W201 hot air out defrost & side vents when AC is on
 

1990 190e 2.6 Recently the AC has started blowing scolding hot air out of the side vents and the defrost vents while the center vents are ice cold. So cold in fact it spits water out from the condensation at times.

Any ideas on what might be causing this?

vacuum
 

sounds like the heater valve supply vacuum is intrerrupted, no vacuum equals valve open and full heat at side vents

Try the defrost button. Thats the one that has all but the water valve vacuum actuator in their no-vacuum state. If any of the vacuum actuators, except the water valve, are leaking you will loose system vacuum and get heat when they are activated.

open the hood and look for a black rubber 90 degree curved vacuum connector just under the corner of the windshield on the drivers side. they tend to rip there...

it is definitely a vacuum problem
 

OK, we're on the the right track it is definitely a vacuum problem. I put a gauge on the intake manifold with the system off and was around 8. With the system on, sank to 5. I also noticed the "economy" gauge reading close to the red while idling in neutral with the A/C on.
I then removed the main vacuum line from the rear of the intake and plugged the manifold. This left the interior without vacuum feed from the engine. The controls responded in the same fashion. (no response).
I then re attached the vacuum line at the manifold and checked vacuum at the "Y" splitter above the fusebox by the windshield. I also checked the vacuum lines that attached. Surprisingly they are all in good shape and there is ample vacuum at the lines.

I read the post about checking action while the defroster is on, but I am unable to follow. When I push the defrost button the fan goes to high and I get hot air out the vents in the same manner, just more out of the top vent. It seems the side vents are always on, no matter what the combination selected.

So I guess the next questions are;
Where to start looking for a leak?
How many vacuum valve flaps are there?

There are 3 vaccum valves situated around the center console and the only way to get at them is to remove the dashboard.

Remove the entire dashboard?
 

I've also been reading about a heat control valve that governs the hot water flow from the engine. I've seen pictures but do not know the location.

I'm hip to removing the dash if necessary, but am wondering if there's anything that might break on removal. This would also give me the chance to fix the trip odo spring.. ;O)

Any reference to legible vacuum/electrical diagrams would be appreciated. I have a subscription to alldatadiy.com but none of the diagrams are worth a $hit.

Success Cold Air!!!
 

For those of you in cooler climates you may not be able to appreciate this unless you climb into a sauna fully dressed and then go to work for the day...

I've spent all week climbing into a car that's been blasting me in the face with scolding hot air all week. It's been in the mid 90's and the temp reading in the 100's when I get into my car. It's in the upper 80's right now. (the best time to work on the car is at night) With the AC on I was getting cold air out the center while hot air out the top. What a tease that was. I found that having the windows down and the sunroof open was more efficient.

After doing some reading on alldata I searched around and looked up the heat control valve that carson356 mentioned and came up with some adjustments. :bulb:

I found this:

Adjusting Mixed Air Flaps

NOTE: For this heating system the heater output is controlled by the position of the mixed air flaps and not by the position of the water valve in coolant circuit.

Open fresh air flap. Set control switch for heater to position "open". Set adjusting screw in such a manner that the mixed air flaps are tightly closing in position "max. heating".

I pulled out the 4mm and adjusted the center flap by two full turns in and all started working as expected. I can hear the flaps inside moving, the air shifts from top to center as expected.

It is for this reason that I really love MB...
In my experience, it's always something simple.

Thanks for all the replies!

Every one I've seen doing this is because the vacuum motor behind the instrument cluster is bad.

Well... Limited success anyway. It made it half way to work this morning and started doing the same thing again...

I my wanten haste perhaps I stumbled across a combination that limped it into working for a bit.

I have changed one of the AC relays, so suppose I should change the other and see if that has any effect.

I would like to know more about the vacuum motor behind the gauges and what it does...

When I mentioned the defrost, I meant the second button from the left, not the super defrost button. My problem was bad floor and defrost vacuum actuators. When they were commanded on the whole system lost vacuum and the water valve opened and the heat came on. Pressing the second button from the left had the defrost vacuum actuator open (no vacuum) and the heater vacuum actuator closed (also no vacuum.) That allowed the good water valve vacuum actuator to close. The heater vacuum actuator is behind the radio and the defrost vacuum actuator is behind the speedo cluster, and is a double actuator.

Another trick I did was to disconnect the vacuum just after the check valve and run it directly to the water valve. If your actuator is bad you could mechanically hold it closed.

I've been poking around and haven't located a "water valve vacuum actuator".

Can anyone tell me where this is located?

Thanks!

The water cotrol valve (to control the heater) is between firewalls located under the hood. This is the same place that holds the wiper motor.You remove the plastic cover to access this area. To acces the vacuum for the defrost vents you need to remove the instruments. This vacuum unit is a small rectangle about 2X3X4. To remove the instruments first remove the left hand vent. Pull out the pannel 1 to 2". With a small hand reach into the vent and disconnect the speedo cable. There are 2 more vacuum elements behind the radio and center section. Any vacuum leak will give all heat in defrost.

I found the water control valve and sure enough the elbow was rotted. I replaced it and am still having the same issue. I checked the valve and it is properly moving. (not sure if it's stopping the water flow, but the lever is moving)

I'm going to put a vacuum pump on the control system and check for leaks. I'm assuming that something is leaking pretty bad.

Vacuum Leak
 

In my recent vacuum follies I have noticed that some lines were tighter than others. When testing with a vacuum pump looking for leaks, I discovered several of the lines I just replaced were leaking! Umm.. I just replaced those...

Appearantly there are two sizes of vacuum hose that are close and will fit, but leak interchangably.

I changed out EVERY single connector I could find using the tighter smaller hose. That cured most all leaks and upped the vacuum (economy) gauge slightly. I'm still getting a vacuum leak from the interior past the "Vacuum Motor" above the fusebox by the firewall under the windshield, so I will start to pull the dash apart when I have the time and it isn't so hot outside. It's sweatsville out there lately. I actually burnt my hand on the steering wheel yesterday. Just enough to have a funny feeling callous on my fingertips...

Meanwhile I've installed a "T" and a vacuum check valve directly to the water control valve to help control the heat in the cabin and THANK GOD it works for now... No more heat in my face.

Aaaah. The price we pay for palm trees.
;O)