Brakes, ABS & Stability Control
Brake fluid, pad thickness, rotor warping, brake fade, ABS operation, and Electronic Stability Control.
- Explain ABS operation and component functions
- Describe Electronic Stability Control systems
- Identify brake fluid types and hygroscopic properties
- Determine brake pad and rotor service limits
- Diagnose brake fade causes and remedies
Lesson 1
Anti-lock Braking System Fundamentals
What Is ABS?
ABS stands for Anti-lock Braking System. The purpose of ABS is to prevent wheel lockup during hard braking, allowing the driver to maintain steering control. Without ABS, locked wheels slide on the road surface, eliminating steering ability and increasing stopping distances on most surfaces.
How ABS Works
The ABS system uses wheel speed sensors at each wheel to monitor rotational speed. When the ABS module detects that a wheel is about to lock (its speed drops significantly compared to other wheels), it rapidly modulates brake pressure at that wheel. The component that modulates brake pressure in an ABS system is the hydraulic control unit (HCU), which contains solenoid valves and a pump motor.
The ABS modulation cycle happens extremely fast - up to 15 times per second. The driver feels this as a pulsation in the brake pedal, which is normal ABS operation. Drivers should maintain steady pressure on the brake pedal and not pump the brakes when ABS activates.
ABS Warning Light
When the ABS warning light is on, the ABS system is disabled but normal (non-ABS) braking still works. The vehicle can still stop - it just will not have anti-lock protection. A scan tool is needed to read the ABS DTC and diagnose the fault.
ABS prevents wheel lockup by having the hydraulic control unit (HCU) rapidly modulate brake pressure using solenoid valves. ABS maintains steering control during hard braking - the pedal pulsation is normal operation.