Used on the 1991 Accord and Prelude, the system is designed to prevent wheel lockup during hard or emergency braking. By preventing wheel lockup, maximum braking force is maintained while maintaining control. Also, some control is maintained when stopped. ABS functions regardless of road surface conditions.
There are conditions in which ABS is of no benefit. When the tires ride on a film of water, losing contact with the road surface, hydroslip is possible. It makes the car completely uncontrollable until contact with the road is restored. Sharp maneuvering at high speeds or when cornering out of the grip of the tire can result in slip that does not respond to vehicle braking. For this reason, the system is called anti-lock, and not anti-slip. Spinning the wheel while accelerating on slippery surfaces can also cause the system to malfunction.
System action
Under normal braking conditions, ABS functions in the same way as a standard braking system. The entire system is a combination of electrical and hydraulic components that control the flow of brake fluid to the wheels as needed.
The ABS control unit - the electronic brain of the system - receives and processes signals from speed sensors. The block starts counter-bocking when it "feels" approaching a blockage in any wheel and immediately takes control of the brake line pressure at the corresponding wheel. The modulator assembly is separate from the master cylinder and amplifier. It contains wheel solenoid valves used to control brake fluid pressure at each wheel.
During anti-lock braking, line pressure is controlled by rapidly cycling electronic valves within the modulator. These valves allow the pressure within the system to increase, remain constant or decrease depending on the situation determined by the control unit. The front wheels are steered separately. The rear wheel circuits have the same pressure control based on information from whichever rear wheel has the highest tendency to lock up.
During the system's anti-lock action, the driver may hear popping or clicking sounds as the pump and/or control valves turn on and off. The sounds occur during normal system operation and do not indicate a malfunction. In most cases, the sounds are barely audible.
When the ABS is in operation, the driver may notice some pulsation in the brake pedal and/or the car body when coming to a sudden stop; this is also a normal operation of the system, but may surprise the driver who is dealing with ABS for the first time.
Although ABS prevents the wheel from locking up due to increased brake pressure during heavy braking, wheel slip can still increase. This slip results in some whistle in the tire when the ABS is applied. The sound should not be taken as a sign of blockage, but rather an indication that the system is holding the wheel just above the point of blockage. In addition, the last few meters (centimeters) when stopped while the ABS is active, the car can overcome with locked wheels; electronic control does not function at very low speeds.
When the ignition is on and the vehicle speed is above 10 km/h, the operation of the system is controlled by the control unit. Finding a fault (loss of signal from the speed sensor, excessive pump run time, etc.), ABS is immediately disabled. Normal brake function remains, but the anti-lock function is disabled. Faults correspond to one or more diagnostic codes, and a flashing warning lamp lights up. Diagnostic codes are stored in the memory of the control unit.