I have a 98 CLK 320 and when I go full throtle, the vents all close up and the defrost kicks in. Any idea on how to diagnose this problem? Or even better....the fix? Thanks guys...

I don't know a thing about newer cars, but I'll guess.

Sounds like the vacuum dashpots are sensing low vacuum from your manifold and reverting to the 'fail safe' mode of Defrost when vacuum is low.

That said, I'll guess there is a vacuum accumulator that is intended to sustain vacuum for the HVAC system during full-throttle low-manifold vacuum conditions, and the accumulator or a dashpot has a leak, giving you the results you report.

In short, a vacuum leak in the HVAC tubing is where I would be looking.

I will delete this post when someone who knows this car better can set you straight, since I'm only guessing.

Best Regards,
Jim

Like Jim's post, this is a guess also, but I believe some designs use a check valve in the vacuum line that is supposed to maintain the vacuum to the HVAC system when vacuum disappears momentarily. If you find such a check valve, you should be able to suck air through it in one direction but not the other.

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95 E320 Cabriolet, 169K

I have the same issue in my 1997 E320. I agree that vacuum is probably the cluprit, just do not know where to start. Has anyone had this problem and solved it?

Vacuum loss is the problem & a VERY common one!

On the CLK try depressing the EC button & see if the center vents still close durning accel, IF not then the fresh air vacuum element is the problem!

The starting point is to find the vacuum block under the passenger under-dash panel & by using a vacuum pump check each vacuum line & vacuum element for leaks.

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I reference the WIS program and located the swithover valve block, the W210 with automatic air has 14 connections. Can you expound a little on the testing procedure using a vacuum pump. Any recommendations on a pump? I assume a rather inexpensive hand pump would suffice.

Does the dash need to be removed to access the vacuum elements?

According to WIS, it is located under the glove box. You probably just need to remove that cover above the footwell and maybe the knee bolster.