Go back and read my first reply about needing a scanner to view live data to see which signal is being lost. For everything that generates a voltage mechanically, meaning a generator / alternator, distributor pickup coil, ABS wheel speed sensor, etc, three things are needed. Those are a coil of wire, a magnet / magnetic field, and most importantly, movement between those two. Faster movement causes an increase in the voltage being developed. Faster movement comes from increasing engine speed for a generator, and increasing wheel speed for a speed sensor. Signal voltage drops at lower speeds, and that's when you say the problem is occurring. The signal drops too low for the computer to see it so it thinks that wheel has locked up and is skidding. To the computer that is a normal condition and is what it is there for to correct. The noise you're hearing is the valves pulsing on and off to do their thing and reduce brake pressure to the affected wheel. Since the computer is reacting to what it considers a normal condition, it thinks there is no defect so it doesn't set a diagnostic fault code. There is no easy way to figure out which wheel is causing the problem other than by watching the wheel speed readouts on a scanner.
Tuesday, December 29th, 2015 AT 3:54 PM