The Symmons Temptrol will not divert to the shower head unless I block the tub filler

Hi Ddayton

It sounds like the diverter knob that switches the water flow from tub to shower may have become stripped... that, or the diverter spindle itself is stripped.

Here, remove the handle and trim plate from around the tub/shower valve and then remove the spindle handle (see how it is mounted to the trim plate) and insert it back into the spindle assembly and move it back and forth. When moving the handle back and forth inside the diverter spindle it should rotate the diverter spindle. If the diverter spindle doesn't rotate then as I mentioned the diverter spindle or diverter handle needs replacement.

See what you can discover and then pop back and we can discuss how to remove that diverter spindle if needed... will need a 5/16" allen wrench, a new spindle, some plumber's grease and a 5/16" x 3" lag bolt, too. Glad to discuss more if you want...

Mark