R129 Blower Air Interupted on Accel

[Mike Miller]

A recent post by another owner about his blower problems reminded me of my question. My controls all work perfectly and the blow is fine at all fan speeds. HOWEVER, under moderate to heavy acceleration, the air will stop coming out the vents until I let off the throttle. Then all returns to normal. It seems like the diverter flap is powered by engine vacuum. When I accelerate hard and the vacuum goes to a low point, it appears like the flap shuts off air to the cockpit until I let up and vacuum returns.

Is this normal? Or has MB been clever and decided to interupt A/C and blow operation to allow all that awesome power to go to the drive train? If that is the case, it might be a little overkill.

[tecqboy]

It's not normal. You may have a bad vacuum check valve in one of the numerous vacuum lines which operate the various flaps and vacuum motors. Having said that, I can't offer any specifics, as it is a very complex system.

[Mike Miller]

Thanks. I will begin the search.

Anybody want to chime in on the most likely suspect??

[Mike Miller]

I took a look at the A/C vacuum system on ALLDATADIY. Yikes! Where do I start? I am leaning toward the vacuum source rather than any individual system within the system. All work fine until I accelerate moderately. Then the air just goes away until I let up on the throttle. Then everything is back to normal.

Where does the system pick up it's vacuum off the engine?

[ArgeMan]

I have been experiencing the same symptoms on my '80 450 SL. I've gotten as far as understanding it's a vacuum issue but don't know where to begin. Did you get very far on your repair? Any word of advise or suggestion as to how I should proceed?

Thanks.

[Mike Miller]

I have been living with the problem. It is low on the priority list as the AC works great and it is just a momentary air flow interuption. I will get to it one of these days.