Subaru 1998 Impreza Owner's Manual page 190

D When you feel the ABS system operating, you should maintain
constant brake pedal pressure. Do not pump the brake pedal
since doing so may defeat the operation of the ABS system.
The ABS system prevents the lock-up of wheels which may occur during
sudden braking or braking on slippery road surfaces. This helps prevent
the loss of steering control and directional stability caused by wheel
lock-up.
When the ABS system is operating, you may hear a chattering noise or
feel a slight vibration in the brake pedal. This is normal when the ABS
operates.
The ABS system will not operate when the vehicle speed is below
approximately 6 mph (10 km/h).
B ABS system self-checking
You may feel a slight shock in the brake pedal and hear the operating
sound of ABS from the engine compartment just after the vehicle
started. These are caused by an automatic functional test of the ABS
system being carried out and does not indicate any abnormal condi-
tion.
B ABS warning light
The ABS warning light comes on when the ignition switch is turned to
the "ON" position and goes out after about two seconds.
This is an indication that the ABS system is working properly.
Starting and operating
HG0346
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