center vent problem 85 300td - PeachParts Mercedes-Benz Forum

kerry · #1 10-12-2002, 05:19 PM

I've been diagnosing the usual climate control problems. Got the 'no heat to the footwell' problem solved by replacing the small rubber hose that connects the plastic line to the actuator. By the way, the actuator for the footwell vents is just to the right of the accelerator. Remove the cover under the steering wheel area and it is readily accessible.
I do have a funny problem with the center vents. No air coming out, door obviously not opening. Pulled a vacuum on the line and it holds vacuum but the door does not open. I can reach up and move the door somewhat with my hand but the actuator wont move it. There are two lines going to that actuator that join in a Y right before the actuator. I'm pulling the vacuum on the line that comes off the switchover valve. Does anyone have any idea why the actuator would hold vacuum but not move the door?

Hour later---I found the problem. Someone had inserted a BB in the line to the center vents!!!! Never underestimate the creativity of mechanics! When I removed it, the line would not hold a vacuum. Previous mechanic probably thought it was the actuator leaking and plugged the line. In fact, the leak was at the three way rubber connector that joins the two lines just before the actuator. Replaced it and now I can pull a vacuum and open the center vents with a Mighty Vac.
HOWEVER, when I turn on the AC, the center vents will not open!! If I give the actuator a litle help with my hand it will open and stay open until I shut it off. My hypothesis is that the switchover valve is not working correctly and not permitting enough vacuum in that line to operate the actuator. Does this seem like a reasonable analysis? Do switchover valves fail in this manner? If not, anybody got any other ideas before I reassemble the whole thing and put off fixing the center vents until next summer?

engatwork · #2 10-12-2002, 10:01 PM

Kerry - When I was going through all this on my 300D I just went ahead and replaced all five of those switchover valves behind the pushbutton unit. You can test it - just remove it, hook up the mightyvac, put 12v to it and see if it switches over or not.

kerry · #3 10-19-2002, 12:41 PM

Update on the problem:
I tested the vacuum at the switchover valve. It is fine and pulls about 10 inches of vacuum. When I hook the line up to the center vent and T into it to test the vacuum, I can pull a vacuum and open the center vents. However, when the ACC operates, what happens is that the vacuum increases up the word 'Zero' on the scale on the Mighty VAc. When it reaches the 'o' on zero it stops and does not open the vents. If I give it a few pumps on the might vac up to 1 or 2, then it increases up to 10 on its own and opens the vents. If I release that vacuum thru the mighty vac back down to 1 it will build up again and work fine. If I hold the release until it goes all the way back to 'z' on zero and hold it there for a second, the same problem arises. It will only go back up to 'o' on zero until I give it some help with a couple of pumps.

If I disconnect the line from the actuator, and test the line, it works fine. If I kink the line to the actuator, let it build up a lot of vacuum and then let it go, the actuator will function on its own.

So, with a small amount of vacuum in the line, the actuator seems to be leaking. My increasing the amount of vacuum or pushing in the actuator about a quarter of an inch, the leak stops and the actuator holds vacuum until it is completely deactivated.

Can someone familiar with the inside of these actuators speculate about what might be causing the leak when the actuator is fully at rest? And further, what has to be done to rebuild them? It looks like the back has to be removed by releasing the three plastic catches. Can this be done in place? If so, has someone done it by feel on the center vent actuator without removing the bottom half and the actuating rod? It seems as though that would be the easier method compared to removing the dash.

kerry · #4 10-20-2002, 10:50 AM

TTT

Surely there's someone with ACC vacuum experience that can comment????

psfred · #5 10-20-2002, 09:28 PM

Kerry:

Either a bad diaphram on the actuator or servo, or a leaking hose (rubber connector) that leaks until you pull some vac on it. Replace the rubber hose and see what happens. If that fixes it, replace all of them, they are all leaking.

You will need to check that the actuator allows enough vac through, too -- connect the Mitivac in place of the servo and see what happens.

Peter

kerry · #6 10-21-2002, 08:15 PM

Here's my solution to the problem. (With thanks to another forum member who came up with a similar solution to a similar problem)

It's a quick fix until I get another reason to remove the dash and replace the actuator. Since the leak was only occuring when the actuator was in the completely closed position and worked fine once the actuating rod was pushed in a quarter inch or so, I took a piece of approx one eighth inch thick flat aluminum stock about six inches long. I bent a 2 inch section of it back on itself into a U kind of shape. I removed the rubber boot on the vent and put this down into the center vent ducts while the flap was open so it is hooked over the edge of the ductwork. When the flap closes, it can't close all the way due to the aluminum stock in the ducting so that now the actuator works just fine. Of course, the flap does not close completely but it is very close to being shut off and if I want to completely shut it off I can use the manual shut offs at the dash vents.

The fix is neither perfect, beautiful or elegant but it is functional and almost certainly the equivalent of the engineering that necessitates the removal of the dash to replace a vacuum actuator.

psfred · #7 10-21-2002, 10:35 PM

Sounds a whole lot better to me than no dash vents! A little warm air out the dash vents in cold weather won't bother you at all, I don't think, and for AC, they are open anyway.

Peter

Alan S · #8 10-21-2002, 10:37 PM

I had the same problem, so I ended up shoving approx. 1 foot of clear rubber hose, which you can find at any hardware store for about a buck a foot, down one of the center vents to manually keep the door open. So now everything works, and is invisable from the cabin. If I don't want air out the center, manually twist the knobs on the eyeball vents.