ABS Hydraulic Modulator
Many ABS systems run self-diagnostic tests when the ignition is turned to the ON/Run position. During this system check, an audible clicking sound can sometimes be heard in the unit. 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.
When a wheel begins to skid, the system senses the rotational speed of the wheel speed sensor. It makes decisions whether to hold or release pressure to the brake circuit. A tire has it's the best traction just before it begins to skid, once skidding a portion of traction and steering is lost.
For Instance, if a driver is in a panic braking situation, and the right front tire begins to lose traction and skid. The EBCM or anti-lock brake control module would sense the difference in the 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 faster than any non-bionic human being could hope to achieve.
The hydraulic control modulator contains the valves, solenoids, and pistons that control the holding and release of the hydraulic brake circuit if a wheel were to begin to skid. During normal operation, the system lies dormant, and the standard brake system is used.