What Is an ABS With a Four-Channel Modulator?

The antilock modulator is responsible for turning computer logic into physical movement.

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While initially developed by French aviation pioneer Gabriel Voisin in 1929, antilock braking systems didn't see use on mass-produced aircraft until the America's Cold War years. The ABS system could help bring heavy nuclear bombers like the B-47 to a stop much more quickly, which meant shorter runways and smaller targets for enemy bombers. Decades later, the modern electronic ABS system uses a computer and a host of sensors to vary brake pressure. But the computer is only half the equation; at some point, electrons must turn into physical movement, and that's where the modulator comes in.

Related Searches: Braking Basics

Brakes are one of the few systems on your car that are too powerful for their own good. An hydraulic braking system starts at the brake calipers, which squeeze the brake pads against the rotors or drums. The calipers get their squeezing power from an hydraulic "slave" cylinder, which fill with fluid displaced by a "master" cylinder attached to your brake pedal. If the slave cylinders push down hard enough on the pads, they'll stop the tire in its tracks and cause it to lock up. An antilock braking system maximizes brake performance by keeping the tire right on the edge of lockup in a process known as "threshold breaking."

How ABS Works

A basic ABS system consists of four basic parts. The wheel speed sensors mounted either to the backs of the wheels or to the center differential send a voltage signal that varies in strength and frequency according to how fast the wheel spins. A computer compares the speed signals from all four sensors; if the input from one sensor suddenly drops, the computer knows that wheel has locked up. It then actuates a valve that releases pressure from the brake line of the locked wheel, which has the same effect as lifting your foot from the brake pedal. Once the wheel starts spinning again, the computer opens another valve from an accumulator that stores pressurized fluid, thus restoring braking power to that wheel.

The Modulator

The Modulator is, in many ways, the heart of your ABS system, the part that initiates and controls all physical actions within the system. The modulator contains the valves and pistons that regulate fluid pressure to the individual wheels, or circuits. Modulators come in several different varieties, but can generally be subdivided into one of two. "Integral" modulators are actually part of the master cylinder -- or, more properly, the master cylinder is incorporated into the modulator -- and "non-integral" modulators are stand-alone units. There's no real performance difference between the two, but the non-integral unit is cheaper to service and replace if damaged or non-functional.

ABS Channels

ABS systems have a few different modes of function: one-channel, three-channel and four-channel. A "channel" is a controlled part of the ABS system -- essentially, it's a valve in the modulator. If a modulator has two valves, it's a two-channel. If it has three valves it's a three-channel, and so on. One-channel systems often see use on the rear axles of pickup trucks, which are very prone to locking up. In a one-channel system, the ABS can only modulate pressure to the rear axle. The common four-channel system uses four valves and four sensors, allowing for independent control of each wheel. A three-channel system is essentially a four-channel system in the front and a one-channel system in the rear; this type may use one sensor for each wheel, or two sensors in the front and one sensor on the rear differential.

ReferencesHigh-Performance Brake Systems; James WalkerRace Car Engineering and Dynamics; Paul Van ValkenburghPhoto Credit Jupiterimages/Photos.com/Getty ImagesRead Next:

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