No voltage output from automatic temp control to monovalve

I have a 1990 560 SEC (owned since new w/45K mi). I can no longer get cabin heat (AC works normally). Upon investigation, I find that there is no voltage output from the automatic temp control (ATC) to open the monovalve. When I disconnect the output from the ATC to the monovalve and apply battery voltage to the valve, coolant flows through it normally and the cabin heats up. There is no temperature control as the ATC modulates flow through the monovalve with on/off voltage to the 'valve (and that can't happen with the output from the ATC to the monovalve disconnected).

My inclination is to replace the automatic temp control. But before I do that, I would appreciate input from this forum on my diagnosis and ways to fix.

Thanks,

Fred Hudspeth
Tyler, TX

1990 SEC (45K mi.)
2104 E550 (17K mi.)
1982 300 SD (150K mi)
(all owned since new)