Let me see if I can explain what you are seeing on the meter. In automatic mode there will be voltage on both. One should be +12v (batt.) and the other should vary (0-11v) realizing that 0-to-11 is more "negative" than 12. Showing 12volts on both pins means there is no difference between the them and no current flow. (Think along the terms of water pressure.)

Have you tried the fan in Hi/Lo vs. Automatic?

If the blower will not work in Hi, Low or Auto, and since it spins fine when connected to the battery, I would check the pushbutton controller. It gets more mechanical wear and they are notorious for intermittent solder cracks at the points where the circuit boards join. Do a search of "resoldering" and you should find pics. It is a pretty simple repair but if needed, find a buddy that can touch it up for you.

The "humming" of the monovalve tends to point to the temp. controller but the pushbutton controller turns it on. Since you are getting a semblance of control by varying the temp wheel, I am thinking it should be working.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )