About ABS

Written by Ken Pratte

Some basic terminology to get started:

ABS - Stands for Anti-Lock Braking System.
ABS Actuator - The work horse of the ABS system, it's the piece that actually releases pressure in the ABS channel to modulate the brakes.
ABS Channel - The channel is the hydraulic line(s) from the ABS actuator to the wheel(s). ABS channel can consist of 1 or 2 wheels that will pulse/modulate when the ABS actuator performs its work.
ABS Sensor - Measures the speed of a wheel. Consists of gear-shaped sensor rotor and a sensor element. The element contains a bar magnet around which a coil is wound. The sensor is installed on the back side of the brake rotor. Sine-wave current is generated by the sensor as the wheel rotates. The frequency and voltage increase(s) as the rotating speed increases.
ABS Pulse or modulation - When the ABS Actuator engages and pulses/modulates the brake pressure being applied to the ABS channel. This is the method by which the system actually works. When the brakes pulse, they are prevented from locking because they rotate for a split second.

ABS comes in about 4 different flavors:

1 or 2 Channel 2-wheel (Rear ABS) - This ABS is usually prevalent on trucks. It consists of 2 ABS sensors on the rear wheels and one or two ABS channels to pulse the rear wheel together (1 channel) or separately (2 channels).
2 Channel 4-wheel Criss-cross - This is the ABS system present on the 91-94 Sentra. It consists of 4 ABS sensors (one on each wheel) and 2 ABS channels arranged in a Criss-cross (Left Front & Right Rear, Right Front & Left Rear). When the right rear wheel locks up, the left front wheel & right rear wheel are pulsed together.
3 Channel 4-wheel - This is the more common ABS system in cars. It consists of 4 wheel sensors and 2 channels in the front (LF, RF) and one channel for the rear wheels. When one of the front wheels locks up, it pulses independently of the other wheels. When one of the rear wheels locks up, it is pulsed together with the other rear wheel, similar to a very fast pulling and releasing of the emergency brake.
4 Channel 4-wheel - This is the ABS system present on the 95+ 200SX/Sentra. It consists of 4 ABS sensors and 4 ABS channels. All wheels pulse independently of each other, like it should be.

So, the long and short of it is that a 4 channel system will work better because only the wheel that is locked pulses (loses braking power), while the other 3 wheels continue to do braking for the car. This results in more stability while braking and possibly shorter braking distances.

Some myths about ABS:

With ABS I came stop faster. - Not necessarily. On a wet road, you MAY be able to stop quicker because your wheels are not locked up. However, in several conditions you can stop faster by locking up the wheels. One example is in deep snow. If you lock the brakes you create a wedge effect and stop quicker. Usually ABS distances are a tad shorter (10 feet) than non-ABS brakes, however, ABS gives you more steering ability then non-ABS brakes, the real advantage.
Pumping the brakes will help ABS - NO. You will probably confuse the ABS system more than anything else. If you need to stop in a hurry then press firmly on the brakes and DON'T pump them. The ABS system will pump the brakes for you.
With ABS I can steer around obstacles while braking - Most of the time. This is the theory, but in practice, if you don't have traction, you can't steer. Take for example ice. You could press on the brakes all you want but the car may not turn, because there is very little traction.

Why buy a car equipped with ABS?

Yes, it does cost $500 - $1000 more to add ABS to a car. With it I've been able to steer around dogs that decided to cross the road (one on snow, one on dry pavement). I've also avoided many accidents with other cars because I could brake and steer away from the other vehicle(s). So, I'm willing to pay for it because it's saved my butt several times, not to mention some peace of mind knowing that I can just stomp on the brakes and not have to worry about pumping the ol' brakes.