With the steering wheel off, we'll look for either dirty contacts or a broken wire and test the horn switch. The horn switch has a roller on the end of it that makes continuous contact with a copper disk on the back of the steering wheel. The disk which has a wire that connects to one of two closely spaced metal plates in the horn button. When the horn button is pressed, it forces the two metal plates together and completes the circuit through the horn switch. Your horn button may be different, but the theory will be the same.
Test the horn switch by touching the roller with the other end of a properly grounded jump wire. If touching the top of the roller on the horn switch doesn't honk the horn (remember that the horns, relay, and circuit to the horns already tested good), and touching the bottom of it (where the wire connects to it) does make them honk, you have a bad horn switch. Look for the break in the horn switch-a broken part or a broken wire, and decide if you can repair it or need to replace it. In this instance, we're able to repair and reuse the 'Cuda's horn, as it only has dirty contact points that were easily cleaned with 600-grit sandpaper. Take some 600-grit sandpaper to the copper disk on the back of the steering wheel and clean everything while you're in there.
Although lots of possible switch/accessory/relay combinations weren't covered, enough was covered to allow the average do-it-yourselfer to understand how everything works, diagnose nearly any problem, and determine a course of action for repair or replacement. There really is no reason to pay an automobile electrician $60 an hour to do the same diagnostic steps covered here.
Next month we'll look at how to diagnose and repair problems with your car's gauges.
Tip O' The DayHate working under the dash as much as we do? Make your life easy and spend 15 minutes removing the front seat (or seats). You'll more than make up that time by not contorting your body into a back-breaking position to work under the dash. With the front seats out, you can lay on the floor of the car and work under the dash comfortably.
Seven Steps to Diagnosing an electrical problem