Siemens-Common Rail System – Vehicle speed signal

1 Wheel speed sensors
2 ABS module

There are two methods available for detecting the vehicle speed:
• Use of a VSS on vehicles with no ABS,
• Via the wheel speed sensors for vehicles with ABS.

The signal from the wheel speed sensors is transmitted via the CAN data bus. The PCM calculates the vehicle speed from this.

The vehicle speed signal is used by the PCM to calculate the gear engaged and as information for the speed control integrated in the PCM.

For calculation of the vehicle speed, the wheel speeds of both front wheels are detected and an average value is calculated.

If one or both front wheel speed signals are faulty, the signals of both rear wheel speed sensors are used and their average is taken as the vehicle speed value. If a fault occurs with the wheel speed sensors (one or both), a reliable vehicle speed signal can no longer be transmitted via the CAN data bus.

Effects of faults

Increased idling speed
Uncomfortable juddering when changing gears
Speed control system inoperative (if installed)
Traction control inoperative (if installed)
Reduction of injected fuel quantity

The vehicle speed signal has only minor effects on exhaust gas emissions and does not exceed the EOBD limits.

The vehicle speed signal is, however, part of the freeze frame data and is therefore classified as MIL active.

Possible diagnostic trouble codes: P0608 (vehicles with VSS), P0500, U2197 (vehicles with ABS).

27. August 2018 by Ron
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