Normal Noises and Pedal Feel

GENERAL INFORMATION
During ABS braking, some brake pedal movement may be felt. In addition, ABS braking will create ticking, popping and/or groaning noises heard by the driver. This is normal due to pressurized fluid being transferred between the master cylinder and the brakes. If 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, ABS will be turned off when the vehicle is slowed to a speed of 3-4 mph. 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 then 3 mph or during an ABS stop where ABS is no longer required. These conditions will exist when a vehicle is being stopped on a road surface with patches of ice, loose gravel or sand on it. Also stopping a vehicle on a bumpy road surface will activate ABS because of the wheel hop caused by the bumps.

TIRE NOISE AND MARKS
Although the ABS system prevents complete wheel lock-up, some wheel slip is desired in order to achieve optimum braking performance. Wheel slip is defined as follows, O percent slip means the wheel is rolling freely and 100 percent slip means the wheel is fully locked. During brake pressure modulation, wheel slip is allowed to reach up to 25 to 30%. This means that the wheel rolling velocity is 25 to 30% 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 total wheel lock-up. Complete wheel lock up normally leaves black tire marks on dry pavement. The ABS will not leave dark black tire marks since the wheel never reaches a fully locked condition. Tire marks may be noticeable as light patched marks.

START UP CYCLE
When the ignition is turned ON, a popping sound and a slight brake pedal movement may be noticed. Additionally, when the vehicle is first driven off a humming may be heard and/or felt by the driver at approximately 20 to 40 km/h (12 to 25 mph). The ABS warning lamp will also be on for up to 4 seconds after the ignition is turned ON. All of these conditions are a normal function of ABS as the system is performing a diagnosis check.