Changing of NC horns

  1. My model of MX-5 is: _NC_SE (2007)

  2. I’m based near: Nottingham

  3. I’m looking for technical help: _Horn wiring technical knowledge.
    I have bought a pair of Ring single electromagnetic horns (Low & High note) I have taken the plastic guard off and mounted the horns. My questions are;

  4. Are the original horns a Positive Acted Type or Negative Acted Type?

  5. The wiring diagram suggests a relay for each of the new horns (no relays in the box) so a separate purchase. Are the new relays required there were 2 horns fitted as standard?


You need to check the amps drawn by old and new horns.
If the new horns draw the same or less, you should be fine with the existing wiring.
If they draw more, you may need to uprate your wiring and relay(s).

some horns mean that the 15A fuse can’t cope and blows.
Some say a 20A fuse is OK
Others say NO

If you have access to a multimeter, measure the resistance in ohms across the horn terminals.
Lets assume you measure 3 ohms.

Assume the cars approximate voltage at 12.5v.
Divide 12.5 by the ohms measurement (in this case 3)
The result is the number of amps drawn.
In this case approximately 4 amps

This will allow you to make an informed decision as to whether or not you utilise a relay.
I didn’t bother, on the basis that the horn gets used once per year for the MOT.

1 Like

i put relays on when i did mine

As I understand it, the horns are protected by a relay as oem but a fairly low-rated one

Thank you to all who replied to my post. I have successfully installed the horns today without the extra relays. I established it was a Positive Acted Type circuit as manufactured. The instructions for the horn said protect with a 10amp in line fuse. The Mazda circuit is protected by a 15 amp fuse so my assumption was my choice of horns did not pull a lot of amps when in use. The result is a much louder horn than the original system ideal as was stated people do not hear horns for stereos etc. Fingers crossed that is my problem solved.