Hi all:

I had a fun episode yesterday where I beeped my horn and then it wouldn't stop.  I was a few minutes from the house so I just drove home (beeping all the way) and figured out which fuse governed the circuit and pulled that one.

It would seem obvious that something went wrong in the turn signal stalk, but before I start taking things apart was wondering if people think a relay could go bad and get stuck in the 'on' position or if it could be something with the horn itself.



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a stuck relay would be a whole lot cheaper that the turn signal stalk assembly. Yes , relays can stick.  It is in the passenger foot well , mounted on a piece of sheet metal with other relays.  I had a picture on my hard drive.  No , idea of the model year.  Replaced mine with a univeral relay from the auto parts store.


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Because I'm incompetent at electrics (at least as they exist in DeTomaso cars), the only way I solve such problems is with a VOM, starting at one end. When electrons stop showing up on the meter, I usually find the problem. When our horn stopped working 20 yrs ago, I did this because to qualify for Concours-type car shows, everything must be working. 

I found the horn stalk button gets power thru the turn signal switch wiring, runs down through the stalk, turn signal switch and steering column, thru the front body wiring loom, goes to the horn on the front left below the headlight, comes ALL THE WAY BACK thru the body to the turn signal switch, up the switch stalk and is solder-grounded about halfway up via a raw hole drilled in the stalk. Facing away from the driver, there's a tiny painted-over lump of solder.

My horn ground wire was bowstrung-tight so sharp turns of the wheel pulled on the connection, eventually cracking it & opening the circuit. Yours could be the same but may be making contact inside the stalk- or somewhere else. Why DeTomaso didn't just ground the horn 2" inches away and save about 8 feet of wire & an hour of wire routing, I don't understand. Italy.....

My experience with the factory horn wiring is not as described above.

The horn relay is a three terminal single pole, single throw relay. The horn button on the stalk is a momentary switch on the ground side of the circuit.

+12v is supplied to one relay terminal via a short red jumper (from an adjacent relay) to the coil and armature (the moving part) inside the relay body. Pressing the horn button provides a ground to the relay coil terminal via the brown wire which causes the relay to throw. The armature makes contact with the third terminal which sends +12v via the white wire to the horns. Release the horn button, no more ground to coil, relay disengages.

The horns are actually the only electrical accessory up front that are grounded to the body, as opposed to the fans, markers and headlights, that all use one skinny little black wire all the way back to the stud behind the dash.

Somehow your brown wire is finding an unintended ground, or the relay is stuck.

Last edited by larryw

Turns out that in this case it was the relay.  New one installed, back in action.  Thanks for your help!

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