PeachParts Mercedes-Benz Forum - View Single Post

tinypanzer · #6 03-23-2009, 07:51 PM

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.