ABS Hydraulic Modulator
The ABS system has a self diagnostic testing system that runs a check on the solenoids and other system parts when the ignition is turned to the ON position. During this system check an audible clicking sound can sometimes be heard coming from the system. This is a normal condition. Older model vehicles will sometimes make this same clicking noise after the vehicle starts moving. This too is a normal condition.
The hydraulic control valve assembly or ABS hydraulic modulator contains 
the valves, solenoids, and pistons. This assembly controls the holding and release of the different brake hydraulic circuits under hard braking situations. During normal operation the system lies dormant and the regular brake system is used. When a hard braking situation occurs the system senses the rotational speed of the wheel speed sensors and makes decisions whether to hold or release pressure to a brake circuit. A tire has it's best traction just before it begins to skid, once it begins to skid both traction and steering can be lost.
For Instance: If a vehicle is in a hard braking situation and the right front tire begins to skid. The EBCM or anti-lock brake abs control module would sense the difference in rotational speed of this wheel compared to the others. It would then release/hold pressure to this brake circuit until optimum traction is achieved. This pumping action is far faster than any human being could possibly accomplish.
The only condition ABS would not be desired is during a light snow. The vehicle would actually be better off with the wedge of snow built up by the skidding tire. This is why some models come with an ABS Anti-Skid disable switch.
