[Richard]

It could be one of two things.

One, low speed blower signal from the tstat is not getting to the board. This may be consistent with the very first post in this thread. NOTHING happens on low. The circuit board has some logic in it, that I am not savvy enough to see in the schematic, that prevents the compressor from starting if the fans are not running. It's logic that prevents the compressor from burning up in case the fans don't come on.

Two, the circuit board logic is all flummoxed because of a failure in the board.

I really like your idea of measuring the voltage at the board coming from the tstat. That verifies both tstat and wiring continuity.

Use that logic table mentioned above to know which pins should be hot under the lo and hi settings. You might also check blower only settings on the tstat. Don't forget, there is a three minute or greater delay before anything happens. And there must be a two degree or higher differential to kick high mode on.

You will soon be the guru of SCS control boards.