On my 1985 300SD I am experiencing an issue where the only three vents that blow air(be it AC or outside when set to that) are my side vents and the rear vent. My center vents just blow a tiny bit of air and if I use the slider on the dash it will stop that. My defroster vents also blow just a tiny bit of air. I am not sure on the floor ones, never noticed air from them but also never put my hand against them to see if at least a tiny bit of air comes out of them as well. This condition is the same regardless of which setting I have the HVAC controls on with the exception of if I have it set to off. I originally thought that perhaps this was an actuator issue and while I have still not dismissed that it would seem after reading about those through various online searches that if an actuator went bad it would blow through the defroster.

I doubt this is in anyway related to my issue but this car has had it's mono valve bypassed since the day I got it, the original owner claimed to not know about that so was of no help in providing history on why that was done. A dealership diagnosed that and told me it was a bad heater core, but I opted not to do it because of the extreme cost to pay someone to do that job. An independent shop I asked to verify if it had a bad heater core while they had the car for some suspension work. The independent shop said that the heater core is not bad but something else within the dash is preventing the heater from working when the mono valve is hooked back up.

The lack of heat is not a primary concern of mine right now but just offered that information in case it in any way, shape or form could have relationship to my vent issues. The vent issues are becoming a tad aggravating this week due to the around 100 degree days we have been having.

Thanks for any help, I did try searching prior to posting and just found an old post about a 500 SEC with a similar issue on the center vents but did not see much in regards to help or completed help.

 

Have similar issues with 1990 350SDL. Can only make air come out defrsoter regardless of settings. Can't get air out of font dash vents at all. Have no clue where to start. Sounds like can of worms.

 

From what I read only out of the defroster means one of the actuators has failed and it is in fail safe mode. I know the actuator behind the glove box can be gotten to with ease to check, not sure on the others though. I think ultimately I need to pull the dash out of this car but not something I look forward to and wish I could find someone local to do it at a "reasonable" price.

 

Thanks for your swift reply, my friend

 

Most likely, from my experience, the actuator pods (there are 5) have failed. There is one right behind the two center vents that operates the cetner vents. Easiest to get to by removing the dash, which isn't as hard as it sounds. The actuator for the defrost is ( I believe) the one that is above your leg when sitting in the drivers seat. Instead of replacing the entire pod you can also replace the diaphragms. I think this is the contact info for replacement diphragms:

PERFORMANCE ANALYSIS COMPANY. 969 OAK RIDGE TURNPIKE, SUITE 258.
865-482-9175. OAK RIDGE, TN 37830. (Voice or FAX).
Mr. George Murphy.

I repaired some of mine by fitting a regular round balloon over the existing diaphragm putting the open end around where the metal rod comes out of the actuator and sealing by tightening with a zip tie, and cutting off the other end othe balloon.

 

Thanks, I originally figured it was a pod but then dismissed when I got to reading that when one goes bad it is supposed to send everything to the defroster.

Are there any online resources(or even printed that I could buy) explaining how to pull the dash on a 126 chassis? I have found some very good write ups but on other model/chassis Mercedes.

 

Thankyou so much my friend

 

I have a 123 body style. On teh benzworld.org forum I found good description of how to remove the 123 dash. Probably very similar. Open doors and find the bolts hiding under a thin flap of plastic from side. Remove panels under dash and remove screws going into the side of the center console. Remove the wood cover where the sunroof and rear dome light buttons are mounted and remove screws from behind that panel. I think that is about it. Once all the screws are out it slides out easily. If it doesn't, keep looknig for that hidden screw. you will also need to remove the instrument cluster. Easiest if you reach from below and push it out. It is also easier if you remove the steering wheel, or drop the steering wheel down bu removing two bolts that are revealed when you remove the isntrument cluster. It isn't as hard as it all sounds. I removed, repaired and replaced dash in a couple of hours. And I didn't even have any bolts left over!

 

I am guessing by your description of the job that this article specific to a 123 is over complicating the job?

http://www.peachparts.com/Wikka/W123Evaporator

I only say that because when I skimmed through it, it looked like it would take a lot more than just a couple of hours.

 

the defroster actuator is located on the right side of the glove box when removed... When you push the button on the dash does it go to defrost??? if not you could also have a vacuum issue possibly a busted hose that needs to be checked as well. Not all cars go to fail safe of defrost only that usually happen when u have more than one go bad because mine did that and the only one that worked in my car as the floor actuator at that point and had to replace all of them because the floor actuator would only work some times but it could be a vacuum issue and as for your heater issue there is a sensor under the hood to the right of the battery make sure that all of the plugs there are hooked up and if that sensor is bad it will cause the heater not to work. If it is hooked up un hook it and check and see if your heater works

 

My defroster never works beyond a slight trickle of air regardless of what button is pressed with the exception of the off button. How does one check an actuator to see if it is working or not?

The sensor you are referring to is what I thought was called the mono valve. I had read how the diaphragms on those will tear with age and the heater will not work any more. I did pull it apart and check for a physical tear. I think the sensor itself was tested by one of the mechanics I took it too but I honestly did not specifically ask them, I just know they did something in that area by visually seeing it and they did disconnect the battery at that time for whatever it was they were doing.

 

1) Is their a possible connection between vent issues and a schredded serpentine belt that over heated the engine for maybe one or two minutes? The vent issues happened shortly after. Now I know I risked cracking the head when I briefly over heated the engine. I put a new belt on and everything seemed okay. It didn't smoke or start using lots of anti freeze without noticeable leakage (indicating hairline head cracking between cylinders). Lately I've noticed it runs pretty hot most of the time. It doesn't go in the red zone, but it flirts with it going up mountain passes, running 3/4 to red zone.
2) Could the snafu heating system have a connection to pretty hot running, but not quite overheating?
3) Note that as mentioned earlier, AC doesn't work regardless of setting and system blows heat out of defroster only regardless of temp setting on dial. Engine runs hot regardless off outside ambient temperature (whether it's 60 degrees f or 95 - always runs that 3/4 plus to redline of temp gauge).

 

Have a pic of where it runs at temp wise? I thought mine was running too hot but then after looking around a good bit noticed that where it sits at is pretty normal.

 

Forget to ask about antifreeze flush, A) What is procedure for flushing and filling 350SDL? B) Is it safe to back-flush cooling system through heater core output hose? I used this technique successfully to fix heater core blockage in different car. C) If so, which hose is out put?
D) Is there any way to completely empty the radiator fluid not just from radiator but from heater core (without draining engine core through freeze plugs? Last time I drained and filled radiator, it seemed like I put in way less than listed spec capacity in manual. This made me think I never got close than 60 or 70% fresh antifreeze mixture in. Also made me think there's a possible heater core blockage in addition to actuator problems.) Thanks in advance for your thoughtful relies! SC

 

There's several lines of demarkation without a number that go from zero to redline. It runs at the last line before redline to half way to redline. Like it's 3/4 to redline (last line) on flats and 4/5 of gauge range redline on steep grades. I really worry about running so hot, the two summers before, it ran much cooler like at the half way mark on the range. I cracked the head on different 300SDL going up Teton Pass with a heavy load of gear. Don't want o do that again.

 

Yeah, sounds a bit hot to me. Here is how mine is when sitting in stop n go traffic for my guess of 20 minutes at least.

 

from your picture, the next demarkation line above needle is where mine sits. On mountain passes it goes to half way from that notch to start of redline zone. Yesterday I drained radiator and put in Redline Company "water wetter", a pink liquid that lowers rad. fluid temperature in a 50/50 mix of antifreeze. It seemed to work and now my gauge reads like yours does on the flats. As noted before, only can get one gallon of fluid in radiator, after draining. Spec capacity is much higher than that.

 

I replaced my radiator a few months ago, prior to my picture being taken. I was told at the time to get three gallons of Mercedes anti-freeze and mix 50/50 with water and that would be if replacing all fluid in the system. I initially got about 1.5 gallons of the benz stuff in there and then one gallon of water and I think actually had a little bit too much in since my overflow did slowly drain over the following week. I did not try to get all of the fluid out of the motor though and knew at the time it was parked that pretty much had 100% water in it since I had to keep topping it off with water when I limped it home. When I replaced the radiator I also did the t-stat and the short hose, those along with the fact that I doubt the motor was full of fluid is probably why I was able to squeeze in more coolant/water.

 

thanks aaron

 

I still do not know which actuator(s) are causing my vent issues but I did recently plug the vacuum line from the engine compartment that feeds them. Discovered I had such a vacuum "leak" to them that my transmission was not getting enough vacuum and shifting harder than it should. Plugging that in a way helped my AC issues since it in turn caused more cold air to head out of my defroster. Probably going to get the actuators and heater fixed by a mechanic in the next month or two once it starts to get "cold" outside. Plugging that did make a world of difference in how the car drives although when the transmission is cold it now hangs some going from 2nd to 3rd but not going to worry about that until I get these other vacuum issues settled.

 

It's kind of weird that I noticed slightly harder shifting from 1st to second around the time I noticed vent issues. Just got a tranny filter from Mercedes and wanted to change fluid. Is it true that MB brand fluid is the only one you can use on 350SDL? I'm having trouble locating drain plug for pan and for torque convertor. I wish I could find a good picture of which bolts are the two drains.

 

Hard shifting out of first gear is exactly what I was getting, plugging that line solved that and it is night and day in regards to how it shifts now. I also sometimes got a hard downshift into first but not near as hard as the upshift. I just did the tranny fluid and did not use MB fluid, never used it in my old 500SEL either and never had a transmission issue with that car. I think maybe the diesel giant website has some pictures for draining the transmission but if they do not sure if there is a difference between models of cars.

 

Thanks Aaron, Is there a picture of the vacuum line in Question? Not sure where you're talking about.

 

Drivers side of the firewall but I do not have a picture. Plus would imagine given the year and model differences that it would be different on your car.

 

found a broken vacuum line near engine and sealed it. Found a second one that was cracked and sealed that.