Table of contents: Operating principle of a switching… ↓ Troubleshooting switching relays ↓ Help with a defective switching relay ↓
The on-board electrical system includes a number of relays and control units located in the central switch or on an additional relay block on the left under the instrument panel or under the rear seat.
A simple switching relay is used primarily for powerful current consumers. This is based on the following: if power is supplied via long cable lines through the corresponding switches, the result will be voltage losses. In addition, the contacts of the switches are subject to a high load due to the high current. With a relay circuit, the switch is used only for low current, and the current consumer is not directly connected, but its relay. If the switching command comes not from the switch, but from the control unit, then sensitive electronic elements cannot conduct high currents without damage to themselves.
Certain relays can perform additional functions. For example, the turn signal relay switches on the flashing pulses, and the windshield washer and wiper relay controls the intermittent action and dry operation of the windshield wiper after washing the windows.
Control units are equipped with more or less complex electronic systems for certain functions, sometimes with a relay installed. An example is the oil pressure control unit or the interior light delay control unit.
Operating principle of a switching relay
When the corresponding power consumer is switched on, the electrical circuit from terminal 86 (incoming "switching current") to terminal 85 (housing) is closed in the relay.
Thanks to this, the electromagnet, overcoming the resistance of the spring, attracts a powerful contact and thus closes the electrical circuit for the "working current".
To prevent voltage losses, the operating current is conducted along the shortest path to terminal 30 of the relay and further, provided the contacts are closed, through terminal 87 to the power consumer.
Sometimes terminal 87a is also found. It is rigidly connected to terminal 87.
Troubleshooting switching relays
There should always be voltage at terminal 30, unless it is a relay whose consumer is dependent on another consumer. Example: voltage is supplied to the rear fog light relay only if the light is on.
To check for voltage, remove the relay and touch the needle electrode of the test lamp to terminal 30 in the relay base. No voltage means a wire break.
Remove the relay, connect terminal 86 to the "+" of the battery, and terminal 85 to the body ("ground"). The electromagnet coil should clearly (audibly) attract the relay contact, otherwise the relay is faulty.
Help with a defective switching relay
Remove the relay from the socket.
Connect terminals 30 and 87 in the relay cell using a paper clip or a short piece of wire. This will provide the corresponding consumer with direct current.
To turn it off, remove this jumper, since the corresponding switch is bypassed in this case.