Repairing Headlight Switches And Horn Relays - Electrical System Diagnostics

Relay and Horn Circuit Repair
Our '69 Barracuda is an example of how to trace the cause of a malfunctioning relay circuit. Using the seven-step diagnosis check list, first check the fuse, then move on to the horns themselves. If the fuse is good, disconnect the wires from the horns, attach one end of the fused jumper wire to the positive battery cable, and briefly touch the other end to the wire terminal on the horn. If the horn doesn't sound, it's probably bad. If it does sound, it probably isn't getting power. To test for power, ground the circuit tester and touch the inside of the horn wire connector with the probe. When your assistant pushes the horn button, that wire should be hot. If the wire is hot and the horn didn't work in the previous test, make sure the horn is properly grounded and all connections are clean. If you've checked the grounds and the horn itself doesn't work even though it's getting power, you'll need to replace the horn.

If, however, the horn did work when directly jumped but there's no power at the horn wire, we need to look further upstream for the problem. We'll test the horn relay next.

On the Barracuda, the horn relay is on the left kick panel, close to the driver's left knee. The location varies by vehicle, so find it by consulting your factory shop manual. If you have trouble finding it (or any other relay, switch, or component), look at the wiring diagram to see how many and what color wires attach to it. The Baracuda's horn relay has three wires; a black 18-gauge wire, a purple 16-gauge wire, and a dark green 16-gauge wire with a red stripe. When we found the relay with that wire combination, we knew it was the horn relay.

The three wire terminals are appropriately labeled on the relay; Power In from the battery "B," Power Out to the horns "H," and Power Out to the horn switch "S." When the horn button is pushed, the horn switch completes the ground and allows power to flow from "B" through the electromagnet, out through "S," into the horn switch, and into the ground. This complete circuit activates the magnet and pulls down the bar. This allows power to flow from "B" through the contact points to "H" and to the horns. The single Power In wire provides power to both the horn switch and the horns. To test the relay, ground the circuit tester and touch the probe end to the Power In wire in the connector. You can touch it from the back where the wire goes into the connector. It should always be hot. If it is hot, unplug the "S" wire and connect one end of the fused jump wire to that terminal on the relay. Briefly touch the other end of the wire to a ground. This accomplishes the same thing as a good horn switch completing a circuit, and a good relay will make the "H" terminal hot and honk the horns. If terminal "H" wasn't hot and the horns didn't honk, the relay is bad-buy a new one or try to repair it the same way you would repair a switch in the previous examples.

If terminal "H" tests hot but the previously-tested good horns didn't honk, that means the relay is OK, but there's a break in continuity somewhere between the relay and the horns. Drag out the wiring diagram and start tracing wires for problems. While you're under the dash, there's one last test to make absolutely sure that the circuit between the relay and the horns is good. Disconnect the connector that covers the "B" and "H" terminals and, with the fused jumper wire, directly jump these two wires together at the connector. If the circuit is good, the horns will honk.

If the horns work when you ground-out the relay but don't work when pressing the horn button, it's time to pull the steering wheel (you'll need a steering-wheel puller).