S4008518810 | Genuine Wabco® ABS Hydraulic Control Module Kit 12V

1. Driver presses the brake pedal

• Brake fluid is sent toward the wheels like a normal braking system

• The HCU is initially just passing pressure through

2. Wheels begin to slow

• Wheel speed sensors monitor each wheel

• If one wheel is about to lock, the ABS system detects it

3. ABS system sends command

• The ABS control module (computer) tells the HCU to act

4. HCU reduces brake pressure

• The HCU releases pressure to the slipping wheel

• This prevents the wheel from locking up

5. HCU holds pressure

• It temporarily holds the pressure steady

• Keeps the wheel rotating at a controlled speed

6. HCU reapplies pressure

• It reapplies braking force once traction is regained

7. Cycle repeats rapidly

• This happens many times per second

• Much faster than a human could pump brakes

1. ABS warning light comes on

• First and most common sign

• The system detects a fault in the ABS (often HCU-related)

• You’ll see the ABS light on the dashboard

2. Traction control / stability control warning

• Since these systems rely on ABS, they may also deactivate

• You might see additional warning lights (traction or stability control)

3. Loss of ABS function

• Brakes still work, but ABS no longer activates

• During hard braking, wheels may lock up and skid

4. Unusual brake pedal feel

• Pedal may feel:

  • Too soft (spongy)

  • Too hard

  • Pulsing when it shouldn’t

5. ABS pump running abnormally

• You may hear:

  • Pump running constantly

  • Or not running at all when it should

• This points to a failing motor inside the HCU

6. Inconsistent braking behavior

• Vehicle may:

  • Pull to one side while braking

  • Brake unevenly between wheels

7. Increased stopping distance

• Especially noticeable on:

  • Wet roads

  • Snow/ice

• Because ABS is no longer managing traction properly

8. Brake fluid leaks (less common but serious)

• Internal or external leak in the HCU

• Can lead to loss of braking performance