I'm trying to debug my sprinkler electrical problem and want to understand how my 3-wire 24vac transformer works. I get 28vac between the yellow and red wires from the transformer (which is a good thing). However I don't get any voltage between either of these wires and the common green wire. Nor do I get any voltage drop between either the yellow or red wire and a known ground (the 3rd prong on a nearby grounded AC outlet). Is this expected?

I'm guessing my control board is bad because when I turn on any zone manually, there is no voltage between the terminal screw for that zone and common. However, when I touch the red or yellow wire from the transformer directly to a zone terminal screw, the sprinklers in that zone turn on.