The most important function of the ABS is to keep the driver in control of the vehicle during heavy braking and on different road surfaces. The major function of this system is to control each wheel's brake line pressure by monitoring their rotational speeds under braking thereby preventing them from locking up. At the right rear corner of the engine compartment, there is an actuator assembly that consists of an electric hydraulic pump and a pair of solenoid valves for each wheel. During ABS operation, hydraulic pressure to charge the reservoirs in the actuator is supplied by an electric pump which, in turn, provides pressure to the braking system. During ABS operation, these solenoid valves are used to modulate brake line pressure. There are four valves in its body, one for each wheel. The actuator assembly contains the pump, reservoirs and solenoid valves. The sine wave current is generated by speed sensors located at each wheel when their sensor rotors are rotating. An analog voltage signal generated from this current is converted into digital signal by the ABS Control Module and can be used to determine wheel rotation speeds. For instance, it monitors the information sent by speed sensors of wheels and controls hydraulic line pressure to prevent wheel lock-up as well as other malfunctions associated with system failure (Ojala & Fawcett 21). The control unit also monitors the system for malfunctions, even when the ABS system is inactive during normal driving conditions. When starting up your car or truck you will see that upon start up of around one second your ABS warning light will come on but once your engine starts it should go off immediately. The first time you exceed 4 mph on a vehicle after starting it up it performs a self-test procedure. You may hear a mechanical noise during this test; there's nothing wrong if you do though. If there's something wrong with this system, then its light will come on and remain lit until it has been fixed. A diagnostic code will also be stored in the control unit, which indicates the problem area or component.