Before beginning electrical diagnosis, check the fuses. First, disconnect the electrical connector from the horn. To test the horn, connect battery voltage to the two terminals using jumper wires; if the horn doesn't sound, it needs replacement. If the horn does sound, check for voltage at the terminal when the horn button is depressed by connecting a voltmeter to the horn wire and ground. If there's voltage at the terminal, inspect for a bad ground at the horn. If there's no voltage at the horn, check the
Relay, noting that most horn relays are either the four-terminal or externally grounded three-terminal type. If the relay is functioning, check for voltage to the relay power and control circuits; if either circuit lacks voltage, inspect the wiring between the relay and the fuse panel. If both relay circuits receive voltage, depress the horn button and check the circuit from the relay to the horn button for continuity to ground. If there's no continuity, look for an open circuit; if there's no open circuit, replace the horn button. If continuity to ground exists through the horn button, check for an open or short in the circuit from the relay to the horn. The horn(s) are mounted behind the radiator grille, and on 2008 and later Expedition/Navigator models, the left
Headlight housing must be removed to access the horns. Disconnect the electrical connectors and remove the bracket bolt to detach the horn(s), and installation is the reverse of removal.