Converting Hunter Controller to Rachio 3

I am trying to convert my Hunter Sprinkler Controller (old SRC model) to the Rachio 3 and I am having some trouble trying to figure out how to wire in the Hunter mini-clik rain sensor. My current wiring configuration in the Hunter Controller is pictured above.
The mini-click sensor has 2 wires. According to the instructions for installing the mini-click on the Hunter controller, 1 of the wires from the sensor should connect to RS and the other should be connected to C and also the valve common should be connected to RS. So normally there would be 2 wires connected to RS and 1 to C. My current configuration has only 1 wire connected to RS not 2. What I believe the original installer did (over 20 years ago) was splice the valve common wire and one of the sensor wires together underground outside and then connected that splice to one wire leading to the Hunter controller on the RS terminal with the 2nd sensor wire going to C.
The Rachio instructions indicate that one sensor wire should go to a S1 terminal and the other to -24VAC. Following that path for my current wiring scheme, I would be connecting one sensor wire and the valve common wire to -24VAC instead of the valve common wire to the common terminal and one sensor wire to -24VAC. Is it acceptable for the valve common to be connected -24VAC? If not what can I do to make the rain sensor work

If your theory about a wire splice is correct, then unfortunately you are out of luck. Commons on Rachio controller(s) is connected to 24v+ bus, whereas zones are “grounded” to 24v- bus while active.
Connecting the commons to 24- bus (via jumper) will short the power supply out and possibly damage the controller.

You seem to have a yellow cable available. If you can find the splice & separate the wires, you could run the rain sensor on a black / yellow pair & commons on the white (which is a standard color for commons).

Good news is that you don’t really need a rain sensor, if you have a good weather station nearby. Rachio is designed to proactively skip irrigation based on forecast, I’ve removed my rain sensor during the upgrade and never looked back.

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Thank you for the response! Glad I asked. I did confirm the wire splice and I have a plan of how to connect the sensor wires to the black and yellow wires as you suggested!