Torque Shop

I have a 16-year-old car that has been reliable so far.

Two months ago, the horn stopped working. A workshop replaced it, but it still did not work well. I was told it may be a wiring issue and that rewiring may be necessary.

I decided to take it to another workshop, which fixed the problem.

What surprised me was that the mechanic took just half an hour and billed me only $40.

He said the "relay" was faulty. What is the purpose of this "relay"?

Several electrical devices in a car run off relays.

As its name suggests, it relays current from the battery to the device. It may seem superfluous as any device would still function if wired directly from the battery. But there is a sound reason for using relays.

A relay is essentially a switch with an electromagnet inside. When energised, it triggers a lever to close the contacts of a circuit.

So, when you press the steering-wheel boss to activate the horn, a small current - less than 0.5 amps - is sent from the horn switch to the relay.

The relay's electromagnet then activates an internal switch, which bridges the main 12-volt supply to the horn's terminal.

Most horns require at least 10 amps to work. If it is a twin-horn set-up, you are looking at 20 amps at least. A relay's switch typically handles 30 to 40 amps.

The horn button therefore does not need to be wired with high-current capacity wires.

Nor does the horn switch need to endure the tens of amperes to power the horn.

This way, over time, wear and tear is confined largely to the relay.

These workhorses are usually installed close to the fuse box. They are easily removed.

A horn relay typically costs about $5.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on October 19, 2019, with the headline 'Torque Shop'. Print Edition | Subscribe