OK, powered off the OpenSprinkler, unplugged the rain sensor wires. Powered unit back on. Observations:
* OS unit indicates a steady/stable “rain indicated” status (I suppose this is expected since OS configured for “normally closed” sensor, and unplugging opens the circuit?)
* DMM indicates steady/stable “open circuit” across the 2 rain sensor wire ends (I suppose this is also expected for a “normally closed” sensor that is currently wet). I checked on continuity setting (reading: no continuity), and on resistance/ohm setting (reading: overloaded).
* Power OS off. Re-attach plug to wires, and reinsert plug to OS. Power ON. Unstable rain sense status again. 🙁
Seems to me like the rain sensor is behaving as expected by providing an continuous/steady open circuit. What should I check next?