(ABS) Anti-lock brake light
(ABS) Anti-lock brake light
Most people are familiar with the term Anti-lock Brakes (ABS), but many do not know much about anti-lock braking, how it works, what if any special maintenance is required, or what parts can be replaced in anti-lock brake systems.
Anti-lock Brakes are essentially an enhanced or improved version of ordinary brakes. Simply put, the anti-lock brake system is designed to prevent the brakes from locking up and skidding when braking hard or when braking on wet or slick surfaces. This adds a significant margin of safety for everyday driving by preventing dangerous skids and allowing the driver to maintain steering control while trying to stop.
Anti-lock brakes do not necessarily reduce the stopping distance, and in fact may actually increase stopping slightly on dry pavement. But on wet or slick pavement, anti-lock brakes may reduce the stopping distance up to 25% or more, which could be the difference between a safe stop and an accident.
There are quite a few different anti-lock brake systems in use today, but one thing they all share in common is the ability to control wheel lockup during hard braking. A tire that is just on the verge of slipping (10 to 20% slippage) produces more friction with respect to the road than one which is locked and skidding (100% slippage). Once traction is lost, friction is reduced, the tire skids and the vehicle takes longer to stop.
The only exception to this rule is when a tire is on loose snow. A locked tire allows a small wedge of snow to build up ahead of it which allows it to stop in a somewhat shorter distance than a rolling tire. That is why some vehicles have an on/off switch for deactivating the anti-lock system when driving on snow.
Directional stability also depends on traction. As long as a tire does not slip, it will roll only in the direction it turns. But once it skids, it has about as much directional stability as a hockey puck on ice. By minimizing the loss of traction, anti-lock braking helps maintain directional stability and steering control.
Another point to keep in mind about anti-lock brakes is that it is essentially an "add-on" to the existing brake system. It only comes into play when traction conditions are marginal or during sudden "panic" stops. The rest of the time, it has no effect on normal driving or braking.
Anti-lock brake systems are also designed to be as "fail-safe" as possible. Should a failure occur in the ABS control electronics, most systems will deactivate themselves. The ABS warning light will come on, but the vehicle should still have normal braking. This does not necessarily make the vehicle unsafe to drive, but it does mean the ABS system will not be there if needed in an emergency.
An ABS warning light should never be ignored, especially if the brake warning light is also on. This could indicate a potentially dangerous loss of hydraulic pressure or a low fluid level!). If both warning lights are on, the vehicle should not be driven until the brakes can be inspected.
HOW ANTI-LOCK BRAKES WORK
All anti-lock brake systems control tire slip by monitoring the relative deceleration rates of the wheels during braking. If one wheel starts to slow at a faster rate than the others, or at a faster rate than that which is programmed into the anti-lock control module, it indicates the wheel is starting to slip and is in danger of breaking traction and locking up. The ABS system responds by momentarily reducing hydraulic pressure to the brake on the affected wheel or wheels.
Electrically operated solenoid valves are used to hold, release and reapply hydraulic pressure to the brakes. This produces a pulsating effect, which can usually be felt in the brake pedal during hard braking. The driver may also hear a buzzing or chattering noise from the ABS hydraulic unit.
The rapid modulation of brake pressure in the brake circuit reduces the braking load on the slipping wheel and allows it to regain traction, thus preventing lockup. It is the same as pumping the brakes, except that the ABS system does it automatically for each brake circuit, and at speeds that would be humanly impossible, up to dozens of times per second depending on the system (some are faster than others).
Once the rate of deceleration for the affected wheel comes back in line with the others, normal braking function and pressure resume, and anti-lock reverts to a passive mode.