Used on the 1991 Accord and Prelude, the system is designed to prevent wheel lockup during hard or extreme braking. By preventing wheel lockup, maximum braking force is maintained while maintaining control. Some control is also maintained when stopped. ABS operates regardless of road surface conditions.
There are conditions in which ABS is of no use. When the tires ride on a film of water, losing contact with the road surface, hydroskid can occur. This makes the car completely uncontrollable until contact with the road is restored. Sharp maneuvers at high speed or when cornering beyond the tire's grip limits can cause the tire to skid, which does not respond to the car's braking. For this reason, the system is called anti-lock rather than traction control. Wheel spin under acceleration on slippery surfaces can also cause the system to fail.
System action
Under normal braking conditions, ABS functions in the same way as a standard brake 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 the speed sensors. The unit initiates anti-lock braking when it "senses" the approach of locking in any wheel and immediately takes control of the brake pressure in the brake system pipeline at the corresponding wheel. The modulator assembly is separate from the master cylinder and booster. It contains wheel solenoid valves used to control the brake fluid pressure in each wheel.
During anti-lock braking, the pressure in the line is controlled by rapidly alternating the operation of electronic valves inside the modulator. These valves allow the pressure inside the system to increase, remain constant or decrease depending on the situation determined by the control unit. The front wheels are controlled separately. The rear wheel circuits have the same pressure control, based on information received from the rear wheel that has an increased tendency to lock.
During anti-lock operation, the driver may hear popping or clicking sounds as the pump and/or control valves cycle on and off. The sounds are normal system operation and do not indicate a malfunction. In most cases, the sounds are barely audible.
When ABS is in operation, the driver may notice some pulsation in the brake pedal and/or the vehicle body when stopping abruptly; this is also a normal action of the system, but it can surprise a driver who is dealing with ABS for the first time.
Although ABS prevents the wheel from locking due to the increase in brake pressure under heavy braking, wheel slippage may still increase. This slippage results in a slight squealing sound in the tire when ABS is in effect. The sound should not be perceived as a sign of locking, but rather as an indication that the system is holding the wheel just above the locking point. In addition, the car may travel the last few meters (centimeters) when stopping with ABS in effect 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 system operation is controlled by the control unit. If a malfunction is detected (loss of signal from the speed sensor, excessive pump operation time, etc.), ABS is immediately switched off. Normal brake operation remains, but the anti-lock function is switched off. Malfunctions are associated with one or more diagnostic codes, and a flashing warning lamp lights up. Diagnostic codes are stored in the control unit's memory.
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