• Wheel Speed Sensors – Each wheel has a sensor that measures how fast it’s rotating.

• Data Processing – The ABS module continuously monitors these sensors.

• Wheel Lock Detection – If it detects that one wheel is slowing much faster than the others (about to lock), it steps in.

• Hydraulic Control – The module commands the ABS hydraulic unit (a system of pumps and valves) to quickly modulate brake pressure on that wheel — reducing it slightly, then reapplying it.

• Rapid Pulsing – This happens many times per second, giving that “pulsing” feel in the brake pedal during an ABS event.

The ABS light (often a yellow/orange circle with “ABS” inside) stays on or flashes.
This is the most direct sign — the module has detected a problem in the ABS system.
A diagnostic scan tool can read the exact trouble code from the ABS module.

2. Brakes Locking Up

Under hard braking, the wheels may lock up instead of pulsing.
This means the ABS system isn’t activating — often because the module isn’t commanding the hydraulic unit correctly.

3. Unresponsive or Weird Brake Pedal

You might feel a hard pedal (no ABS intervention) or a soft/sinking pedal if the valves are stuck open or closed.
Sometimes, the brake pedal pulses at the wrong time or vibrates excessively.

4. Speedometer or Traction Control Problems

In some vehicles, the ABS module shares wheel-speed data with other systems.
If the module fails, you might also see:
Traction Control / Stability Control lights
Speedometer acting erratically

5. ABS Pump Continuously Running

If you hear the ABS pump motor running constantly (even when the car isn’t braking), the control module may have a stuck relay or internal short.
This can drain the battery and burn out the pump motor if left unchecked.