Since you have DC-powered OpenSprinkler, please note that you should only measure DC voltage and DC current — because the controller outputs DC-only voltage. So “0.02V AC” doesn’t mean anything because the controller does NOT output AC voltage.
VIN always outputs 5V — the VIN pin is for powering certain sensors, NOT for solenoids. You should be using the COM (common) pin, as the user manual clearly states. So each solenoid should have one wire that goes to the COM, and the other wire goes to an individual zone port.
Also, VIN is NOT (I repeat: NOT) for input voltage — as the user manual clearly state, the input power should be supplied through the black-colored power barrel. I’ve seen at least one case where the user tried to feed 12V to VIN, that caused damage to the controller.
The way DC-powered controller works is by generating a high impulse voltage to energize the solenoid, then lower it to input voltage to provide holding current. When you use 12VDC input supply, that means after a zone is turned on, at stable state, the voltage measured between COM and that zone should be roughly 11VDC (a bit lower than 12VDC).