For example, in order to turn on a consumer using a manual switch, you need to manually press it or move it to the desired position. In this case, the relay "receives an order" to supply power to the corresponding consumer.
Of course, the voltage from the battery to the consumer could be supplied directly through the switch. But in the electric circuit of all devices consuming a large current (headlights, windshield wipers, fog lights), a relay is included as an intermediate link in order not to overload the switch, or to shorten the current path.
Checking switching relays
When the corresponding consumer is switched on, the voltage supplied to the relay closes its electrical circuit.
The magnetic coil inside the relay attracts the contact, thus closing the circuit of the "working" current. The "working" current flows from the relay to the consumer.
The easiest way to check the relay's functionality is to replace it with a known good relay. This is what workshops do. But since a car enthusiast does not always have spare relays at hand, the following approach is recommended when checking them.
1. Remove the relay from the holder.
2. Turn on the ignition and the corresponding switch.
3. First check with a voltage tester whether voltage is supplied to the (+) terminal on the relay holder. To do this, connect the tester to the "ground" (-), and carefully place the other contact rod on the (+) terminal.
If the tester LED lights up, there is voltage. If the tester does not register voltage, find the open circuit from the positive pole of the battery to the (+) terminal of the relay holder using the circuit diagram.
4. Make a jumper from a piece of insulated wire, stripping its ends.
5. Connect this jumper to the (+) terminal on the relay holder (voltage is supplied here constantly) with a relay closing contact. This technique does almost the same thing that a working relay should do. The location of the terminals on the relay holder can be determined by the color of the insulation of the connected wires or by the electrical diagram.
If, for example, the high beam lights up when the jumper is connected, then we can assume that the relay is faulty.
If the high beam does not light, find out if the connection of the headlight to the car body "ground" (-) is correct. Then, using the circuit diagram, find and eliminate the break in the circuit from the relay terminal to the main headlight.
6. Install a new relay if necessary.
Attention! If the relay functions intermittently, then the fault, as a rule, should be looked for in the relay itself. Sometimes the so-called gluing of the relay contact occurs, as a result of which it does not open, although the relay otherwise works perfectly. If this fault is detected, simply knock on the relay body. If such a relay short circuit occurs again, then replace this relay with a new one.