A simple switching relay is used primarily for powerful current consumers. This has the following reason: if power is supplied through long cable lines through appropriate switches, then voltage losses will be the consequence. In addition, the contacts of the switches are heavily stressed due to the higher current. With a relay circuit, the switch is used only for low current, and not the current consumer is connected directly, but its relay. If the switching command does not come from the switch, but from the control unit, then sensitive electronic elements cannot conduct high currents without damage to themselves.
Certain relays may perform additional functions. For example, the turn signal relay switches on flashing pulses, and the windshield washer and wiper relay controls the intermittent operation and operation without water of the wiper after washing the windows.
Control units are equipped with more or less complex electronic systems for certain functions, sometimes they also have a relay installed. Examples include the control unit for oil pressure control or the control unit for delaying switching off the lights in the passenger compartment.
Operating principle of the switching relay
When the corresponding power consumer is switched on, the electrical circuit from terminal 86 is closed in the relay (incoming «switching current») to terminal 85 (frame).
Due to this, the electromagnet, overcoming the resistance of the spring, attracts a powerful contact and thus closes the electrical circuit for «operating current».
To prevent voltage losses, the operating current is conducted along the shortest path to terminal 30 of the relay and further - provided that the contacts are closed through terminal 87 to the power consumer.
Sometimes there is also terminal 87 a. It is hardwired to terminal 87.
Troubleshooting switching relays
There must always be voltage at terminal 30, unless it is a relay whose consumer is dependent on another consumer. Example: The rear fog light relay is only energized when the light is on.
To check the presence of voltage, remove the relay and touch the needle electrode of the control light to terminal 30 in the relay base. No voltage means broken wire.
Remove relay, connect terminal 86 to «+» battery, and terminal 85 with housing («weight»). The electromagnet coil must clearly (heard) pull the relay contact, otherwise the relay is faulty.
Help with defective switching relay
Remove relay from socket.
Connect terminals 30 and 87 in the relay cell with a paper clip or a short piece of wire. Thanks to this, the corresponding consumer will receive a direct current.
To disable, remove this jumper, since the corresponding switch is bypassed in this case.
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