Audi A4

since 1994 release

Repair and operation of the car

A number of the relay and control units which are placed in a protective box under a control panel or under facing of a steering column at the left belongs to the onboard electrotechnical equipment.

• The simple switching relay is used for powerful consumers of current. If current arrives in the long bypass ways on wires via the corresponding switches, there is a loss of tension. Besides, contacts of switches are exposed to strong loading at the expense of current of big force. At relay switching switches are used only for small switching current thanks to what the consumer is connected not directly, and via the relay.
• If the signal of switching goes not from the switch, and from the electronic control unit, the same law works: sensitive electronic knots cannot carry out currents of big force without damages.
• Certain relays can carry out additional functions. For example, the relay breaker of faltering impulses, the relay of switching of intervals in work of system of washing/clarification of glasses operates an interval in work and the movement of screen wipers dry after washing of glasses.
• Control units possess more or less extensive electronic schemes of switching of certain functions, also one more relay sometimes is installed. As an example it is necessary to call the control unit of the central lock and systems of the prevention of breaking.