The a/c unit air volume in my 1996 s420 works perfectly until you accelerate, and then the air volume from the center vents goes down. The outside right and left vents work perfectly no matter what speed. When you see the vacumn guage (fuel economy) in the dash and it's about 10-15 inches while accelerating, that's when the flow of air slows down. Also my glove compartment button on the dash doesn't work, is it vacumn operated also, and would that have anything to do with the a/c air flow. Should I be looking for a vacumn leak? Where would you suggest I start looking?

I have a slightly older MB(see signature) with a similar problem. When I accelerate, the air volume from the center vents drops off. Once the accelerations tapers off, the air flow returns to the center vents. During all of this, I never lose air flow to the side vents. My MB has a fuel economy gauge on the dash. I've noticed that when I accelerate , it pegs and that's when I lose center vent air. When this gauges needle drops back more to the other side, center vent air returns.

It's been said that a gas engine develops more vacuum at idle or slower speeds. As you accelerate, there's less vacuum.

In my case, there are a number of vacuum pods within the dash that control air flow, etc. I suspect that what I have is a weak pod. I also get air flow out of the defroster area when the defroster cycle is switched to off, so I have one or maybe more than one weak vacuum pod.

I wonder if your model is also equipped with these vacuum pods?

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Sounds like your description is the same. My thought is that the door that controls air flow to different areas is vacumn operated, and that there is a leak, hence the lower volume because the "diverter"door is not all the way open, but I don't know that for sure, that's why I am seeking advice here amongst all these tech gurus! Mybe one of them can give us a place to start on both our cars to seek out the problem.

I am having the same problem with my S350. Must be a vacuum leak somewhere. The diesel has a vacuum pump and does not use engine vacuum; and the pump is relatively new. All the EGR and turbo boost controllers are removed; so all that vacuum goes to is the a/c, brakes, and engine kill. Any ideas??

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K

Be aware that these pods do not really "get weak."

The diaphragms inside tear and they leak. Then you lose vacuum, the leaky ones fail and it also affects the ones that don't leak.

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2012 E350
2006 Callaway SC560

I can understand the pods leaking all the time and the air not going to the right places all the time. What puzzles me is it only diverting the center outlet air to the defrost windshield vents under acceleration. When you let off the gas, it immediately blows harder through the center outlets.

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K

It can depend on how big the leak is in the diaphragm. If you have a small tear in the center vent pod, for example, it could do just as you describe.

The default setting for the whole system is defrost vents open. You must have vacuum to close them.

W124 climate control vacuum pod replacement

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2012 E350
2006 Callaway SC560

On my "to do" list is the pods (I'm having the same problem with the vents) and since I'm going to have the dash apart I'll tend to all of them (4 I think). maybe they'll go another 16 years

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1991 300SE (my ride, 279,000 miles, looks almost new
1954 Cadillac (21 yo son's car, he bought when age 15)
1972 SeaBird 19 ft runabout (old but solid, slant six, Volvo sterndrive perfect condition, undergoing complete overhaul and refit)
1998 Toyota Rav4 (my sons daily driver when he is in the Continental US, PROUDLY serving in US Navy)

I would also make sure that your check valves are working properly - the vacuum reservoir should maintain the vacuum for a significant amount of time, even if you have a slight leak, unless the check valves are not working, which will allow the vacuum to bleed back towards the engine under heavy acceleration. I had two bad check valves in my '79 450SL, they were passing air in both directions. When I replaced them with new ones both my heater system and the door locking system worked much better.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

No "it's been said" about it. A gas engine develops considerable vacuum at idle, and more at cruise. Almost none during acceleration. At full throttle, any vacuum generated is working against your goal of getting as much air as possible into the engine.

I could understand it better if I had a gas engine, but I have a diesel. Vacuum is gererated by a gear driven pump on the front of the engine. I would think that the faster the engine speed - the greater the amount of vacuum. Upon acceleration (not necessarily full throttle) the air immediately diverts from the center vents to the defrost vents. Let off the pedal and it diverts back. Looks like a leak in the system would be the same always, not just at acceleration. I have noticed, especially at night, that the windshield fogs up on the bottom by the vents and has done this for years. I think I have had this problem a long time and it has just recently gotten worse. I noticed something was wrong on a hot day driving on the interstate, everytime I let off the gas the air would all the sudden blow harder out the middle vents. I ordered a Mity vac kit and as soon as it comes in, I will get to the bottom of this problem and post what I find. I attached a file of my hose routing but I have removed all the ones going to the egr and vacuum transducers associated with turbo boost regualtion.

Attached Files

Vacuum Diagram.pdf (40.2 KB, 369 views)

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K

Ok, I received the Mity vac and did some testing. No leaks from the pods under the dash. Had some minor leaking at some hose connections under the hood that when fixed helped the problem but did not cure it. I was just not having enough vacuum under acceleration. I think it was bleeding some off in the injection pump where the vacuum line goes in there, I think for the engine kill. But I got to thinking about a '76 Olds Cutlass I had that had a vacuum ball on it. It was a vacuum storage ball with two nipples on it. One going to the pump and one going to the vacuum devices on the car. I went to Napa and ordered one and installed it yesterday. It completely cured the problem. No drop in vacuum under acceleration inside the car now at all.

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K

Good thinking!

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2012 E350
2006 Callaway SC560