures from the magazine article. The article also goes into detail about other things to check and repair. http://db.mercedes.cx/mb/199601/0106.html

One item that I never see mentioned is the electrical unit for temperature control. It’s located behind the glove box next to the blower speed unit. Never can recall selling one, however, it does play a role with center vent control, and is tied into your pushbutton unit. Possibly the temperature control unit may be resoldered?????
I have seen them on the Fastlane site and ***************[/I]
As mentioned by Irg, the switchover valves are located behind the pushbutton unit. Five valves with colored vacuum lines and electrical wires. Example: red/white means red with a white tracer.

Valve 1 defrost red/white vacuum line, brown ground, blue/red 12V
Valve 2 footwell red/yellow vacuum line, brown ground, blue/violet 12V
Valve 3 center red/white vacuum line, (ties into defrost line) brown/blue 12V AND black/red 12V (tied in with pushbutton unit)
Valve 4 fresh air/recirculated air green vacuum line, blue/green/yellow 12V AND black/red 12V
Valve 5 main air red vacuum line, brown ground, yellow/black 12V

Defrost pod is located behind the console, remove left console cover and left underdash pad. You will also see the footwell pod.
Fresh air pod is behind glove box, right side
Center pod is, well you know where it is
Main air is beside temp control unit, behind glove box.

I would pull a vacuum on each line, make sure the arms move and hold. I would then let the car run at idle, and watch each one in operation as you push the buttons.


I do have the pdf file that shows vacuum/electrical schematics for the Type III ACC in 123 cars from the second version CD set. Someone in this forum sent it to me a while back. It’s about 2 megs and could e-mail it to you. It may be too large a file to post here. It does not go into detail about checking the electrical control units, although it may be on the second version CD set. Mercedes omitted a lot of information on the first versions of their CD’s, and made corrections on the second versions.

Hope this helps. Alan

Thanks
 

Thank you guys for the inputs...I will post later the developments...but from the post of the other members, seems like it is normal, but I will still check and post it for other members' future reference.

arsoria My center vent will only blow cold air with the a/c on, but when i use the heater, it shuts off, hot air only blows on the side vents...Is this normal? I'm now confused...so which is which?

I need to pay attention to the dates these questions are raised. I was answering Steve's question, and just noticed it was posted over a year ago!!!

Arsoria, it would help if we know the year and make of your Mercedes to better try and answer your question. Alan

thanks
 

I have a 94 E 320...thanks for your advise though :)