Relays and control units (Audi A4 B5)

The on-board electrical equipment includes a number of relays and control units, which are placed in a protective box on the left under the instrument panel or under the steering column trim.

A simple switching relay is used for powerful current consumers. If the current comes in long bypass paths through wires via the corresponding switches, then a voltage loss occurs. In addition, the contacts of the switches are subject to a strong load due to the high current. With relay switching, switches are used only for weak switching current, due to which the consumer is not connected directly, but through its relay.

If the switching signal comes not from the switch, but from the electronic control unit, the same law applies: sensitive electronic components cannot conduct high currents without damage.

Certain relays can perform additional functions. For example, the intermittent pulse interrupter relay, the interval switching relay for the windshield washer/cleaner system controls the interval in operation and the movement of the dry windshield wipers after washing the windshields.

Control units have more or less extensive electronic circuits for switching certain functions, and sometimes another relay is also installed. An example is the control unit for the central lock and the burglar alarm system.