The automobile depends on electricity for fuel ignition, headlights, turn signals, horn, radio, windshield wipers, and other accessories. A battery and an alternator supply electricity. The battery stores electricity for starting the car. The alternator generates electric current while the engine is running, recharging the battery and powering the rest of the carís electrical needs.
Early automotive electrical systems ran on 6 volts, but 12 volts became standard after World War II (1939-1945) to operate the growing number of electrical accessories. Eventually, 24- or 48-volt systems may become the standard as more computers and electronics are built into automobiles.
Almost everything in a car is wired through a fuse. Fuses are designed to fail when too much current is drawn through the device. This prevents heating of the wires and subsequent melting of the insulation, followed usually by fire!
Fuses are simple in design. Inside a fuse is a soft wire with a specific cross-sectional thickness. This dimension dictates how many amps can be carried before the wire melts. Too many amps and the fuse fails, saving the rest of the circuit from damage. Pretty neat, huh?
Most of any car's fuses are located in the fuse panel, but some are in-line. In-line fuses are found under the dash and in the engine compartment.
Let's look at a fairly typical horn circuit and see how various components are put together to form a working system:
Fusible Links, another kind of fuse, are used in many cars and are almost always found in the wiring harness in the engine compartment. These are molded, single-purpose links in the wire which are designed to melt under extreme conditions (usually a crash which might crush wires together, causing a huge short circuit). Your car's schematic will show their use and location
Notice that battery voltage travels through a high current wire (red) through the relay to the horn and also through a smaller wire (blue) through the ignition switch to the relay's low-current coil. The first thing you should be aware of is that the horn circuit is always "hot" or "live" when the ignition switch is turned on and all that's needed is a path to ground.
That path is completed when you push in the horn button. When the button is pushed the ground connection is made, energizing the relay's coil "A". The coil's iron core (in this particular design) pulls down arm, connecting high-current contacts "B". High current then flows from the battery to the horn (the horn is connected to ground because it's mounted to the chassis of the car).
Actually, one thing is missing from this circuit. There has to be a fuse somewhere in the circuit! The high-current wire from the battery might go through an appropriate fuse on the fuse panel or there might be an in-line fuse near the horns (it depends upon the production engineering decisions as to the most economical placement, but your schematic drawing will show its location).
Also, your car's designers might have fused the low-current side of the relay as well. Check the schematic.
Suppose the horns don't work. Where do you start your troubleshooting? Here's a good procedure:
1. Check the fuses.
2. Check for voltage to the horns at the horn connector. Push the horn button or jump the wire to ground to actuate the relay. If you have voltage the horns should be operable, so search elsewhere for the problem.
3. Check for voltage at the horn button. While there, check to be sure the button's contacts touch each other when pushed. If everything's ok, go to the relay.
4. With someone pushing the horn button, check for voltage (on the low-current wire coming from the dash) on the relay. If there is voltage, the relay isn't working, right?
That's right. Now you have isolated the problem to the relay and only two things can be wrong: either the relay's coil isn't energizing (due to an internal broken wire) or the high-current contacts are being drawn together but no current is passing through. If the coil is bad the relay must be replaced. If the contacts are charred, file them smooth.