Mercedes-Benz Forum banner

w123 - complete climate servo bypass with a/c

9075 Views 14 Replies 6 Participants Last post by  UncleDirtnap
I've read every climate and servo thread I can find, but still don't have enough to piece together what I need.

First, the car - 1980 300cd, 110k. It's solid but the florida sun and 20 years under an oak tree has left it with some rust, terrible rubber, and lots of neglect. It's my daughter's 'learn to work on cars and drive it when I'm 16' car. It's nice enough to be nice but not nice enough to restore, if that makes any sense.

The climate control system is toast. Defrost comes on, but that is it. The servo was a solid corroded mass of nonsense inside - no moving parts inside at all, like something recovered from the titanic. I removed the servo and plumbed the heater core in manually.

Now that the days are in the 90s, I need to go the other direction and have some AC, but this is where I can't quite find the info I need. I have a handful of colorful vacuum lines, the electrical connection to the servo, and a bunch of questions.

Does someone have a document that details the outcomes of the buttons? i.e push bottom button, green and red wires get power, and vacuum goes to yellow and blue? I know that the servo controlled things like fan speed and recirculation, but the rest should be constant.

If I can push a button, have the fan come on full, air route to the vents, and the AC come on, then I will be happy.

Ultimately, it would be fun to reverse engineer part of the servo with an arduino and some small vacuum selenoids, but thats later on.

1 - 7 of 15 Posts
Thank you all! This is all the information I need.

There are always other options - an arduino, some 12v relays, a temp sensor, a hot water selenoid, and some vacuum solenoids should get you 100% there for less than $75 and give you computing power to spare (keyless entry, anyone?) but you'll be building it yourself. If thats your idea of a good time, then awesome-

If you have a gorgeous stock example, it seems wrong to 'hack' it- just fix it right. But if you just spent the weekend welding in new floors and are debating trying to get the sunroof to ever open, maybe your w123 won't be any worse off :D

Does anyone know where under the dash the vacuum switches for the blower and AC are? I see them on the schematics, but can;t find them in real life. I guess it was a 1980 and below thing?

Thanks! I think that was for a later year. Usually the pages with the diagrams says something along the lines of 'unless you have a 79 or 80, this should work for you' :)

I'm going to end up pulling the center console anyway, I just need to bite the bullet and accept my fate-

I'm know I'm kicked a slighty old horse, but I had a status update and question.

First, I can get the compressor to come on and can have cold defrost. Yay! I assume the solid state servo replacement folks are going down the same path, so I followed their install for a time and bypassed the amplifier and have configured the vacuum lines the way they specify.

I'm at the point where I need to figure out the blower, and I am lost. Looking at the functional diagrams, there is a number 19 'Vacuum switch - Main' that I need to evaluate. Any idea what they are referring too?

I'm going cross eyed zooming into these diagrams. I want the fan on full, so I need to connect pin 6 and 7 from the servo in the functional diagram....I think.

Looks like you have the classic servo bypass, done by folks in colder climes. The gizmo undoubtedly turn flow through you heater core on and off - basically, full on heat, or not. I completely bypassed the heater core myself, although this last winter, I did try it out for a bit for kicks. It seems to oooze heat constantly, so you will probably want to figure out the off position of your gizmo.

I had to toss a belt on my AC as well, but it is working - I just can't get the fan to work except for defrost. Defrost is it's own circuit, and the normal fan buttons go through a labrythine path through servos and mystery vacuum switches to bring power to the blower. I'm still figuring that one out.


I had a small delay while I worked out vacuum issues (hose fell off behind center console). So to review, I had no servo in the engine bay (I removed, it was a solid hunk of corroded metal inside), i have bypassed heater core (Florida), and have put a fresh belt on my compressor. Defrost only would work, but the AC would come on with defrost regardless of switch.

I followed the steps in the ACCII installation manual as a starting place - I removed the amplifier and connected the connecter pins as directed, and connected the vacuum hoses in the engine compartment as directed in the manual. Since I didn't actually HAVE a ACCII, I connected the purple/white hoses to each other just for kicks.

Lo and behold, it works! I can turn on high or lo with the compressor AC switch working. bi-level doesn't seem super effective, but at this point life is beautiful. I have AC and some fan speed control (high is higher than lo, but not as high as defrost. It's a good balance between airflow and noise).

Come winter, a manual valve plumbed in to the heater core would be the fancy solution. Last winter I just hooked the hoses back up again for heat and left it until February when it got to warm. Other folks may not have that luxury.

If I was going to be extra special, I would get a 12v value to control the heater, a relay to drive it, and a arduino with a thermoprobe under the dash. You can refine from there, but the guts would be there. I may not bother at this point. What is interesting is how unimportant the wiring connector is in the engine compartment. I understand that I am at 20% of true functionality, but still.

I am also confident that the ACCII product is fantastic and would restore full functionality.
See less See more
1 - 7 of 15 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.