Mercedes 6.9: AC blows hot air when going uphill

[jllgd]

I have an issue with my 6.9 AC system.
Its the notorious Chrysler ACI variant designed for the USA models. 

When going steady uphill, or accelerating on a flat road, the ac system switches over to provide hot air only.
I can clearly hear a sound, like a flap is closing (or opening) when this occurs.

When coming over the hill the ac and cold air comes back on.
Any ideas to whats wrong? Thanks!

[daantjie]

Likely you have a vacuum leak.  There is a large vacuum actuator that controls the fresh air flap, they are notorious for leaks.  You will need to remove the knee roll and plastic ducting at the passenger side, then look up from below and you will see this large actuator.  You can test it in situ if you have a Mityvac. Last time I checked they were still available.  If you are shaped like me, this is a very "fun" little caper trying to squeeze a 250lbs 6 foot frame into this space