W126 heat/AC blowing at same time

[merbz]

I have a 1987 560 SEL and am having issues with the climate control (surprise!)

The heat and A/C blow at the same time
Makes a loud "clank" noise when 'switching' from heat to A/C (even though both blow at the same time)
Will not stay off, even when "0" is selected it comes off/on intermittently

Any suggestions are appreciated, I've tried switching climate control units and that doesn't make a difference so it's not the unit itself.