I have a 300e that blows Hot Air out the defrosters no matter what I do. Is this a Climate Control problem or a vaccum issue, or?

Any help would be greatly appreciated.

DeFaults to DeFrost when BAD
(DeFrost is the only HVAC function the Engineers know you NEED)

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'84 300SD sold
124.128

Thanks for the reply.

So are you saying the climate control unit is bad? The whole unit or only parts that it contols. I have seen other threads talk about heater values.

If so, does anybody know where I can get documentation on how to replace as well as the unit?

I have the same problem. Heat just comes out defrost.
Good DIY about blower motor removal. I think that's where our problems are. After cleaning engine compartment with compressed air my unit fired up for a couple of times (I mean all functions worked fine) then zip! I think maybe I got the squirl cages to turn by shooting air down a vent and it unstuck it some how. I'm going to tear into it this weekend.

Since it is blowing hot air, the blower motor is not the problem.

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J.H.
'86 300E

It could easily be a vacuum issue.

The defroster is the only vacuum pod that defaults to open, as a fail-safe. Clever engineering if you discount the fact that these fail safe so often.

So, lack of vacuum to the switchover valve could cause this as well as a bad switchover valve, a bad ccu, or multiple bad vacuum pods.

What I would do is this:

Get a mityvac. Make access to the switchover valve. There is one vacuum hose supplying vacuum to the entire system, it's the one on the end. The other hoses go to the various vacuum pods. Start by removing the vacuum supply. Plug the hose. Attach the mityvac to the vacuum supply input of the switchover valve, and suck it down.

Now try the climate control. If it functions now with manually supplied vacuum, then the problem is a vacuum leak upstream of that supply hose. If the syptoms are the same, then we need to look further. First, does the vacuum stay down, or does it rise slowly. Keep in mind that it will drop when a valve opens, but if you suck it back down after turning on the CCU it should stay there as long as you are not changing the vent selections.

If the vacuum starts bleeding off, then there is probably a leaky vacuum pod somewhere. You can use your mityvac to test the hoses to the pods one by one until you find the leak(s).

If the vacuum testing shows no problems, then it may indeed be the CCU.

Let us know what you find with your vacuum tests, that is the place I would start.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

Maybe BLOWING is the wrong word. Hot air comes out the defrost slowly. Pushing the fan speed buttons does nothing, zip!!!

Thanks for all the input.

To be clear on the problem. Hot Air blows out the defroster ALL the time. So I don't think its a fan as it is blowing. When I turn on the A/C, it still blows hot air out the defosters.

So I think the vaccum is a good place to start. Thanks a bunch. I will update with my results. It will be a few days as I need to get an minvac and my daughter to stop driving long enough to work on it. LOL