ABS fluid pressure control

ABS control cycle

  1. The ABS-ECU calculates the speed and deceleration of each wheel based on the signals from the four wheel speed sensors and the G sensor <4WD> incorporated in the ABS-ECU, and estimates the vehicle speed at that time.
  2. When the brake pedal is depressed, the brake fluid pressure applied to the wheel cylinder increases, and the wheel speed decreases. When the difference between the wheel speed and vehicle speed increases, and the vehicle deceleration goes below the specified value (Point A), ECU determines that the wheels are about to be locked. At this time, ECU reduces the brake fluid pressure by outputting the pressure decrease signal to the solenoid valves (IN, OUT). (between a and b)
  3. When the vehicle deceleration and wheel speed begin recovery, and the vehicle speed reaches the point B, ECU outputs the pressure hold signal to maintain the wheel cylinder fluid pressure. (between b and c)
  4. When the wheel speed deceleration is further recovered and overpasses the point C, ECU determines that the wheel lock possibility has been eliminated and increases the brake fluid pressure by outputting the pressure increase signal again. (between c and d)
  5. Brake fluid pressure is controlled by repeating the increase and hold of the pressure. (between d and e)
  6. When the wheel deceleration goes below the threshold again, ABS-ECU controls the brake fluid pressure by repeating the cycle (Step 2 to 5).

EBD fluid pressure control

EBD control is activated in a range with lower slip ratio where ABS is disabled. EBD calculates vehicle deceleration and slip amount of the four wheels based on the wheel speed sensor signal. If the rear wheel speed differs from the vehicle speed by a certain level or more, EBD increases, holds, and decreases the pressure at the rear wheel control solenoid valve in the hydraulic unit, and then adjusts rear wheel brake fluid pressure fairly close to an ideal distribution curve.


ABS-ECU performs the following initial checks using the diagnostic functions. ABS-ECU illuminates the ABS warning lamp for 3 seconds (including the initial check) * after the ignition switch is turned ON. If any malfunction is detected, ABS-ECU continues illuminating the ABS warning lamp and disables ABS control.
note *: The ABS warning lamp may stay on after the ignition switch is turned ON until the startup vehicle speed reaches approximately 10 km/h. As far as ABS-ECU memorizes any diagnosis code related to the wheel speed sensor malfunction recorded during the previous ignition ON status, ABS-ECU continues illuminating the ABS warning lamp until it verifies that the malfunction for that code is resolved (startup check).


When the startup vehicle speed reaches approximately 10 km/h, ABS-ECU performs the following checks.
1.Motor, solenoid valve check (Initial startup* only)
Turns ON the motor relay in ECU, and checks the pump motor operation. At the same time, ABS-ECU sequentially energizes each solenoid valve in a very short period and checks the valve operation.
note *: Initial startup indicates a first startup after the system has started.
2.Wheel speed sensor check
ABS-ECU checks for any wheels that have not received wheel speed sensor signal from the startup.


ABS-ECU constantly checks the following items.
(1) Performs self-diagnosis in ECU.
(2) Checks for abnormal output voltage of G sensor, and detects open or short circuit in the G sensor <4WD>.
(3) Checks the output voltage for G sensor, and determines that the G sensor is stuck when the output voltage exceeding the specification continues for a certain period or more <4WD>.
2.ECU power supply
Checks if ECU power supply voltage stays within the operational range.
3.Wheel speed sensor
(1) Monitors the output voltage of the sensor signal wiring harness and checks for abnormal output voltage (open/short circuit).
(2) Checks for any wheels that do not send pulse signal while the vehicle is in motion.
(3) Checks if wheel speed which is abnormally higher or lower than the vehicle speed is input.
4.Pump motor, solenoid valve
Checks that the ABS-ECU output signal and the operating conditions of the pump motor and solenoid valve agree with each other.


If any malfunction is detected by the self-diagnostic function, ABS-ECU illuminates the ABS warning lamp and brake warning lamp*, and it disables ABS and EBD control.
note *: The brake warning lamp is used as EBD control warning lamp.


ABS-ECU has the following functions for easier system checks. The following items can be diagnosed using M.U.T.-III.