Hey Guys, scratching my head trying to figure out how to install the Rachio System where I have an existing K-Rain 2520.
Appears that there are only 2 positives, 2 negatives, 2 grounds, and the rain sensor running to my current unit.
Any pro-tips for how to wire the rachio 2nd gen?
If I have to guess, I’d make a wager that you have some sort of irrigation pump tied in with some soft of an indexing valve.
If I am right (and you do have a pump), you would need to install a pump relay, something like this (link)). If you don’t have a pump, pictures of the rest of your setup would be helpful.
If I am also right about the indexing valve, which looks something like this (link), than you will be able to use it with Rachio (thanks to the recent updates), but will not be able to take advantage of the flex daily schedule, which is the most economical option available.
It will be always possible to setup using your existing equipment, or convert everything to an individual zone model for most savings. Just tell us what the rest of the system looks like.
Cheers,
Gene
@Bwoodham These are my favorite timers that we replace 
They definitely belong in the irrigation hall of fame.
@Gene, Thank you for your detailed reply. You are correct, indexing valve is on top of well pump.
So, I will just need the pump start relay to get this setup working? How is this wired in?
Is there any way to get the flex daily schedule with an indexing valve?
@Bwoodham = indexing valves and flex daily are incompatible as flex can skip zones, but the indexing valve can’t. One can replace the indexing valve with 24 VAC valves and use flex. Here are some posts that show how that is done, the first post also should have pictures on how to wire in the pump start relay (Rachio M and C(ommon) to the pump start relay, and activated in the application and 120/220 VAC to the pump start relay).
How many HP is the well motor?
@DLane is right, while Rachio can help skip watering due to recent and forecaster rain, indexing valves require all of the zones to be run in sequence. Flex schedule is best when soil and plant types determine the zone requirements and allow individual control over each of the zone run-times.
No matter what, you will need to figure out how to run the irrigation pump off of Rachio.
Get a pump relay, like this (link). This would essentially replace your old timer and you would install one White / Black cable to the Input wires and the other White / Black cable to the output wires (they are labeled). Which one is which (polarity) doesn’t matter since relay is bi-directional. Make sure you don’t short anything by connecting the same while and black cable to both input and output sides, they should always either be connected onto to input, or only to output to prevent shorts.
The remaining blue wires within the pump relay would connect to Rachio’s M and C terminal(s).
