I am trying to implement a dual horn setup for my bike. The stock came with a single horn. I have with me a 3-pin relay(H-B-S) with me. I am connecting the positive from the battery to the B terminal, Horns connected to the H terminal (horns are grounded on the other end). By what the relay box specifies i need to add the S terminal from the relay to ground via a switch (not the stock switch provided on the bike). So when the switch is turned on the horn sounds. This works as intended, but I want to use stock horn button to activate the horns. What should i do for that.

PS: I tried connecting the stock switch in place for the switch in the diagram (by connecting a terminal to the S and the other to the ground), but it does not work. I dont know what polarity is being supplied by the stock switch. I tried connecting the switch both ways to the S terminal, these are the outcomes for that scenario

  1. The horns starts sounding continuously (with out the switch being activated, bike still in ignition off position) untill i pull out the power wire from battery
  2. No response even when activating the switch

Attaching the wiring diagram of the Relay.

Attaching the wiring diagram of the Relay.

Can this be done on a 3 pin relay? If not I Should go forth and get myself a 4 point relay as I know this can be done with a 4 point relay. Thankss

  • Do you have a diagram for the existing switch? Is there a relay involved in the existing circuit? Are you able to access both sides of the existing switch wiring? Switches do not have polarity, although one must observe polarity of the wiring being attached to a switch and to ground.
    – fred_dot_u
    Jul 8, 2018 at 10:38
  • No i dont have a wiring diagram. I dont think there is a relay already present. I cant acess the other side as its in the handle bar and i dont want to mess with it. Jul 8, 2018 at 11:26

3 Answers 3


I would assume that the stock switch on the bike has a permanent live supply and switches the supply to the existing horn, if there the horn has an earth terminal then that is the way it is working.

To use the existing switch you will need a 4 terminal relay : the coil driven by your existing switch and a battery supply which is fused.

Edit based on comment: if you get to the switch wires, disconnect the supply side and put that to earth then it would work as per your diagram. However, as you specifically stated you did not want to interfere with the standard switch I gave you a different workable solution.

  • I was guessing at that configuration as well, but didn't want to make an answer out of a question. Your solution is not particularly expensive and easy to implement.
    – fred_dot_u
    Jul 8, 2018 at 15:01
  • @AthulGeorge see my edited answer, your first comment was "I don't want to mess with the switch..." so I answered accordingly.
    – Solar Mike
    Jul 9, 2018 at 7:53

The Horn relay that you have shown is made for cars and trucks, where the horn button only connects to the body. Grounds it. But from what you say, the switch on your bike seems to supply +ve, hence, as Ed comment you need to rewire it, to supply ground. If you do not want to do that, then go for the four terminal relay. That circuit i assume you already worked out. 4 Terminal plastic body relays are available plenty in the market. All the best.


Connect in this series and try.. H to horns, (other horn pins grounded ), B to +ve supply of stock wire, S to -ve supply of stock wire.. To identify +ve and -ve of stock wire.. use a 12V test lamp.. connect to one wire and ground the other.. press the horn switch.. if light glows the wire u have connected is +ve.. if not try reverse.. (usually +ve will be red, yellow colors and -ve will be in blck)

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