ABS operation is available at vehicle speeds above 5 mph (8 km/h). If a wheel locking is detected during a brake application, the brake system enters the ABS mode. During ABS braking, hydraulic pressure in the four wheel circuits is modulated to prevent any wheel from locking. Each wheel circuit is designed with a set of electric solenoids to allow modulation. Wheel lockup may be perceived at the very end of an ABS stop and is considered normal. During an ABS stop, the brakes hydraulic system is still diagonally split. However, the brake system pressure is further split into four control channels. During antilock operation of the vehicle-s brake system, the wheels are controlled independently and are on separate control channels. The system can build, hold and release pressure at each wheel, depending on signals generated by the wheel speed sensors (WSS) at each wheel and received at the controller antilock brake (CAB).
This section covers the physical and operational descriptions and the on-car service procedures for Electronic Stability Program (ESP). The purpose of this four-channel design ESP is to prevent wheel lockup under braking conditions on virtually any type of road surface. Antilock braking is desirable because a vehicle that is stopped without locking the wheels retains directional stability and some steering capability. This allows the driver to retain greater control of the vehicle during braking. The ABS system features the following components:
The system carries out a self-check after start-up. At the same time, the return pump in the hydraulic control unit (HCU) also starts up briefly (this may be felt in the brake pedal). If a fault is found in the process, the antilock warning indicator lamp illuminates. Any system faults which occur while driving also cause the antilock warning indicator lamp to illuminate and stay on constantly. If the antilock warning indicator lamp comes on while driving and then goes out again after a short time, the cause may be battery under-voltage.
Noise & Brake Pedal Feel
During ABS braking, some brake pedal movement may be felt. In addition, ABS braking will create a ticking, popping, or groaning noises heard by the driver. This is normal and is due to the pressurized fluid being transferred between the master cylinder and the brakes themselves. If an ABS operation occurs during hard braking, some pulsation may be felt in the vehicle body due to fore-and-aft movement of the suspension as brake pressures are modulated. At the end of an ABS stop, the ABS is turned off when the vehicle is slowed to a speed of 3 mph (5 km/h). There may be a slight brake pedal drop anytime that the ABS is deactivated, such as at the end of the stop when the vehicle speed is less than 3 mph (5 km/h) or during an ABS stop where the ABS is no longer required. These conditions exist when a vehicle is being stopped on a road surface with patches of ice, loose gravel, or sand. Also, stopping a vehicle on a bumpy road surface activates the ABS system due to -wheel hop- caused by bumps in the road.
Premature ABS Cycling
Symptoms of premature ABS cycling include: clicking sounds from the solenoid valves, pump/motor running, and pulsations in the brake pedal. Premature ABS cycling can occur at any braking rate of the vehicle and on any type of road surface. The amber antilock warning indicator lamp may illuminate but no fault codes are stored in the controller antilock brake (CAB). Premature ABS cycling is a condition that needs to be correctly assessed when diagnosing problems with the antilock brake system. It may be necessary to use a DRB IIIT scan tool to detect and verify any premature ABS cycling. Check the following common causes when diagnosing premature ABS cycling: damaged tone wheels, incorrect tone wheels, damaged steering knuckle wheel speed sensor mounting bosses, loose wheel speed sensor mounting bolts, excessive tone wheel runout, excessively large tone wheel-to-wheel speed sensor air gap, or a damaged speed sensor head face. Give special attention to these components when diagnosing a vehicle exhibiting premature ABS cycling. After diagnosing a defective component, repair or replacement is required. When the component repair or replacement is completed, test drive the vehicle to verify that any premature ABS cycling has been corrected.
There is a antilock warning indicator lamp in the instrument cluster. When the ignition is switched on (ignition switch in position 929), the antilock warning indicator lamp in the instrument cluster illuminates, this is referred to as a bulb check the antilock warning indicator lamp then goes out after the engine is started. If the antilock warning indicator lamp illuminates when the engine is running, there may be a fault in the ABS system. The system is at this point inoperative, but full operation of the normal braking system is retained without ABS protection.
Tire Noise And Marks
Although the ABS system prevents complete wheel lockup, some wheel slip is desired in order to achieve optimum braking performance. Wheel slip is defined as follows: -0- percent slip means the wheel is rolling freely. -100- percent slip means the wheel is fully locked. During brake pressure modulation, wheel slip is allowed to reach up to -25-30- percent. This means that the wheel rolling velocity is -25 - 30- percent less than that of a free rolling wheel at a given vehicle speed. This slip may result in some tire chirping, depending on the road surface. This sound should not be interpreted as a total wheel lockup. Complete wheel lockup normally leaves heavy black tire marks on dry pavement. An ABS stop will not leave heavy black tire marks since the wheel never reaches a fully locked condition. However, tire marks may be noticeable as light patched marks. This is a normal ABS occurrence.