My W123 has center vent heat! - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 02-11-2007, 03:26 AM

My 1985 300D (ACC III) now can blow hot air as well as cold air from the center nozzles. Here's a DIY showing how you can add this feature to your W123. It may also apply to other cars using the ACC III system.

[part 1 of 2 parts]

This is an easy mod that is also easily reversible -- that is, if you don't like it, you can put it back the way it was before. It doesn't involve destroying anything, is both invisible and transparent to the user, and has only one minor negative: the low-volume "bleed" of air to the defrost nozzles is lost in some ACC modes. (Regular defrost mode works normally.) The cost to you is a few dollars and an hour or so of your time.

This modification came about because I wanted heat from the center nozzles, like most of us do, but I didn't want to hack up the flap or its vacuum pod and I certainly didn't want to remove the dash. I also didn't want to do anything irreversible.

Having just purchased the factory service manual for the ACC system, I started trying to understand how the thing worked. The various vent flaps are operated by vacuum pods and the vacuum is switched by electric "switchover valves" that themselves are controlled by the ACC electronics.

With the electrical schematic in front of me, I started looking for a way for the system to open the center vents when in "heat" mode. The footwell vacuum pod is controlled by switchover valve #14 and the center nozzles are controlled by valve #13. Unfortunately, the valves aren't operated the same way (electrically) -- relays would be required. I also didn't want to risk damaging the electronics of the ACC. Therefore, I looked for another way.

Reading the vacuum diagrams for the ACC system, I realized that all I had to do was to replumb the system so that the vacuum "command" to open the footwell pod would also open the center nozzles. To do this, all that was required was to "tee" the lines together.

The first challenge was to find the switchover valves. They turned out to be in the center of the dash, behind the panel holding the several accessory switches (sunroof, etc.). First, after disconnecting the battery, I removed the ACC pushbutton control unit. I forgot to take pictures of this part of the operation but it should be documented somewhere. Remove the lights from the back, unplug the two big cables at left and right, and set the ACC switch aside.

Next, I removed all of the switches from the panel above the ACC unit. (See the first picture.) To remove a switch, reach up behind the panel and push the switch out, along with its cable. Carefully wiggle the switch out of its socket and then gently pull out the fiber optic light pipe (it looks like a wire). Push the light pipe and the cable back through the hole in the panel and continue until all of the switches are out. You may wish to number the cables and the switches so you can get them back correctly. With all the switches out, pull up at the bottom of the wood switch panel and remove it from the dash. Now gather up all of the switch cables and carefully pull them to the left and out of the way. See the second picture.

[to be concluded in part 2]

Jeremy5848 · #2 02-11-2007, 03:28 AM

[part 2 of 2 parts]

With the ACC and switches removed, you can finally see the switchover valves and all of the vacuum lines. The third picture shows the lines that we want to tee together.

To do this we need the thing in the fourth picture -- a pair of three-way vacuum connectors ( MercedesShop #B2025-26557) joined by a check valve. The check valve (O4040-32232) allows the footwell pod to also open the center vent pod but not the other way around. Test the pod before you install it. If you get it the wrong way, it won't work. The arrow shows the direction of air flow -- you should be able to blow through the check valve in the direction of the arrow. Note how one leg of each connector is cut short -- that's because there's not a lot of room and the switchover valves are close together.

Once you have assembled the mod, install it by removing the rubber elbow connector at switchover valves 13 and 14. The valves themselves are not numbered but the electrical cables are numbered. You want the valves that have cables 2 and 3. Notice how all of the switchover valves are connected to a common vacuum source at the top. The other rubber connectors on valves 13 and 14 are the ones you want to remove. On valve 13, the connector holds a plastic vacuum line (red with a black stripe) and valve 14 has a line that is red with a yellow stripe. Remove one line at a time and replace the rubber elbow connector with two legs of one of the three-way connectors. Then do the same with the other valve. Make sure the check valve arrow is pointing in the right direction. Picture five shows the installed mod.

Now put everything back together. After you're sure that you didn't accidentally disconnect something, put the switch panel back. Then gently pull each cable through its hole, plug the switch into its plug, and carefully plug the fiber optic cable into the hole in the back of the switch.

Next, plug the two cables back onto the ACC pushbutton unit, return the lights to their sockets, and put the ACC back in its place. Put the wood panel back, reconnect the battery, and test your finished mod.

Enjoy!

#3 02-11-2007, 03:31 AM

So far so good. I just wired the center flap permanently open in my wagon.

Now all we need is a guide for the MCC equipped 240's.

MBeige · #4 02-11-2007, 05:28 AM

Disabling the center vent flap worked for me so far, I did it when replacing the foam hose behind the glovebox. Always have cold air on the center vents, or warm air as needed.

Ara T. · #5 02-11-2007, 06:04 AM

Always wondered why there was no heat coming out from my center vents. Actually, after about 2 or 3 minutes of heat on the maximum speed, my passengers are usually begging me to turn it off, so I guess it's not really needed.

JWJ · #6 02-11-2007, 06:36 AM

Quote:

Originally Posted by Ara T.

after about 2 or 3 minutes of heat on the maximum speed, my passengers are usually begging me to turn it off

But see before that time as the driver of the car (being in the front seat) you are super heated and turn the heat off anyway. So the passengers in the backseat are cold. Its either burn yourself alive or everyone else is cold.

Ara T. · #7 02-11-2007, 07:49 AM

True, but in a minute or so the whole car becomes pretty hot. Actually it could be my car which runs about 10 degrees hotter than it should (90 deg) if my temp gauge is accurate.

pizzachef · #8 02-11-2007, 12:54 PM

Quote:

Originally Posted by ForcedInduction

So far so good. I just wired the center flap permanently open in my wagon.

Electrically wired or tied open with a wire? Is it hard to do?

#9 02-11-2007, 06:29 PM

Quote:

Originally Posted by pizzachef

Electrically wired or tied open with a wire? Is it hard to do?

I tied it open from behind. I got to it by removing the glovebox.

t walgamuth · #10 02-11-2007, 09:36 PM

somebody called me on the phone earlier today with a question about a problem similar to this.

i have no expertise with the automatic climate control system.

sorry.

tom w

DeliveryValve · #11 02-12-2007, 11:40 AM

On a friends 300d, I used a piece of a radiator hose about 3 inches and stuck it between the flap to hold it open. I basically remove the glove compartment to get at it. Works pretty good!

Sev · #12 09-09-2010, 03:40 AM

wow, rad idea. going to follow these instructions to get warm air through the center vents. thanks jeremy

libbybapa · #13 09-29-2011, 12:14 AM

I tried this today and it worked for a little while, but then stopped working and went back to the stock operation. I did a bit of testing to determine the cause, and it appears that the #13 solenoid opens the line to atmosphere when it is not energized. That makes sense as the unused pods need to be released from vacuum. Anyway, the check valve I used must have had enough resistance that when coupled with the valve open to atmosphere, didn't allow enough vacuum to the center vent element to pull the flap open. It could be done with two check valves, one in the leg to the #13 valve and a second between the two tees. That much additional hardware might be a little cramped behind the switches. I then had the thought of just adding two 12v diodes between the the two connections of #13 and #14. Maybe I'll try that instead in the next couple days.