Based on signals from the speed sensors, the Skid Control ECU senses the rotational speed of the wheels as well as the vehicle speed. During braking, although the rotational speed of the wheels drops, the amount of deceleration will vary depending on both the vehicle speed during braking and the road surface conditions, such as dry asphalt, wet or icy surface, etc. In other words, the ECU judges the amount of slipping between the wheels and the road surface from the change in the wheel’s rotational speed during braking, and controls the solenoid valves of the brake actuator in the following 3 modepressure reduction, pressure holding, and pressure increase modes, to optimally control the speed of the wheels.
The ECU continuously receives wheel speed signals from the four speed sensors, and estimates the vehicle speed by calculating the speed and deceleration of each wheel. When the brake pedal is depressed, the hydraulic pressure in each wheel cylinder begins to increase and the wheel speed begins to decrease. If any of the wheels seems about to lock up, the ECU decreases the hydraulic pressure in the wheel cylinder of that wheel.
(1) Wheel speed control
- Section A The Skid Control ECU sets the solenoid valves to the pressure reduction mode according to the deceleration rate of the wheels, thus reducing the hydraulic pressure in the wheel cylinder. After the pressure drops, the ECU switches the solenoid valves to the ”holding” mode to monitor the change in wheel speed. If the ECU judges that the hydraulic pressure needs to be reduced further, it will reduce the pressure again.
- Section B When the hydraulic pressure inside the wheel cylinder decreases (section A), the hydraulic pressure applied to the wheel falls. This allows the wheel that was on the verge of locking up to speed up. However, if the hydraulic pressure is held down, the braking force acting on the wheel will become too low. To prevent this, the ECU sets the solenoid valves to the ”pressure increase” mode and ”holdingmode alternately as the wheel which was on the verge of locking up recovers speed.
- Section C As the hydraulic pressure is gradually increased in the wheel cylinder by the ECU (section B), the wheel tends to lock up again. Therefore, the ECU again switches the solenoid valves to the ”pressure reduction” mode to reduce the hydraulic pressure inside the wheel cylinder.
- Section D Since the hydraulic pressure in the wheel cylinder is decreased again (section C), the ECU starts to increase the pressure again as in section B.
(2) Initial check function
The Skid Control ECU operates the solenoid valves and the pump motors in sequence in order to check the electrical system of the ABS. This function operates each time when the ignition switch is turned ON, and the vehicle is traveling at speeds greater than 6 km/h (4 mph) with the stop light (brake light) switch OFF. It operates once only each time the ignition switch is turned ON.
(3) Diagnostic function
If a malfunction occurs in any of the signal systems, the ABS warning light in the combination meter will light up, as indicated in the table at the left, and warns the driver that a malfunction has occurred. At the same time, the DTCs (Diagnosis Trouble Codes) are stored in memory. The DTCs can be read by connecting the hand-held tester to the DLC3 to communicate with the ECU directly or causing a short between TC and CG terminals of the DLC3 and observing the blinking pattern of the ABS warning light.
This system has a sensor signal check function. The sensor signals can be read by connecting the handheld tester to the DLC3 or causing a short between TS and CG terminals of the DLC3 and observing the blinking pattern of the ABS warning light.
For details on the DTCs that are stored in the Skid Control ECU memory and DTCs that are output through the sensor check function, refer to the Repair Manual.
The DTCs can be cleared by connecting the hand-held tester to the DLC3 or causing a short between TC and CG terminals of check connector and depressing the brake pedal 8 or more times within 5 seconds.
(4) Fail-safe function
If the Skid Control ECU detects a malfunction in the signal system or there is a malfunction in the relay, electrical current to the actuator from the ECU is turned off. As a result, the brake system operates as if the ABS were not operating, there by assuring normal braking functions.