Center vents problem
 

My car is 1985 300D Turbo.
When AC is tuning on, no air is coming out from the center vents.
When heater is turning on, hot air coming out from side vents plus the center vents (instead from the footwell)
What could be the problem?
How can I permanently leave the center vents open without removing the dash board?

Thank you

Steve

Bump, I'll be watching for answers.

P E H

I pulled the glove box liner from mine rolled the rubber boot back and knocked the pushrod off mine. Then put the flap in the open posistion and went at it. Full on air unless the vent wheels were closed, worked great.

I also shoved a golf t into the lines running to the center vent element and the defroster closed fully after that. (mine failed at the beach and this was a supoosedly "temporary fix" that I liked.



Joe

Joe p,

Were you able to access the preumatic cylinder thru the glovebox?

P E H

I could touch it but not remove it. I've got BIG hands and barely got in there.



Joe

PEH
I assume by "pneumatic cylinder" you mean vacuum actuator that drives the flap behind the center vents. It is possible to access that with the glovebox removed. I've actually replaced mine that way (on the 123) without removing the dash. It's not easy but it is possible. Obviously, if you can replace the actuator itself you can get at the vacuum lines.

Steve
If the center vents are working correctly they should open only for cold air and not with hot. It sounds like your vents are working but the vacuum lines may have been reversed at some point. When the actuator on the center vent fails it tends to work only intermittantly and/or leaks enough that you'll notice the whole system (locks, engine shutoff, etc.) working poorly. If you can hear the vent opening and closing and don't have a vacuum leak problem then I think you may be able to fix this just by tracing the vacuum lines and looking to see if they got reversed at some point. It's worth a try.

LRG

Jeo p,

Thanks.

Irg,

Yes I meant the pneumatic cylinder (spelled wrong) or vacuum actuator. Mine is a '87 126 (300SDL) and the center vent never opens. Do you know if I can get at that one thru the glove box opening?

P E H

PEH
I don't know about the 126. It's possible on the 123 but not the 124 so even though there might be more space behind the dash of the 126 it's largely related to the design. On the 123 you have to remove the glovebox and center vents. I found with my smallish hands I could reach the actuator with my right hand and use my left hand to manipulate tools through the center vents. The main problem is that 80% of the work is done by feel (unless your x-ray vision is working). On the 123 the job wasn't hard or time consuming as much as it was just fussy and frustrating. Depending upon what you need to do you might have just enough room in your 126 if you get a bit creative. As you probably know, a common failure point is the rubber bladders in the actuators that crack and leak. If all the vacuum plumbing looks good and nothing is obviously broken you probably need a new actuator (the early ones are rebuildable but the later ones generally are not). I have to admit it's great having those center vents working on a hot summer day. Sounds like it's time to break out the mityVac. Good luck.
LRG

When the A/C is on, are the middles vents SPPOSED to blast air as much as the side vents? If so I want them to, its hot here! I have the ice cold A/C, but its only cool enough for front passengers. The middle ones running stronger could cool the whole car down quicker.

Irg:
Thank you for the reply.
I don't know how the vacumm lines got reversed. This just happened the past winter when heater was on, heat coming from side and center vents (instead of side vents and floor wells).
Now when AC is on, air coming from side vents and floor wells, but not from center vents.
I don't have vacuum leak problems. Door locks, engine shut off etc. are fine.
Do you know the locations of the vacumm lines to the floor wells and center vents?

Thank you

Steve

Why not have the center vents on for hot air?

As for the flap, will wedging it open affect any other vents?

I actually moved the lines...
 

I changed the electric couplers on the switchover valves 4 and 5 in the 81 126 and now when it is hot outside (ambient air) the center vents come on. I do not have any recirculating air now but I can live with that.
I have not been able to daignose the causes to my system yet btu this was my quick fix.

Steve,
There is a vacuum terminal of sorts behind the climate control unit on the center console that services all the major vacuum lines. I'm not sure if it's easier to get at by removing the ACC or by removing the passenger side kick panel, it's been quite a while since I've done it. Maybe someone else here has done it recently and can offer more specific help. Once you get at it you'll want to remove the lines one by one and test them with a MityVac to see which line operates which actuator. You should be able to hear them open and close (with the engine off obviously). Switch the line for the center vent with the one that operates the side vents. You may need to experiment a bit to get it right. Take some notes so you can put things back as they were in case you need to. Good luck.
LRG

The vacuum pod for the center vents on the 126 is above the drivers knee. You have to remove the knee bolster and maybe the left center console side cover.

The vacuum pod is controled by relay number 4 found on the right side of the console. I can actuate the pod by applying vacuum to the line. The temperature controll module is not turning on the vacuum relay. Therefore I may just put a switch on the side of the console and control the relay manually.

According to the Owners Manual (in German)for my 1984 300D (Euro spec), and the 1981 English version, the center vents are fresh air (not heated / not cooled) only.

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 :)