Miata Mailing List: March 1995, Message #168
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   From: "Drake A. Daum" 
Subject: Relay Advisable for Air Horns
   Date: Fri, 3 Mar 1995 15:31:31 -0500

     
     Regarding the recent question concerning the installation of a 
     dedicated relay in the electrical circuit of aftermarket air horns:
     
     Although the Miata's stock horn circuit utilizes a relay, the power 
     source is shared with the brake light circuit via a common fuse.  The 
     stock horn draws so little current that this is not a problem.
     
     However, there have been reports that those of you who have merely 
     connected the stock horn wire (+12vdc) directly to the positive 
     terminal on the air horn compressor have experienced blown fuses when 
     applying the brakes and using the horns simultaneously.  This, of 
     course, takes out your brake lights and horns until you replace the 
     fuse.  Don't be lulled into thinking that a higher rated fuse is the 
     answer.  Mazda sized the fuses in each circuit for a reason.  Don't 
     second guess the factory on this.
     
     The solution is to install your own dedicated horn relay to keep the 
     high amperage compressor current isolated from existing Miata 
     circuits.
     
     The CONTROLLER side of the relay (the two terminals USUALLY marked 
     "30" and "87") should be connected to the stock horn wire and chassis 
     ground.  The CONTROLLED side of the relay (terminals USUALLY marked 
     "85" and "86") should be connected to a +12vdc source in the underhood 
     fuse box (via a dedicated 20-25A inline fuse) and the (+) side of the 
     compressor.  The (-) side of the compressor should be connected to 
     chassis ground.
     
     Now, only the low amperage current to merely ACTIVATE your dedicated 
     horn relay is routed thru the stock Miata wiring.  The compressor 
     current comes directly from the +12vdc underhood source through its 
     own fuse.
     
     Brake lights and air horns are now on separate circuits.
     
     I take no responsibility for your modifications.  As always, your 
     mileage may vary.
     
     Cheers,
     DRAKE A. DAUM, President
     Miami Valley Miata Club
     PO Box 33503
     Dayton, Ohio 45433-0503