CAR CLINIC | Understanding how relays work

The past 20 years have seen a proliferation of relays on cars. From a DIY point of view this is good news. Not only is a good-quality relay a very reliable component, but its working is also easy to understand for anyone with high school knowledge of electricity.

Once you understand how it works, you can do your own troubleshooting if you suspect a fault in a relay.  

In essence a relay is an electro-magnetic switch which uses a small current – the “control current” – to close a circuit in which a much bigger current can then flow. At the heart of a relay is a little electromagnet, consisting of a coil of fine wire around an iron core.

If a tiny current flows through the coil of wire, the iron core becomes magnetised and can then move a spring-loaded contact to touch another fixed contact, thus completing the pathway for the heavy current to flow.

These parts are enclosed in a small plastic box from which flat “pins”, usually four or five, protrude to make contact with female connectors in the relay holder when the relay is plugged in. Wires coming into the holder are attached to the female connectors.

On most cars the relays are situated in the fuse box, usually somewhere in the engine compartment. To appreciate the need for relays, imagine what would happen if you didn’t have them.

To supply power to spotlights, for instance, which draw a relatively large current, you would have to bring thick (hence expensive and heavy) wires all the way from the battery positive pole to a switch on the dashboard, and from there all the way back to the spotlights at the front of the car.

The same applies to the radiator fan, also a big current consumer, except in this case the engine control module will decide when to switch it on. An ECU, however, operates with low voltages and small currents. It was never meant to switch large currents directly.

The pins on a relay are marked with specific numbers which have been standardised across the auto-electrical industry. The pins may not always be in the same positions on the back of the relay, but each pin will be numbered and the number tells you the purpose of the pin. Thus, on a four pin relay, pin 86 will always bring the control current into the relay and pin 85 will take the control current out to earth. Pin 30 receives the heavy current from the battery and pin 87 takes the heavy current out to the load as long as the control current is doing its job of energising the electromagnet.

Two points raised by an experienced auto-electrician to whom I spoke about relays are worth repeating:

• There are inferior relays available at bargain prices. They don’t last long.
• Observe the volt and amps ratings shown on the relay. When these are exceeded, the relay will be damaged.
• Most electrical problems are caused by someone tampering with the electrical wiring (perhaps to fit a tracker or sound system and so on). The electrical system of a modern car is highly sophisticated and sensitive. Be very cautious about who you allow to work on your car.

Excellent videos on relays are available on the internet:  This one has good illustrations to explain the working principles of a relay:                   

These two explain how a relay can be tested using a multimeter and simple, readily available tools: