Don't know how much response I'll get but here goes.

When I have climate control on, actually just vent since the AC is still defunct but EC is not illuminated, the position of one or more of the doors inside the plenum under right side dash will change position under various acceleration rates. If I push the car hard I hear a definite change in something under the right side dash, but when I approach say a light and am slowing down I hear it return to where it was, or at least change again.

I assume this is most likely a vacuum issue that I'll need to tear into and find but I thought I would put it out there for comment before I started in on it.

Unfortunately the W140 FSM is not available on the net like some, and I haven't brought myself to the point of forking over the money for one yet.

I will say that it would appear that one or several people that have worked on this car in the past, before I bought it, should not have, so the problems could be entirely man made and it wouldn't surprise me in the least.

Any help or advice would be great, and if what I've written isn't entirely clear LMK.

Thanks
Steve A

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

I think you hit the nail on the head. Revving the engine would crank up the vacuum pump overcoming a leak threshold that prevents the vent door from changing position. I'd venture that once a certain RPM is passed, whether under hard or mild acceleration, that the door would change positions and stay there until the reverse threshold is passed.

It could be as simple as a loose connection on a changeover valve, or a pinhole leak in a pod diaphragm. You won't be able to tell until you isolate down to the vent and break into the vacuum system.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

There should be a manifold from which come all of the vacuum lines to the various pods. In the early W124 it is accessible through the glove box. I don't know the 140, don't know if it has an airbag in place of the glove box. In any case, if you can access that manifold, you can pull the lines one by one and test them with a Mityvac or other vacuum testing tool. That will tell you if any of the pods have leaking or ruptured diaphragms.

From there it is a matter of replacing or rebuilding the defective pod(s). Some are easy to access, others require pulling the dash, unfortunately. In any case, the first thing is to find out which ones are bad. There may be ways to work around a defective pod so that the system mostly works and you don't have to pull the dash.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Thanks for the assist guys. I will pull the lower dash cover under the glove box. There is a passenger airbag but also a glove box below that which I think would be closer to the plenum / heater housing etc.

As I said, there have been some pretty inadequate repairs done to this in the past and from what I have found that extends both in and outside of the car.

Steve A

__________________
Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others