Checking the relay (BMW 5 Series E39)

Many electrical circuits have a built-in relay. The switching relay works as a switch. When the high beam is turned on manually, the relay receives a command to supply current to the high beam headlights. Of course, it would be possible to supply current to the high beam headlights from the battery directly through the light switch. However, all consumers that need a large current include an intermediate relay in order not to overload the switch and to protect the target from a short circuit. Along with switching relays, there are also, for example, relays for intermittent switching of the windshield wiper.

Checking the switching relay

The relay is activated when the corresponding consumer is switched on. At the same time, due to the switching current, the coil of the electromagnet inside the relay attracts the contact and thus closes the electric circuit of the working current. The working current then flows to the consumer.

The easiest way to check the functionality of a relay is to replace it with a working one. This is also done in a workshop. Since a car enthusiast only very rarely has a new relay at his disposal, the following sequence of operations is recommended with so-called switching relays. The terminal designations given below, especially for serially installed relays, may differ.

Remove the relay from the board.

Turn on the ignition and the corresponding switch.

First, use a voltage tester to determine whether voltage is supplied to terminal 30 (+) of the relay board. To do this, connect one electrode of the tester to ground (-), and carefully insert the other electrode with a sharp end into terminal 30. If the LED of the tester lights up, then voltage is supplied. If the device shows no voltage, use the electrical diagram to find a break in the circuit from the positive pole (+) of the battery to terminal 30.



Make a jumper from a piece of insulated wire, stripping the ends of the jumper to a shine.

Connect terminal 30 of the relay board using this jumper (this terminal is always supplied with voltage from the positive pole of the battery) with terminal 87 at the relay output. This operation does practically the same thing as a working relay does. The location of the relay board terminals is indicated on the relay or on its pin contacts.

If the current consumer works with the jumper installed, we can conclude that the relay is faulty.

If the current consumer does not work, it is necessary to find out whether its connection to the ground is correct. Then, using the electrical diagram, find and eliminate the break in the wiring from terminal 87 to the consumer.

If necessary, install a new relay.

Note: If a fault in the electrical circuit with a relay occurs only from time to time, the cause is usually due to a defect in the relay. In this case, the relay contact remains closed or open, while the relay functions perfectly the rest of the time. If this type of fault occurs, lightly tap the relay housing. If the relay operates as a result, it should be replaced.