Electronic Control Unit

In the sedan, the ABS control unit (arrow) is located on the left under the rear seat. In the station wagon, this control unit is located under the left side trim of the luggage compartment.

Similar wheel speed sensors (arrow) are installed on all four wheels of vehicles equipped with ABS.
The control unit is located on the left under the rear seat and processes the signals received from the wheel speed sensors. At the same time, it controls the hydraulic unit so that the wheels do not lock. In addition to complex signal processing and the logic that follows, the control unit also contains a monitoring circuit. With it, the unit can check itself, detect external faults and monitor the operating voltage. If faults are detected, the ABS is switched off and the warning lamp illuminates on the instrument panel.
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A quick Audi 80 quiz: every answer comes with an explanation and a link to the manual page.