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 and makes decisions whether to hold or release pressure to the brake circuit. A tire has it's best traction just before it begins to skid, once skidding a portion of traction and steering will be lost.
For Instance, if an driver is in a panic braking situation and the right front tire begins to loose traction and skid. The EBCM or anti-lock brake 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 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.