Wheel speed

The ABS control unit receives wheel speed information from a sensor on each wheel. Other systems in the car which require information on road speed obtain this information from the ABS control module.

Fig 1: Identifying Control Module Communicating Devices

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Fig 2: Wheel Speed Circuit Diagram

Courtesy of SAAB-SCANIA OF AMERICA, INC.

A transistor in the ABS control module grounds the relevant output 29 times per wheel revolution. The output is supplied with current by the systems receiving the signals.

Pin 23 sends speed signals from the left-hand rear wheel to:

The main instrument unit (MIU), the speedometer
Sensonic

Pin 26 sends speed signals from the right-hand front wheel to:

Motronic 2.10.3
Trionic
EDC 15 (Diesel)

Pin 25 sends speed signals from the left-hand front wheel to:

CC (Cruise).

If voltmeter readings are taken on any of the pins concerned, the readings obtained should alternate between 0 V and about 12 V when the relevant wheel is slowly rotated.

The signals are also sent if the ABS control module is out of order and the ABS lamp is on. The information is sent as long as the respective wheel sensor is in working order.