Car (South Africa) - - TECH -

Au­to­mo­tive tech can be puz­zling. To ease the con­fu­sion, each month we ex­plain how a com­po­nent in your ve­hi­cle works

A wheel-speed sen­sor is an in­duc­tive pick-up that sends pulses to the ABS con­trol unit to cal­cu­late wheel speed in real time. The electric pulses are gen­er­ated when a per­fo­rated ring or me­tal teeth passes close to the sen­sor. The num­ber of teeth per rev­o­lu­tion (as well as wheel sizes) is stored and avail­able to the ABS con­trol unit as part of the in­put to the cal­cu­la­tions. All four wheels’ speeds are needed for ABS and elec­tronic sta­bil­ity con­trol func­tion­al­ity. The ve­hi­cle speed is usu­ally cal­cu­lated as the av­er­age of all four sen­sor val­ues to still be ac­cu­rate around bends where in­di­vid­ual wheel speeds dif­fer. A faulty wheel-speed sen­sor is the most com­mon fault present when the ABS light is on in the in­stru­ment clus­ter. Us­ing a spe­cialised di­ag­nos­tic scan tool, all four ve­hi­cle-speed val­ues can be read from the CAN bus of a ve­hi­cle through the di­ag­nos­tic OBD port to aid the fault-find­ing process.

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