The Anti-lock brake system is to enable the vehicle to have manoeuvrability and direction stability, and enhance the best degree of deceleration in extreme braking situation on most surfaces of the road by constantly detecting the rate of the wheel's rotational speed and controlling the brake hydraulic pressure to avoid early locking of the wheels. Two types of systems are utilized: Rear Wheel Anti-lock Brake System (RABS): Controls lock-up on the rear wheels 4-Wheel Anti-lock Brake System (4WABS): Lock-up prevention on all 4 wheels Hydraulic control unit is placed in the left front corner of the engine compartment and the control module, which is considered to be the 'brain' of the system, takes analog voltage inputs from the speed sensors to regulate the hydraulic line pressure and to prevent the wheels from locking up. The engine control module and the transmission control module are always on the lookout for system faults, which trigger the ABS warning light and may even turn off the anti-lock system if the problem is severe enough; a diagnostic trouble code is retained for the technician. A speed sensor produces an analog voltage signal that must be processed into digital data by the control module to measure wheel rotation speed; there is one rear wheel speed sensor in the differential housing and the extra front-wheel sensors in 4WABS. The brake light switch informs the control unit when the brake pedal is depressed to turn on the anti-lock system which is turned off when the brake pedal is released. Preliminary checks for the ABS warning light on, should include checking if the parking brake is on, checking the condition of the brake pads and calipers, inspecting the electrical connectors on the control unit, checking fuses for the system, making sure there are secure connections and no damaged wires to the speed sensors as well as brake light switch. In the event that these checks do not settle the problem, then it is advisable to seek professional diagnosis.