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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]