Wheel Speed Sensor
The wheel sensors are the magnetic contactless type. As the gear pulser teeth rotate past the wheel sensor's magnetic coil, AC current is generated. The AC frequency changes in accordance with the wheel speed. The ABS control unit detects the wheel sensor signal frequency and thereby detects the wheel speed. See Fig 1 . When the wheel speed drops sharply below the vehicle speed, the outlet valve opens momentarily to reduce the caliper fluid pressure. The pump motor starts at this time. As the wheel speed is restored, the inlet valve opens momentarily to increase the caliper fluid pressure.
When the front wheel speed rises sharply above the vehicle speed, the TCS Normally Closed (NC) valve opens, the TCS Normally Open (NO) valve closes, and the pump motor starts to intensify the front caliper fluid pressure. When the wheel speed drops, the front inlet valve closes momentarily to retain the front caliper fluid pressure. When the wheel speed drops further, the front outlet valve opens momentarily to reduce the front caliper fluid level pressure. See Fig 2 .