CENTER A/C vents don't blow air - PeachParts Mercedes-Benz Forum

greenfoot · #1 06-07-2007, 12:25 AM

Gentlemen you all probably heard of this issue in the past.

I have a 85 MB 300 TURBO DIESEL A/C BLOWS COLD THRU THE SIDE VENTS AND THE DEFROST VENTS. THE CENTER VENTS DO NOT BLOW A LICK OF AIR.

WHERE SHOULD I START TO FIX THE PROBLEM.....

Knightrider966 · #2 06-07-2007, 12:40 AM

Quote:

Originally Posted by greenfoot

Gentlemen you all probably heard of this issue in the past.

I have a 85 MB 300 TURBO DIESEL A/C BLOWS COLD THRU THE SIDE VENTS AND THE DEFROST VENTS. THE CENTER VENTS DO NOT BLOW A LICK OF AIR.

WHERE SHOULD I START TO FIX THE PROBLEM.....

Remove your glove box and pull the rubber hose off the centre vent pod and plug it and see if that doesn't make your vents operate. Your center vent wiil remain closed until you open it by hand and leave it there, which is what I did. when i want defrost, I just close the centre vent manually.

Biodiesel300TD · #3 06-07-2007, 10:59 AM

Sounds like the vacuum operated actuator that opens those vents is shot. Pull the glove box and hook up a mity vac to the actuator, it's directly behind the two center vents. Then pump the mity vac and see if it moves AND hold vacuum. If it doesn't do both then it needs replacing. You can get new diaphragms for the pods from - www.autoluxmbz.com. It's much cheaper to replace the diaphragms than the whole pod. As long as the plastic isn't cracked or damaged you can replace the diaphragm. If the plastic is damaged you'll probably need a whole new actuator.

jshadows · #4 06-07-2007, 03:46 PM

Autolux no longer sells the parts as the owner got sick (as he tells me, very nice guy).

George Murphy of Performance Analysis in TN has taken over the sales (this is in another post too).

His phone number is 865-482-9175

TX76513 · #5 06-07-2007, 03:50 PM

Georges Email is [perfanalysis (at) comcast . net] Super guy. Diaphrams are $6@ some take one other take 2

jshadows · #6 06-07-2007, 05:06 PM

Quote:

Originally Posted by TX76513

Georges Email is [perfanalysis (at) comcast . net] Super guy. Diaphrams are $6@ some take one other take 2

awesome. I noticed his e-mail (compuserve) advertised elsewhere no longer worked.

I noticed the single action pods have an upper and lower. do you need one of each?

I also assume that if you tell George which pod is leaking he'll be able to tell what you need?

pselaphid · #7 06-07-2007, 05:14 PM

Having done the pod job, I suggest getting enough diaphragms to do all the pods and make a list of anything else that might need to be done under the dash, then take the dash out, hopefully for the one and only time you'll ever have to. Its a horrible, tedious, and only marginally possible chore, but very rewarding the first time you hear the pleasant sound of the pods opening and closing on their own and air comes out the center vents.

Or, just open and close it manually from the glove box opening as suggested.

TX76513 · #8 06-07-2007, 05:44 PM

Quote:

Originally Posted by jshadows

awesome. I noticed his e-mail (compuserve) advertised elsewhere no longer worked.

I noticed the single action pods have an upper and lower. do you need one of each?

I also assume that if you tell George which pod is leaking he'll be able to tell what you need?

Here is some info from George

The single hose actuator takes one 2010 diaphragm
The dual hose actuator takes a 2010 and a 2020 diaphragm.
If you need to replace ALL the diaphragms in the 126 models, you will need 6-2010 and 3-2020 diaphragms

The parts are $6.00 each plus S&H
We accept Visa/MC .

LaughingGravy · #9 07-12-2007, 09:03 PM

Quote:

Originally Posted by Knightrider966

Remove your glove box and pull the rubber hose off the centre vent pod and plug it and see if that doesn't make your vents operate. Your center vent wiil remain closed until you open it by hand and leave it there, which is what I did. when i want defrost, I just close the centre vent manually.

How do you open the center vent by hand, from the back with the glove box removed, or some other way, through the round vents from the dash side?

POS · #10 07-12-2007, 10:04 PM

First, find out why it's not working.

Just tonight, I took out my glovebox and banged around there with a flashlight because my center flap wasn't opening either. I was fully expecting the pod to be shot because everyone says "it's the pod that's not getting vacuum."

Not true in my case - the arm that comes from the vacuum pod was no longer connected to the flap - the pin broke and fell into the AC ductwork. A zip-tie to connect to two together again and voila my center vents work. Diagnosis and solution took 20 mins. Get in there and find out why it's not working - it's very fixable.

LaughingGravy · #11 07-12-2007, 10:46 PM

Thanks, I'll certainly get in there once the car is back from getting the oil cooler hoses replaced, maybe even this weekend.
Thanks for what else to look for first, even if the rubber hose was disconnected.
As far as the pin, this wouldn't be the first time a zip tie has solved a problem with my 300TD. The wiper motor in the back hatch has a plug that was fixed with that little piece of plastic magic.
We'll see, and I'll post back.

I can do stuff like this by myself, but my driveway is on a hill. I don't have a ton of tools, and I have a good relationship with my local mechanic who I trust to do the oil cooler hoses. We talked about it at length before I brought it to him.

Anything that severely disables the car if I screw it up, I'm not doing without help unless I'm positive I can do it right.

arew264 · #12 07-12-2007, 11:44 PM

Okay, the first issue here drives me NUTS:
If the center vents don't open as they should, IT DOES NOT MEAN THAT VACUUM ACTUATOR IS SHOT.
The center vents not operating generally is caused by a vacuum leak somewhere, anywhere, in the ACC system. There are other symptoms, but people (including myself) don't generally notice anything but the center vents not working.

To DIY fix this, do as advised and remove the glovebox. You can see right behind the center vents where a plastic air duct comes up and a rubber piece connects it to the actual vents. Behind there, connected to a flap in the plastic duct, is the center vent pod.

Just a note, this only applies to 123's.

It is a PITA to remove that pod, a bigger PITA to disconnect and reconnect the vacuum line to that pod, and the biggest PITA to remove the dash in order to remove that pod.

Verify that with the temperature wheel set to minimum and the ACC on (in economy or normal mode), the pod is pulling the flap open. If it isn't, you have a vacuum problem. If it is, you have the aforementioned problem of the pin.

The pin problem is easy to solve, pull off (partially if possible) the rubber duct and use a plastic zip tie or small machine screw (or basically whatever you have on hand that will stay in place) to connect the metal piece from the duct to the metal piece on the flap. It is fairly self explanatory.

If it's a vacuum problem, the fun starts.
If you don't have a mityvac, you will need one to do any vacuum troubleshooting on this car. As they are durable and quite handy, most forum members will advise you to buy one. Just make sure to get the metal one and not the plastic (hopefully another forum member will chime in here).
The company that makes them is called silverline, search for it on google if you don't know where to look.

Once you have a vacuum tester of some sort, the best way to approach this is to just do a full test of the ACC system. The way the system works is that the CCU (Climate Control Unit, AKA that thingy with the buttons you push) controls 5 switchover valves that open and close to operate the vacuum pods. When you see them, it makes a lot more sense.
The switchover valves are located behind the wood in the center console. To get at them, gently pop the wood panel off of the CCU.
I did this with a small flathead screwdriver, starting at the radio. The thing to be careful of is that you are pulling up the metal plate under the wood and not the wood itself. As my wood has started to curl up at the corners, I had to watch very carefully.
Don't bend the panel as you remove it, just be gentle and patient.

Once that's out, you see a plastic cover underneath. This is held by four screws. Remove them and gently remove the plastic cover.

Now for the upper panel (the one with the switches, one of which is for the rear dome light).

Gently push the switches out from behind. Once they are popped out of position, remove the wires from behind them (there is a plastic wiring harness that pops/slides off of the actual switch). You wil see that each switch has one or two wires on the top and/or bottom that stay with the wiring harness, but are separate. You may want to tape these to the wiring harness, as they have a habit of popping out and getting lost in the tangle back there. These oddball wires are the fiber optics that light your switches.

Once you have the switches out and separated from the wiring harnesses, pop out the wood panel. It literally pops out, the bottom comes out, and then the top. I don't remember too much about this process, just push from behind and be slow but firm.

Now that the panels are out, the switchover valves are exposed. You may have to move those wiring harnesses aside to access them (this is where those fiber optic wires have a tendency to get lost).

As you can see, there is a rubber piece above them that connects them all to the main climate control line (the green line that starts in the hood). They are allowing air to flow or blocking airflow to the lines that come out of the valves towards you, and then curve off to the sides.
These are the lines to your vacuum pods. Now comes the trouble shooting.

In order from left to right, the lines go to these pods:
1 - Defrost full
2 - footwell
3 - center vents and defrost halfway connection
4 - recirculation pod
5 - main air pod

To test a pod, simply remove the rubber connector that connects the line to the switchover valve from the switchover valve and connect it to your vacuum tester. Evacuate the line (and the pod on the end) to about 10 inches of mercury. See if it holds this vacuum.
If you have a leak, it will be very noticeable (as in you will see a noticeable drop in under 10 seconds). The one thing to be careful of is the recirculation pod. It can trick you into thinking you have a leak when you don't because it has a very thin line connecting it to the system that restricts airflow, causing it to operate very slowly. With that, see if the vacuum stops draining, as it should once the pod is fully actuated.

All these pods, except the center vent pod, are readily accessible and easy to fix. What actions you should take depend on what is leaking and what is not.
The pod itself consists of a rubber diaphragm, a connecting piece, and a plastic shell. As air is evacuated from the rubber diaphragm, it shrinks and pulls on the connecting piece, thus opening or closing whatever vent it is connected to.
With age, the rubber diaphragms weaken and crack, thus allowing air into the system and keeping all pods from actuating.

For now, just test all five lines and report back with the results. With all but number three, you know which pod to replace.
With number three, however, the vacuum line goes to the venter vent pod AND the halfway connection of the defrost pod (the defrost pod is different from the others in that it can be actuated the full way or halfway).
If you get a leak on this line, don't panic(tm), there's a good chance it's not the center vent pod at all, but the halfway defrost connection. To test this, you must remove the driver's side kick panel (I can detail that if you need it), and disconnect the line from the middle of the defrost pod. If you lie on the floor (doing the limbo through the door) looking up at the stuff above the driver's kick panel, it is directly above you. You can see the red/pink cap.
There are two lines connecting to it, one directly above you (connecting to the pink cap) and one connecting to the middle (that you need a flashlight to see). Remove the middle one and plug it with a golf tee or something that will form an airtight seal with the rubber piece.

Now that the halfway connection defrost is removed from line three, test it again. If it was leaking before and it doesn't leak now, your halfway connection is leaking, as you can easily conclude.

At some point, I should probably get pictures and do a wiki write up of this (or someone should).

I would be happy to answer any questions (even the ones you may think are stupid - I've asked my share of stupid questions in the past

).

Hope this helps.
-Andrew

POS · #13 07-13-2007, 12:07 AM

Nice writeup, Andrew!

Mismost · #14 07-13-2007, 01:40 AM

The dis-connected flap and "simple" zip tie fix would be way cool....problem is you had better have a tiny little hand to thread the zip....unless you remove the dash which I don't even want to think about.

I just glued the flap open for center vents and then close the vents if I don't want to use them. But, I am a hack just looking for the easy way out and cold air! I got both.

arew264 · #15 07-13-2007, 11:35 AM

My pin was missing AND I had vacuum problems.
I replaced the pin with a machine screw and nut with plenty of locktite to keep it from vibrating apart.