How to test relays (and how relays work)

Checking fuses is simple, but it is much less so for relays: ultimately, a relay is nothing more than a switch, and there are many of them in a car. Its main function is that of a remote switch, the relay allows you to control a high-power circuit (for example, headlights or the starter motor) using a low-power switch, such as the one on the dashboard.

This way, you avoid passing the entire current of the main circuit through the switch, reducing the risk of overheating and wear.

Furthermore, a single switch can activate multiple relays simultaneously, efficiently controlling different functions: for example, a single relay can activate both the starter motor and fuel injection during engine startup. In case of overload or short circuit, the relay can interrupt the flow of current in the protected circuit, avoiding damage to the components.

It also provides electrical isolation between the low-power control circuit and the high-power circuit, increasing safety and reducing the risk of interference. On the back of the relay there are numbered pins, and on the relay we have the diagram printed, showing which pins are for the control circuit and which are for the load circuit, which are usually parallel to each other on the outside of the relay, while the control pins are perpendicular to each other; if the relay is rectangular, the diagonally placed pins are connected; if it has 5 pins, one of them is normally closed.

We test it by clicking it. If it doesn't click, it means it is defective, and we can confirm this with the resistance, which should be between 50 and 200 Ohms, otherwise the coil wire must have broken somewhere. There are then different combinations of open/closed to try with the multimeter; a test for the car is to replace one relay with another in the circuit, as they are all more or less similar to each other; this will give us a test of functionality.