When the ECU receives signals showing one or more wheels about to lock, it sends an electrical signal to the solenoid valve(s) within the actuator to release the brake pressure in the line. The solenoid moves to a position which allows some pressurized brake fluid to flow out of the brake line into a holding reservoir. As the wheel unlocks or rolls faster, the ECU senses the increase and signals the solenoid to re-apply hydraulic pressure. Additionally, the ECU can signal the solenoid to hold present line pressure, allowing neither increase nor decrease if the situation requires it.
This cycling occurs several times per second when ABS is engaged. In this fashion, the wheels are kept just below the point of lock-up and control is maintained. When the hard braking ends, the ECU resets the solenoids to its normal or build mode. Brake line fluid pressures are then increased or modulated directly by pressure on the brake pedal. Fluid released to the ABS reservoirs is returned to the master cylinder by the pump and motor within the actuator.
On most systems, the front wheels are controlled individually, although the Sentra and NX Coupe's system links control with the opposite rear wheel. Depending on the model, the rear wheel solenoid circuits may receive the same electrical signal or be under individual control.
The operator may hear a popping or clicking sound as the pump and/or control valves cycle ON and OFF during normal ABS operation. The sounds are due to normal operation and are not indicative of a system problem. Under most conditions, the sounds are only faintly audible. If ABS is engaged, the operator may notice some pulsation in the body of the vehicle during a hard stop; this is generally due to suspension shudder as the brake pressures are altered rapidly and the forces transfer to the vehicle.
Although the ABS system prevents wheel lock-up under hard braking, as brake pressure increases wheel slip is allowed to increase as well. This slip will result in some tire chirp during ABS operation. The sound should not be interpreted as lock-up but rather as an indication of the system holding the wheel(s) just outside the point of lock-up. Additionally, the final few feet of an ABS-engaged stop may be completed with the wheels locked; the electronic controls do not operate below approximately 6 mph.
DEPRESSURIZING THE SYSTEM
This is a low pressure system, No special system depressurization is necessary. Follow normal service procedure for bleeding, filling and repairs to the system.
The brake fluid reservoir is located on top of the master cylinder. While no special procedures are needed to fill the fluid, the reservoir cap and surrounding area must be wiped clean of all dirt and debris before removing the cap. The slightest dirt in the fluid can cause a system malfunction. Use only DOT 3 fluid from an unopened container. Use of old, polluted or non-approved fluid can seriously impair the function of the system.
On vehicles equipped with anti-lock brake (ABS) systems, turn the ignition switch to the OFF position and disconnect the negative battery cable. All other steps apply to both ABS and conventional brake systems.
For the specific procedure, refer to bleeding of the conventional brake system as outlined in this section.