Classic Car Automotive Electrical Systems — Part 6: How Automotive Relays and Fuses Work
We've gone through basic theory and major components over the last several articles, so by now you should be fairly comfortable with overall circuitry. Now, we'll cover just a few more common devices and then draw a simple circuit and follow an electron through it. Ready?
This was touched-on before but it's worth special attention. Remember that certain devices require considerable current (amps) and that, in turn, require thicker wire. High current devices require big, heavy switches to handle the current. Unfortunately, these would be ugly and expensive, so engineers use relays.
A relay consists of a small coil of wire around a central iron core. When the actuating switch energizes the coil this core moves heavy-duty contacts together, thus allowing high current to be passed to the device. That's how a small switch can control a high-current device.
You already know the starter solenoid is a high-current relay. Other devices that typically utilize relays are the horns, power antenna, air conditioning compressor, power seats, power windows, engine cooling fans, and power tops. Sometimes, headlights and accessory driving lights use them too. It's important to know this because many electrical failures occur in the relays themselves!
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.
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.
Let's look at a fairly typical horn circuit and see how various components are put together to form a working system:
Horn Relay Diagram
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). See how it works?
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 (remember high-resistance connections?). If the coil is bad the relay must be replaced. If the contacts are charred, file them smooth.
Automotive circuits are quite simple in design. Remember always that the factory used as little wire as possible, so look at the schematic diagram to see where multiple connections are made. Remember also that your friendly electron travels through several devices on its way to doing its work so you need to systematically trace the path.
Schematic diagrams are printed way too small in the manuals. Take your drawings to a copier that enlarges and blow them up to easily-readable size. Tape all the sheets together into one big drawing an you will find that tracing electrical paths becomes very straightforward.
With a little practice and patience you will no longer fear your car's electrical system.