1992 W124 (400E) A/C setting blows hot air from the side vents while getting cold air from center vent.

I've tested all the A/C system sensors and components. I've performed all 21 tests in the manual. Everything tests OK - except the monovalve remains open (i.e. 0v = no ground = open = water path to heat exchanger = full heat = hot air from the side vents).

I measured 12V to body ground at the monovalve connector (power to monovalve is OK), 0 ohms between the monovalve connector and LHS pin 9 on the control unit (monovalve ground wire is OK), grounded pin 9 with A/C on to force-close the monovalve and got cold air from the side vents as expected (monovalve is working OK), put 12v directly to the monovalve itself (again, monovalve solenoid engages, OK). But when the climate control unit calls for cold air, the monovalve (pin 9) never sees a ground signal (even at max detent A/C, auto setting).

I’ve investigated both sides of the pc board and all the components look to be OK (no burns, tested for cold solder, cracks, etc). Is the IC not triggering the monovalve ground signal? How & where exactly is the ground signal triggered? Stumped. Any ideas would be appreciated. I’ve searched high and low - does anyone have the pc board schematic (detailed circuit level)? Thx.