Climate Control Servo or More?

[jtwoods4]

I may need to order some parts for my 1980 300SD Mercedes. The car is in outstanding condition with 130,000 miles on it. However, I started running into problems with the air conditioner vents LAST summer.

First, I noticed the center 2 vents closed and would not open. So my air conditioner would blow cold air out of the defrost and the far left and right vents. Cold air was still blowing.

This winter I was able to adjust the climate control wheel and turn on the heat. Used the heat all winter. Over the winter I noticed there was a small 1 inch crack in the climate control server housing. I put JB Weld on the crack to keep it from getting worse.

Now that the weather has warmed up here in Atlanta, I tried to turn on the AC today. I adjusted the climate control wheel to 65 degrees, turned on the AC button and I heard the compressor engage, however the climate control servo did not turn.

Today I did some diagnostics and I disconnected the servo vacuum and power plugs and manually connected 12 volts power right to the 2 servo power pins. This successfully turned on the servo and I heard the gear turn all the way. Assuming this would activate the AC system I plugged the vaccum and power connectors back to the servo, got back in the car and went for a test drive. Still no cold air. Heat only from the defrost vent and side vents.

Seems like the system is stuck in Heat mode and the center vents are closed and defrost vent is always open.

Are these systems of a bad climate control servo or is there more to it? (vents not opening)

[thegreg92]

My initial thought is that the electronics in the servo are toast as it has already cracked. As far as the vents opening and closing, you probably have one or more bad vacuum elements.

I dealt with my climate control last fall. I had wildly swinging temperature regulation as well as no change in airflow. It was constantly on defrost. I found that I had two leaking elements: one for the center vent and one for the fresh air flap. I eliminated the center flap and element altogether and replaced the fresh air element. After I replaced them I found that the temperature regulation was more accurate.

I would recommend vacuum testing each element and replacing the bad ones. It might need more after that though.

[jtwoods4]

OK thank you. Can you explain where the 2 elements are that I need to vacuum test?

[daantjie]

Might be a dumb question but is it actually warm enough to trigger a cooling cycle?
When the system is in heating cycle, no air will come from centre vents, they only work in the cooling cycle.
There is both an in car as well as external temp sensor which tells the system what temp to "make". If either of them are toast you will get incorrect info to the system.
With key in "ON", work the temp dial from full hot to full cold. You should hear the servo motor run in the same fashion as when it "parks" when you turn the key off.

[jtwoods4]

The servo does not turn when I turn the temp wheel. Can you tell me where the outside air temp sensor is? I have inspected the one in the car and actually replaced the foam tubing connected to it under the dash.

[thegreg92]

There are 5 elements to test IIRC. 2 behind the center vent, 2 for the fresh air flap under the dash on the right side, and one for the floors in the lower plenum. There is a vacuum hose to it that is accessible from the right side with the underdash panels removed. I have found that a vacuum leak anywhere will impair the whole system. You could try blocking them off as a diagnostic measure to see if you can get temperature regulation back online.

The outside air temp sensor is on the firewall under the hood with a 2 pin connector to it.

All the sensors including the temp wheel are wired in series and their total resistance tells the servo how to adjust. If one is bad, the whole system will not regulate.

[jtwoods4]

today I disconnected the power plug from the servo and then connected a 12 volt battery source to pins 4 and 5 and I could hear the servo turn all the way down. but the servo will not turn when everything is connected and I use the temperature wheel. maybe the small crack in the servo housing is what's wrong with the entire system and just replacing the servo will fix everything. I looked around at all the vacuum lines and connectors and everything under the dash looks brand new and in perfect condition. before I start poking around too much I'm just going to replace the servo and the amplifier. hopefully this will fix the system talk about over engineered..... give me a break Mercedes.

[daantjie]

Actually the ACC is a Chrysler system which MB used to appease the US market.
Yup, it is a POS and was installed in tens of thousands of Benzes bound for the US.

[jtwoods4]

well it's definitely a piece of shit. I'm an engineer by day and professionally restore muscle cars as a hobby. this is the most ridiculous over-engineered piece of shit system I have ever encountered. seriously I'm considering just selling the car rather than troubleshooting this.

[beagle2022]

The non-US version is really simple and bullet proof.  Fan speed control and a simple thermostat.  I am sure you could get one from an Aussie wrecker.  Seems sad to sell a great car because of this mess.

[daantjie]

Agreed.  There is also a replacement servo kit from Unwired Tools.  Have no experience with it, so cannot vouch for it.  I for one like to keep my car as original as I can, so I put up with it.  Luckily here it does not get smoking hot in the summer, plus winter is mild, so for me not such a big deal.

[Malpaso44]

Well sir I also live in Georgia and the heat here can be unbearable. After going through 2 servos I purchased the replacement servo kit. I've had no further problems what so ever! My other w116 is still originally intact.

[jtwoods4]

well here's the latest update I purchased George Murphy's performance analysis servo that has the aluminum housing. I install it in the car and I also replaced the amplifier behind the glove box. while I was at it I replaced all the hoses that connect to the servo because they look really old and original.

I started the car and like the instructions say I pressed the defrost button and made sure heat was coming from the vent. I then rolled the climate control wheel to 65 and turned on the AC and sure enough the air conditioner was working again so I took the car around the block for a test drive.

after about 2 miles I noticed the temperature gauge on the dash quickly started to rise and then got very very hot so I immediately pulled over and turned off the engine. when I popped the hood there was steam coming from the area underneath the servo on the bottom of the servo where cooling appeared to be venting and the servo was extremely hot too hot to touch.

the car is now safely in the driveway after I trailered it home with my pickup truck. I removed the servo and inspected it and everything looks OK on the outside I think I'm going to have to send it back maybe something is defective with it.

[jtwoods4]

well I finally got the climate control servo connected properly and the hose is connected properly so there are coolant no leaks. now the car will not turn off with the AC on and the blower on but if I press the off button on the control panel the car will stumble for a few seconds and then turn off.

[daantjie]

Not sure if you have the vacuum switching valve on the firewall, but if you do, one of the connections runs to the distributor so that you get "idle up" when you switch on the AC.  Might be something to do with that.  There are 2 connections on the switching valve, center connection goes to contour hose between WUR and idle air control, off center connection goes to distributor vacuum diaphragm for idle up.
Or could be the main AC switch behind the glove box that is acting up, not sure how to check this though.