Relay — purpose and fault finding (Mercedes-Benz E-Class W210)

What are relays used for? Relays are used primarily in the circuit of powerful energy consumers. There is the following reason for this: when current is supplied along a long wire through a switch, voltage losses occur. In addition, the contacts of the switch experience a strong load due to the high current. When a relay is connected to the circuit, small currents appear on the switch, and it is not the consumer itself that is directly connected, but its relay.

Relay function

If the command to turn on comes not from the switch, but from the instruments in the engine compartment, the following happens: sensitive electronic devices cannot transmit high currents without the risk of damage.

When the corresponding consumer is connected, the current supplied to contact "86" is closed in the relay to contact "85" of the "ground".

At the same time, the magnetic coil closes the contact, overcoming the resistance of the spring, and closes the circuit for the "working current".

To avoid voltage drop, the "working current" is supplied via the shortest route directly to the relay contact "30" and from there further - with the switch contacts closed - through contact "87" to the power consumer.

There is also a contact "87a". It is firmly connected to contact "87" and has the same function.

Troubleshooting Relays

1. Contact "30" must always be energized, unless it is a relay whose consumer depends on another. For example: the fuel pump relay is energized only when the injection system is operating.

2. To check for voltage on the relay, remove it and use a contact tester with a lamp to check contact "30" on the relay base. If there is no current, the wiring is faulty.



3. Remove the relay, connect contact "86" to the positive terminal of the battery, contact "85" to the "ground". The operation of the contact in the relay magnetic coil should be clearly audible, otherwise the relay is faulty.

4. Help with a faulty relay: remove the relay from the contacts. Install a jumper in the relay contact board between contact "30" and "87" from a paper clip or a short piece of wire. In this way, the corresponding energy consumer will be under constant voltage. To turn it off, you need to remove the jumper, since the corresponding switch is then blocked.