When mine failed it was full heat all the time. It was crusted to the point that I couldn't believe that it even worked to move up and down.
Maybe this is part of the reason why some fail open?

I have always been led to believe that the default on these is full heat and defrost if you have a power and vacuum failure...

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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