1985 300sd (126 body); climate control unit problem...

switchover valve #3 only getting 8.79 volts in heating mode. Gets 11+ in cooling mode. I've swaped CCU's (refurbished) with no change. I've swaped Elec Temp Control box (an old one I had) with no change. Installed new Aux. Water pump. Monovalve works fine.

Where else should I look?

Wife's car, can't put it back together till it's fixed (or I'm dead!)