No air flow from 00 SL500 center vents

[E Benz]

The 129 'vertible only has 48K mi but is out-of-warranty time-wise. The center vents were just fine one day and the next day they were shut.

The recirculate does not appear to engage either- at least there is no apparent change when the button is pushed. All the air flow is from the corner vents and the defrost vent on top of the dash. When the defrost button is pushed, the air gets hot but continues coming from the same three places. The three vent dials located below the center vents feel "loose" or very easy to turn as if not engaged and have no effect on the airflow. The iconic directional flow button and the "extra" cold/hot buttons with the red l.e.d.'s located between the center vents also have no effect on the air flow. The A/C is good and cold, heater is hot, and the climate control seems fine.

The center vents are completely shut, no matter what.

As noted in another thread, I pushed gently on the vent flap with a clean take-out chopstick through the vent grille with the system on but the flap feels very firm and substantial- no give/response at all- and promptly gave up the probing rather than risk possible damage.

Other threads refer to glovebox removal to access the center vent vacuum hose and possibly have it plugged up for permanently open center vents on other pre-SRS bag models (124s etc.). This '00 129 has SRS bag in place of glove box so that access point is probably not prudent.

Is there something to check under the dash or hood? Maybe a loose vacuum hose some where? Another thread referred to a vacuum check hose under ther hood which had a disintegrated plug or stopper- on a 124- and the dealer provided a fresh plug free of charge and the center vents responded again.

Fuses on the 129 all look good.

I'm hoping that an expensive dash-teardown scenario for vacuum element/actuator replacement can be avoided but I'm not holding my breath. Other threads have indicated cost estimates of up to $1,300.00 on other models.

I wouldn't mind a non-destructive fix that could permanently open the center vents with a significantly reduced cost. I have already resisted the chop-stick assault.

I haven't had a chance to take it to my mechanic yet- he's over 60 miles away.

Any thoughts on this puzzle are greatly appreciated

Thanks,

[E Benz]

Reprinted from "Tech Help" section:

Thank you, Brian.

It turned out to be a quite simple fix but I would not have tried to find the problem without your analysis and references to the diagram.

Your diagnosis that the system was apparently in default mode due to no vacuum pressure made perfect sense and I was actually relieved to think the fix might not involve a dash tear-down.

The diagram from the website link seems to show an inline-6 but I took another thorough look at anything coming off the manilfold (left and right) on the 129's v-8. I thought I would at least inspect anything and everything up in plain view before trying to undo the right wheelwell to find the vacuum reservoir.

Then I saw it.

A small white vacuum hose, 4 to 6 inches long, coming straight out of the upper/center area of the firewall. I also saw an apparently corresponding black rubber hose coming from behind the head just under the cover.

All I had to do was plug the ends of these two hoses together and the problem was solved. All the vent flaps fly.

I can't believe I missed that disconnected hose during the initial inspection. During my initial inspection, based on a search for a vac res hose, I was only looking around the right front underhood area and the wheelwell. When I finally saw the offending disconnected vacuum line, after reading your post, I was looking on the left manifold/firewall area.

Thanks again for you detailed response