My '99 S420 has been giving me problems with the climate control system. The ventilation system defaults to defrost when I'm driving, and won't let air come out of the two center vents. I replaced a check valve in the gray plastic line that is on the passenger side of the engine compartment before it enters through the firewall. This seemed to help as my old check valve was bad, but every now and then the car still defaults to the defroster vents. I was just wondering if anyone else has experienced a similar problem.

Thanks in advance.
Dave

Not that this is a very timely reply to your post but I have had numerous diesels and now a S420 and the only thing I can think of is that there is a vacuum leak somewhere in your system. In the diesels the default for no power and no vacuum is full defrost with full heat.

I have noticed that in mommies 99 S420 that occasionally in the morning when pulling away from a stop it seems to shift over tot he defrost and then back to the footwell vents. I have not dug that deeply but my assumption is that the vacuum in these is supplied by the manifold and not a vacuum pump as in the diesels therefore it could be due to a temporary loss of vacuum on acceleration that makes this happen.

I would think though that MB would have used a vacuum resevoir on these cars to eliminate the temporary loss of vacuum incurred during full throttle acceleration.

Sorry if this doesn't help much but I am just starting to explore this S class gas thing...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Mine was doing this and I found a loose electrical connection to the vacuum distrubution thing behind the glove box..

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Bill

Backyard Shadetree Mechanic, 30 years running.

1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
On my list to buy: R129, for me, and a R107 for my wife.

I have been paying more attention to mommies car and the thing I find in her's is that it tends to change over to defrost for a few minutes or seconds upon acceleration which would lead me to believe it is a vacuum issue. Once you back out of the throttle it takes a second but then shifts back to the normal footwell heat.
I'll have to check the vacuum supply and see how much the car is putting out. I would have thought that they have a reservoir for the climate control if it is managed like the diesels.
Will let you know if I find anything interesting.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique