S400-851-150-0 | Genuine Wabco® Hydraulic ABS Modulator

This module is typically found on:

1. Mack Trucks medium-duty and heavy-duty trucks equipped with hydraulic brakes and WABCO SmartTrac ABS.
2. Volvo Trucks trucks using the same SmartTrac hydraulic ABS/ATC system.

The ABS module continuously monitors each wheel’s speed.
If it detects a wheel about to lock (stop spinning), it reduces brake pressure momentarily.
This keeps the wheel rotating and maintains traction with the road.

Maintain Steering Control

By preventing wheel lock-up, the driver can steer around obstacles even while braking hard.
Without ABS, locked wheels cause skidding and loss of directional control.

Improve Stability on Slippery Surfaces

The module adjusts braking force to each wheel independently.
This helps the vehicle stay straight and balanced on wet, icy, or uneven roads.

Optimize Brake Pressure

It regulates brake fluid pressure using solenoid valves and a hydraulic pump.
This results in shorter stopping distances and smoother braking under emergency conditions.

Communicate with Other Systems

Modern ABS modules are integrated with:
Traction Control System (TCS)
Electronic Stability Control (ESC)
Hill-Start Assist
The module shares data with the engine and transmission control units to adjust power and braking for overall safety.

1. ABS Warning Light Is On

The ABS light (or sometimes “Brake” or “Traction Control” light) stays illuminated on the dashboard.
This is the most common and earliest sign of a control module fault.
Diagnostic scan will often show ABS-specific trouble codes like C0265, C0110, or U0121.

2. Brakes Lock Up Under Hard Braking

If the ABS module fails, the system can’t regulate brake pressure properly.
You might notice wheel lock-up during sudden stops — especially on slippery surfaces.
This defeats the purpose of the ABS system and increases stopping distance.

3. ABS Pump Runs Constantly (or Not at All)

A stuck internal relay or shorted circuit in the module can cause the ABS pump motor to run continuously, even with the key off.
In other cases, the pump might not activate at all, leading to no ABS function.

4. Unresponsive or Pulsating Brake Pedal

The brake pedal might feel unusually hard, soft, or pulsate abnormally during braking.
Sometimes the pedal sinks slowly to the floor due to poor hydraulic modulation.
These symptoms often indicate the module’s valve control system isn’t working correctly.

5. Traction or Stability Control Lights On

Because the ABS module communicates with the traction control (TCS) and electronic stability control (ESC) systems,
a fault in the ABS module can trigger those warning lights too.
You may lose features like traction assist, hill-start assist, or electronic stability control.

6. Inconsistent Speedometer or Loss of Communication

The ABS module reads data from wheel speed sensors.
If it fails, the speedometer may fluctuate or stop working, and the vehicle may store communication codes like U0121 – Lost Communication with ABS Module.

7. Vehicle Goes into Limp or Fail-Safe Mode

On some vehicles, when the ABS module fails completely, the system may limit performance (limp mode) to protect the drivetrain and braking system.