THE electrical system in your car is a maze of colourful spaghetti. There are miles and miles of it and they all control one thing or another. Sometimes things don’t work (like my power windows) and we are basically ‘lost’ when it comes to this maze of wires. But this maze is actually quite straightforward. Electricity needs to make a full circle (or circuit) from positive to negative and things don’t work when one part of the circuit is interrupted. Luckily the manufacturers have built-in failure points to make it easier for us to isolate a fault. We shall take one example to illustrate this concept later. But first we need to know what it is we’re dealing with.


Most everything in a car is being wired through a fail-safe called a fuse. They are designed to fail when too much current is drawn through a device. Otherwise heating of the wires and subsequent melting of the insulation would be a fire hazard. A fuse is a soft wire with a specific cross-section or thickness. This dimension dictates how many amps can be carried before the wire melts. Exceed the rated amps and the fuse will fail, usually saving the rest of the circuit and the component from damage.

Most fuses are located in a fuse panel (check your owners manual), but some are in-line. In-line fuses are found under the dash and in the engine compartment. The ones in the fuse panel are colour-coded to simplify identification. Do not put a different rated fuse, use the correct rating every time. You may risk damage to a circuit or component if you ignore this. Sometimes, you may discover that a bigger rated fuse does not blow when the correct rating does. This is an indicator of a problem in the circuit and should be investigated further.

Fusible Links are another kind of fuse and are almost always found in the wiring harness in the engine compartment. These are moulded single-purpose links in the wire which are designed to melt under extreme conditions (usually a collision which might crush wires together to cause a huge short circuit). You may safely ignore these fuses but the wiring diagram will show their use and location.


Some components in your car require considerable current (amps) to function and that usually requires thicker wire and big, heavy-duty switches to handle the current. Relays are used to minimise the use of thick cable and enable the use of small, dinky switches.

A relay consists of a small coil of wire around a central iron core. When you energize the coil through a switch, this core moves heavy-duty contacts together, thus allowing high current to be passed to the component. Therefore a small switch can control a high-current device like a starter, foglamps or a horn.

Other devices that typically utilize relays are sound system amplifiers, the air-conditioning compressor, power seats, power windows, engine cooling fans, and convertible tops. But electrical failures occur in the relays too so it is important to understand how to recognise relay failures as well.

For example, here is a typical horn circuit and how the various components are put together to form a circuit:

Horn Relay Diagram

Notice that from the battery a high voltage travels through a thick high current wire (red) through the relay to the horn and also to 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 live when the ignition switch is turned on and all that is needed is a path to the negative of the battery. This path is provided when you push the horn button. Th e complete blue circuit energizes coil “A” and pulls down an arm thus connecting high-current contacts “B”. The 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 (and the chassis is connected to the negative pole of the battery). A complete circuit is made in a high current component through a small current. The fuse is not shown in this diagram but it should be in either the high current circuit or in both circuits.

If any of the car’s components stop working, you should:

1. Check the fuses first. Replace with a correct rated fuse if it is blown. This should correct the problem. If the fuse is intact but the horn isn’t working;

2. Check for voltage to the horns at the horn itself using a circuit tester. Push the horn button and if there is voltage at the wire then the horn is unusable or damaged. Replace the unit. If there is no voltage at the horn, first check the horn button for contact problems. If your button is okay,;

3. Check for voltage (on the low-current wire coming from the button) on the relay. If there is voltage, then the relay’s internals are damaged and it should be replaced.

Simple when you know how. A circuit tester is actually a probe with a crocodile clip that is grounded to the chassis and a bulb inside the probe (which lights up when voltage is apparent). Get one at the parts shop and try it out this weekend. You never know if you might have a knack for it. See you next service!

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