Relays — general information and checking the correct functioning (Mercedes-Benz M-Class W163)

Relays are used to supply power or a control signal to many of the electrical consumers in a car (such as fuel injection system components, horn, starter, cooling fan(s), fog lights, etc.). In fact, a relay is an electric key that ensures the closure of the high-voltage circuit by a low-voltage signal. If a relay fails, the corresponding consumer will not function. Most relays are located in the engine compartment, while others are "scattered" throughout the car. A description of how to check the proper functioning of a relay is given below. Failed relays must be replaced.

Examination

1. The correctness of the relay connection can always be determined by Electrical connection diagrams.

2. Usually, the control circuit is always connected to two of the relay contact terminals. When voltage is applied to these terminals, the current begins to circulate through the relay control winding, which results in the closing of the large contacts of the working circuit of the consumer of electric power. The remaining terminals are terminals of the working (BB) circuit and are closed with each other when power is applied to the control circuit.

3. To facilitate identification of the relay terminals, explanatory markings with an image of the key connection diagram are usually applied to its body. If such markings are missing, you should refer to the electrical connection diagrams.

4. To check, connect an ohmmeter between the two terminals of the relay's high-voltage circuit - in a normally open relay there should be no continuity. Now connect a jumper wire equipped with a fuse between one of the relay's control terminals and the positive terminal of the battery (B+). Using another jumper wire, ground the second control terminal - the relay should click, after which the ohmmeter will record the presence of continuity (normally open relay). Some relays require that the polarity of the connection be strictly observed.

5. If the test result is negative, replace the relay.