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The ABS operates by monitoring the speed of each wheel. If one is turning more slowly than the others, then it's assumed to be 'locking up' and the brakes are released on that wheel to allow it to come back up to speed before being reapplied.
So if you already have a computer that knows the speed of the wheels, what's the point in fitting a speed sender to the gearbox? The ABS computer sends the speed signal to the dash (probably over the CAN multiplex) and the dash computer sets the speedo needle to the appropriate position.
Of course if a sensor fails, and it happens to be the one that the ABS uses for the speed reading, you lose your speedo too
Currently in Horatio's garage:
2010 Laguna III Initiale.