I re-checked several times, tested by connecting to the screws on the plug, as well as on the raw wires (green plug removed), and I was always getting 4-5MΩ reading.
I didn’t quite understand this: do you mean that when it’s connected to OS, it always appears as open circuit (i.e. 4-5Mohm reading), even when you don’t press the self-test button? Because it’s normally closed, it should be almost 0ohm when self-test button is released. I can’t imagine how connecting it to OS would affect this.
No, sorry I was not clear. In all cases the rain-sensor was disconnected (i.e. unplugged) from the OpenSprinkler controller itself when I measured with the meter.
What I’m saying is that I checked, when the self-test button was depressed:
(a) by leaving the green plug on the rain sensor wires and touching the meter probes to the two screws on the plug
(b) be removing the green plug from the rain sensor wires and touching the meter probes to the wires directly
I wanted to rule out that the possibility that the plug itself was defective and was somehow shorting the two rain-sensor wires making it appear closed (i.e. no rain) when it should have been open (i.e. rain detected). But that wasn’t the case — I got the 4-5MΩ reading both ways (i.e. using ‘a’ and ‘b’ methods). So I don’t think the plug is defective.
So I think we’re in agreement that the rain-sensor is operating correctly and that it should work with OpenSprinkler? In that case, does this suggest I might have a defective unit? Is there anything else I can check here?