How to replace the transformer

Transformers rarely fail,and when they do, it’s usually caused by people not turning the power off when they are moving wires and the 24VAC wires touch.  Sometimes a failed solenoid could cause transformer problems but the timer usually shuts off and issues a FUS message before the transformer is damaged.  Some models also have fuses on the back terminal board that help protect it.

A note of caution – the transformer is connected to your household 110/120VAC power, which is dangerous.  Be sure to take precautions by turning off the power before working on the unit.  You should not attempt it unless you are experienced enough to do so.  This post does not contain sufficient information to work with these voltages safely; If in doubt, contact someone who is qualified. 

Most of the time people think that the transformer is bad when it isn’t,  Follow the DIY page topic on ‘How to test if the transformer is good’ to know for sure.

If you conclude that the transformer is bad, you can use the following information to help you understand how to replace it. I generally start on the left side and work my way around counter-clockwise.

After removing the case, you’ll see how the transformer is connected into the case.  You can often find them on ebay or at a local landscape supply store.It should be rated for 1.25A, 24VAC.



The transformer typically looks like this.

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Good luck,

Brian, Jen, Meghan and Ben – the rainDialDoctors

2 thoughts on “How to replace the transformer

  1. We have purchased a new transformer, and a new controller separately…. the new transformer has three wires, whereas the old one had two… the question is “if the third wire is a ground wire ” , what is the place on the controller to attach it?

    • Hi Harold,
      It’s good to hear from another Keller. My Grandfather’s name was Harold as well.

      I do not usually seen a transformer in these systems that has an extra ground wire, usually just the red and yellow 24VAC wires come out, and the white, black and green come into the transformer. If you have a ground wire coming out of the transformer along with the 24VAC wires, you can connect it to the GND terminal on the newer back-terminal boards, or to the VC terminal of the older back terminal boards (along wiht the common wire to the valves).

      If you are referring to the green, white, and black wires coming out of the sprinkler timer transformer, these are intended to be connected to the 120VAC ‘mains’ along with the ground, just like a three terminal plug that you’d put in an power outlet.

      Hopefully that answers your question, if not, let me know. If you can send me a photo of what you’re looking at, that would help too. Send it to

      Brian – aka the RainDial Repair Doc.

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