Fiats through the mid seventies all use a three terminal horn relay, which is now long discontinued. The most common brand is Fiamm, though you'll also find Sipea or Mixo. Here's how the original relay works: Terminal 87 connects to the horn. Terminal 30 is the power supply from the battery. As you can see, it is bridged inside the case to provide power to the electromagnetic coil as well. Terminal 85 is connected to the horn button, which grounds at the steering wheel when depressed, thus completing the circuit and activating the electromagnetic coil in the relay, causing the points to close and the horn to blow.
This is how you substitute a readily available four terminal relay in place of the three terminal relay:
Power lead to horn on terminal 87 as original
Power supply from battery on terminal 30 as original
Ground lead from horn button on terminal 85 as original
Connect a jumper wire from terminal 86 to terminal 30, thereby providing power for the electromagnetic coil.
Beep Beep-You're done!
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