Function, structure, operation of brake

1. ABS overview

The ABS is a system that prevents wheel lockup when braking and maintains the stability and maneuverability of the vehicle. If a system malfunction occurs, the fail-safe function activates, the ABS control is inhibited, and the warning light illuminates.

2. EBD overview

The EBD controls the braking force distribution of the front wheels and rear wheels according to the load status, and optimizes brake fluid pressure of the rear wheels. If a system malfunction occurs, the fail-safe function activates, EBD control is inhibited, and the warning light illuminates.

3. Brake assist control overview

The brake assist control is a system that assists in generating a strong braking force even when the brake pedal is operated weakly. If a system malfunction occurs, the fail-safe function is activated, the brake assist control is inhibited, and then the warning light illuminates.

4. TCS overview

The TCS is a system that controls engine output and braking and prevents wheel skidding during start-up. If a system malfunction occurs, the fail-safe function activates, the TCS control is inhibited, and then the warning light illuminates.

5. ESC overview

The ESC is a system that controls the braking of each wheel and the engine output when it detects an unstable vehicle condition because of a sudden vehicle posture change while the vehicle is running, and ensures the stability and steerability of the vehicle. If a system malfunction occurs, the fail-safe function activates, the ESC control is inhibited, and then the warning light illuminates.

Running mode	Operation of the brake control function
	ABS	EBD	TCS	ESC
2H (2WD)	Operation	Operation	Operation	Operation
4H (4WD High)	Operation	Operation	Operation	Operation
4L (4WD Low)	Operation	Operation	Operation*	Not in operation
*: Only brake control is performed, but the engine output control is not performed.

6. Trailer sway control overview

The trailer sway control is a system that controls engine output and braking in order to maintain vehicle stability while towing a vehicle. If a system malfunction occurs, the fail-safe function is activated, the trailer sway control is inhibited, and then the warning light illuminates.

7. Hill Start Assist overview

The Hill Start Assist is a system that retains the brake pressure and prevents the vehicle from moving down, when switching from pressing down on the brake pedal to pressing down on the accelerator pedal at vehicle start on an ascending slope. If a system malfunction occurs, the fail-safe function activates, the Hill Start Assist control is inhibited, and then the warning light illuminates.

8. Hill Descent Control overview

The hill descent control is a system that ensures stability while running on a steep downslope at a fixed speed. If a system malfunction occurs, the fail-safe function is activated, the hill descent control is inhibited, and then the warning light illuminates.

9. Emergency Stop Signal (ESS) overview

The Emergency Stop Signal (ESS) is a system that rapidly flashes the rear right and rear left turn signal lights during sudden braking or during ABS operation to prevent a following vehicle from colliding. If a system malfunction occurs, the fail-safe function activates, the ESS control is inhibited, and then the warning light illuminates.

10. External braking request (AEB/ACC) overview

The external braking request is a brake control system performed by the EHCU based on a braking request from the stereo camera. If a system malfunction occurs, the fail-safe function is activated, the external braking request control is inhibited, and then the warning light illuminates.

11. Multi-collision brake (MCB) overview

The multi-collision brake (MCB) is a system that activates the brake when the airbag is deployed due to a collision, etc., and reduces a secondary collision and departure from the road. If a system malfunction occurs, the fail-safe function is activated, the multi-collision brake control is inhibited, and then the warning light illuminates.

1. ABS control

The ABS controls the brake fluid pressure appropriately by having the EHCU retain, reduce, or increase the brake fluid pressure during braking based on signals from the wheel speed sensors.
The EHCU calculates the wheel speed, wheel acceleration/deceleration, and vehicle speed using signals from the wheel speed sensors. When braking, if the wheel speed decreases so rapidly that the difference between the wheel speed and the vehicle speed becomes more than or equal to the predetermined level, the system determines that the wheels are tending toward lock-up and retains the brake pressure. The system determines that if the wheel speed decreases further in this condition, the wheels will tend toward lock-up, and thus reduces the brake fluid pressure. If afterward the system determines that locking of the wheels has been avoided, it returns to increasing, retaining or decreasing the brake fluid pressure.

2. EBD control

EBD is a system that appropriately controls braking force distribution between the front and rear according to the change of the vehicle loaded condition and the load shift caused by deceleration, and the EHCU controls the braking force distribution between the front wheels and the rear wheels during braking.
The EHCU detects the brake pedal stroke based on signals from the master cylinder pressure sensor and activates the EBD control. Once the EBD control is activated, the EHCU calculates the optimum brake force for each wheel based on signals from each wheel speed sensor and controls the hydraulic unit.

3. Brake assist control

For the brake assist control, the EHCU performs the automatic pressurization when the vehicle is suddenly braked and continues increasing the brake pressure up to the ABS operation pressure in order to maintain a high braking force. The EHCU calculates the depressing speed of the brake pedal based on signals from the master cylinder pressure sensor. During the brake assist control, as long as the brake pedal is depressed, the brake pressure increases up to the ABS operation pressure even if the depressing amount is small or constant. When the brake pedal is released, the automatic pressurization stops and the brake assist control is canceled, and then it returns to the normal brake control.

4. TCS function

Sudden acceleration on a slippery road surface may cause the drive wheels to skid, leading to impaired startup acceleration and maneuverability. The TCS suppresses a skid by controlling the drive wheel brake and engine output and maintains the driving force appropriate for the road surface.

5. ESC control

Due to road conditions, vehicle speed, sudden sharp turns in the case of an emergency, external factors, etc., oversteer (strong rear wheel skidding) or understeer (strong front wheel skidding) may occur. When the ESC detects this kind of condition, it warns the driver as well as controls the brakes and the engine output of each wheel to mitigate oversteer or understeer and ensure vehicle stability.

1) Oversteer

If the EHCU detects oversteer, it controls the engine output and brakes mainly the outside wheels at the turning to generate moment against the turning direction, and thereby suppressing the oversteer.

2) Understeer

If the EHCU detects understeer, it controls the engine output and brakes mainly the inside wheels at the turning to generate moment in the turning direction, and thereby suppressing the understeer.

6. Rolling prevention function

When large lateral G on the vehicle is generated, and the vehicle loses stability as its gradient angle increases, this function alerts the driver and performs the engine output control and brake control. It performs the controls to decrease the vehicle speed in order to suppress the lateral G, and illuminates the stoplight.

7. Trailer sway control function (For Australia)

The EHCU detects the width of the sway transmitted from the towed vehicle at the towing vehicle side based on the signals from the yaw rate sensor, and starts the trailer sway control. If the vehicle runs at a speed of 50 km/h {31.1 mph} or more while towing a vehicle, a swaying phenomenon of the towed vehicle may occur depending on road conditions, etc. When the trailer sway control function detects this condition, it illuminates the brake lights and controls the brakes and engine output of the towing vehicle to reduce the swaying phenomenon, and ensures the vehicle stability.

8. Hill Start Assist function

The Hill Start Assist retains the brake pressure for approximately 2 seconds and prevents the vehicle from moving down, when switching from pressing down on the brake pedal to pressing down on the accelerator pedal at vehicle start on an ascending slope with an inclination of 5% or more. This function is activated as well when ascending a slope in the reverse gear and deactivated by turning the ESC OFF.

9. Hill Descent Control function

The hill descent control is a system that ensures stability while running on a steep downslope at a fixed speed. It starts operating when the accelerator pedal is released while driving at a very low speed of 30 km/h or less. The EHCU performs brake control of 4 wheels while monitoring the vehicle speed and gradient. The desired hill descent speed can be adjusted by operating the accelerator pedal or brake pedal. During the hill descent control operation, the stoplight illuminates together with the flashing of the hill descent control indicator light.

1) Hill Descent Control start conditions

2) Hill Descent Control stop conditions (system control status changes to standby status, indicator flashing to illumination)

3) Hill Descent Control system stop conditions (complete termination, indicator OFF)

10. Emergency Stop Signal (ESS) function

While the vehicle is running at a vehicle speed of 60 km/h or more, if a deceleration more than the predetermined level is detected or when the ABS is activated, the EHCU sends the ESS to the BCM via the CAN communication circuit and rapidly flashes all the turn signal lights.

1) ESS control start conditions

While the vehicle is running at a vehicle speed of 60 km/h or more, if any of the following are met, the ESS operates.

2) ESS control stop conditions

After the ESS starts operating, if any of the following are met, the ESS is canceled.

11. External braking request function (Models with stereo cameras)

1) Brake pressurization function

When the stereo camera determines that emergency braking is necessary, it sends a brake request to the EHCU. After the emergency braking is started, the brake fluid pressure for each of the 4 wheels is fully increased, and enhances the braking force. The stoplight illuminates while the brake pressurization function is in operation.

2) Brake retention function

When the own vehicle stops as the vehicle ahead stops while the adaptive cruise control is operating, the stereo camera sends the brake retention request to the EHCU. Based on the request from the stereo camera, the EHCU drives the valve to retain the brake fluid pressure for 2 seconds. While the brake retention function is operating, the brake operates and the stoplight illuminates even though the brake pedal is not depressed.

12. Multi-collision brake (MCB) function (Models with stereo cameras)

When the airbag is deployed due to a collision while driving, etc., the brake is operated automatically to reduce a secondary collision and departure from the road. The stoplight illuminates while the brake is operating by the multi-collision brake.

1) Multi-collision brake operating conditions

The multi-collision brake is activated when all of the following are met.

2) Multi-collision brake non-operating conditions

The multi-collision brake may not be operated if any of the following is met.

3) Multi-collision brake cancellation conditions

If any of the following is met during the multi-collision brake operation, the multi-collision brake is canceled.

1. Hydraulic circuits diagram

<Models with stereo cameras>

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<Except models with stereo cameras>

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2. Normal braking

During normal braking operations, the solenoid valves are not energized, and the hold solenoid valve is opened while the release solenoid valve is closed. Brake fluid pressure passes through the hold solenoid valve and is then force-fed to the wheel cylinder.

3. When the ABS is in operation

The EHCU detects sudden deceleration based on the signal from the wheel speed sensors and drives the solenoid valve of each wheel to retain, decrease, or increase the brake pressure, preventing wheel lock-up.

1) Brake pressure retention

When sudden deceleration signals are input from the wheel speed sensors, the hold solenoid valve closes, brake fluid from the master cylinder is cut off, and the pressure in the wheel cylinders is retained.

2) Brake pressure decrease

When the wheels are about to lock up due to braking, the release solenoid valve is opened, and the brake fluid that has been retained in the wheel cylinders flows into the reservoir to reduce the pressure.

3) Brake pressure increase

When the wheel speed increases due to the pressure being reduced, the hold solenoid valve opens and the release solenoid valve closes. The brake fluid that is force-fed by the pump motor and the brake fluid from the master cylinder are force-fed to the wheel cylinders. At this time, the driver may feel a slight pulsation in the brake pedal, but this is not a malfunction. If the wheel approaches a locking state again with this pressure increase, the EHCU will repeatedly retain, decrease, and increase the fluid pressure.

Operation description

Valve operating condition	When not controlled	During ABS control
		Hold mode	Depressurization mode	Pressurization mode
Hold solenoid valve	Open	Close	Close	Open
Release solenoid valve	Close	Close	Open	Close

4. During TCS operation (Models equipped with ESC)

The TCS control applies the brake by leading hydraulic pressure generated in the pump in the hydraulic unit into the wheel cylinder of the driving wheel and prevents slippage of the vehicle wheel due to driving force.

Operation description

Valve operating condition		When not controlled	TCS control
			Pressurization mode	Hold mode	Depressurization mode
Master cylinder cut valve 1		Open	Regulated	Regulated	Regulated
Master cylinder cut valve 2		Open	Regulated	Regulated	Regulated
MAX oil pressure wheel for 2-wheel control in the same system, or control wheel for 1-wheel control in the same system	Hold solenoid valve	Open	Open	Open	Open
	Release solenoid valve	Close	Close	Close	Close
MIN oil pressure wheel for 2-wheel control in the same system	Hold solenoid valve	Open	Open	Close	Close
	Release solenoid valve	Close	Close	Close	Open
Non-control wheel	Hold solenoid valve	Open	Close	Close	Close
	Release solenoid valve	Close	Close	Close	Close

5. During ESC operation (Models equipped with ESC)

1) In case of the over steer

The brakes are operated by the hydraulic pressure generated by the pump inside the hydraulic unit being guided into the wheel cylinder of each wheel, controlling the side skidding of the rear wheels.
In the case of oversteering, the front and rear wheel brakes on the outer side of the turn are operated.

2) In case of the under steer

The brakes are operated by the hydraulic pressure generated by the pump inside the hydraulic unit being guided into the wheel cylinder of each wheel, controlling the side skidding of the front wheels.
In the case of understeer, the brakes of both rear wheels, or if necessary, the brakes of the front wheel outside of the turn are activated.
As an example, the illustration shows the front left/right brakes and the rear left brake as the controlled wheels, with the rear right brake as the uncontrolled wheel.

Operation description

Valve operating condition			During ESC control
			Pressurization mode	Hold mode	Depressurization mode
Control system	Master cylinder cut valve		Close	Close	Close
	Control wheel	Hold solenoid valve	Open	Close	Close
		Release solenoid valve	Close	Close	Open
	Control wheel	Hold solenoid valve	Open	Close	Close
		Release solenoid valve	Close	Close	Open
Control system (Diagonal wheels)	Master cylinder cut valve		Close	Close	Close
	Control wheel	Hold solenoid valve	Open	Close	Close
		Release solenoid valve	Close	Close	Open
	Non-control wheel	Hold solenoid valve	Close	Close	Close
		Release solenoid valve	Open	Open	Open

1. Wheel speed sensor

The wheel speed sensor uses a Hall IC sensor. The wheel speed sensor for the front wheels is installed to the knuckles, and the wheel speed sensor for the rear wheels is installed to the rear axle case. Voltage is generated in the sensor by the rotation of the front and rear wheels. The frequency of this voltage changes proportionately with the rotation speed, thereby enabling the wheel speed to be detected.

2. Steering angle sensor (Models equipped with ESC)

The steering angle sensor is installed to the combination switch and it detects the steering angle and steering direction. The signal detected by the steering angle sensor is sent to the EHCU via the CAN communication circuit.

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3. Yaw rate sensor (Models equipped with ESC)

The yaw rate sensor is comprised of the semiconductor-type yaw rate sensor and G sensor, and is built into the EHCU. It detects rotating angular speed and lateral acceleration speed due to the changes in vehicle behavior, and sends them to the EHCU.

4. ABS warning light

The ABS warning light notifies the driver of an ABS malfunction. When the EHCU detects an ABS malfunction through the self-diagnosis function, it illuminates the ABS warning light.

5. Brake system warning light

The brake system warning light notifies the driver of an EBD malfunction. It also illuminates if the brake fluid level gets low or other brake-related abnormal conditions are detected.

6. ESC warning light (Models equipped with ESC)

The ESC warning light informs the driver of an ESC malfunction. When the EHCU detects an ESC malfunction through the self-diagnosis function, it illuminates the ESC warning light. The ESC warning light flashes during TCS, ESC, or trailer sway control operation.

7. ESC OFF indicator light (Models equipped with ESC)

When the ESC operation is stopped while the ESC OFF switch is being operated or the 4L (4WD Low) is selected, the ESC OFF indicator light illuminates.

8. TCS OFF indicator light (Models equipped with ESC)

When the TCS operation is stopped while the ESC OFF switch is being operated or the 4L (4WD Low) is selected, the TCS OFF indicator light illuminates.

9. Hill descent control indicator light (Models equipped with ESC)

The hill descent control indicator light illuminates when the system is in a standby status, and flashes when the system is in operation.

10. Hill descent control switch (Models equipped with ESC)

The hill descent control switch turns the hill descent control function ON/OFF.

11. ESC OFF switch (Models equipped with ESC)

In an attempt to escape from mud, etc., depressing the accelerator with the TCS or ESC operating may limit the driving force and make the escape difficult. The TCS and ESC can be deactivated by operating the ESC OFF switch.

1) When 2H (2WD) or 4H (4WD High) is selected for the driving mode

Briefly pushing the ESC OFF switch will switch the TCS to TCS OFF mode, deactivating the TCS. Also, the TCS OFF indication light in the instrument panel cluster illuminates at this time.

Pressing and holding the ESC OFF switch for about 5 seconds or more with the vehicle stopped switches the ESC to ESC OFF mode, deactivating the TCS and ESC. Also, the TCS OFF indicator light and ESC OFF indicator light in the instrument panel cluster illuminate at this time.

Each mode can be canceled by pressing the ESC OFF switch briefly or turning the ignition switch OFF.

2) When 4L (4WD Low) is selected for the driving mode

The ESC function stops operating.
Pressing and holding the ESC OFF switch for about 5 seconds or more with the vehicle stopped switches the TCS to TCS OFF mode, deactivating the TCS. Also, the TCS OFF indicator light and ESC OFF indicator light in the instrument panel cluster illuminate at this time.

The TCS OFF mode can be canceled by pressing the ESC OFF switch briefly or turning the ignition switch OFF.

Running mode	ESC OFF switch operation	Control allowed/prohibited				Indication light
		Engine TCS	Brake TCS	Engine ESC	Brake ESC	TCS OFF	ESC OFF
2H (2WD), 4H (4WD High)	No	Permission	Permission	Permission	Permission	OFF	OFF
	Short press	Inhibition	Permission	Inhibition	Permission	Illuminated	OFF
	Long press (5 seconds or longer)	Inhibition	Inhibition	Inhibition	Inhibition	Illuminated	Illuminated
4L (4WD Low)	No	Inhibition	Permission	Inhibition	Inhibition	Illuminated	Illuminated
	Short press	Inhibition	Permission	Inhibition	Inhibition	Illuminated	Illuminated
	Long press (5 seconds or longer)	Inhibition	Inhibition	Inhibition	Inhibition	Illuminated	Illuminated

12. Rear differential lock switch

The rear differential lock switch switches the differential lock between activation and deactivation.

When the rear differential lock switch is turned ON, the following brake controls are deactivated and the warning light and indicator light are illuminated.

<Brake control not to be operated>

<Warning light/indicator light to be illuminated>

13. EHCU

The EHCU consists of the control unit part and hydraulic unit part. The control unit is comprised of a brake control part, malfunction detection part, and fail-safe part. It drives the hydraulic unit, etc., based on signals from the various sensors. If a malfunction occurs in the system, it has the ability to inhibit each function using the fail-safe function. Using the self-diagnosis function, it also displays defective areas when performing trouble diagnosis. The hydraulic unit is comprised of the motor, pump, solenoid valves, reservoir, and master cylinder pressure sensor, etc.

1) Solenoid valve

When the brake fluid pressure is retained, reduces, or increases, the oil passage is switched.

2) Reservoir

The brake fluid is stored when pressure decreases.

3) Pump

The brake fluid stored in the reservoir is force-fed to the master cylinder.

4) Motor

The pump is driven.

5) Master cylinder pressure sensor

The master cylinder brake fluid pressure is detected.

1. External component location diagram

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2. Internal component location diagram

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1. Braking device system diagram

2. Master cylinder and brake booster

3. Vacuum hose

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4. Front disc brake

5. Rear drum brake

6. Rear wheel cylinder

1. EHCU general circuit diagram

2. EHCU terminal layout

Pin No.	Pin Function
1	BAT (Motor power supply)
2	Vehicle speed signal
3	-
4	FR Wheel speed sensor (-)
5	-
6	-
7	-
8	FL Wheel speed sensor (-)
9	Hill descent control switch
10	-
11	-
12	-
13	GND (Ground)
14	CAN Low
15	ESC OFF switch
16	FR Wheel speed sensor (+)
17	RR Wheel speed sensor (+)
18	RL Wheel speed sensor (-)
19	FL Wheel speed sensor (+)
20	-
21	-
22	-
23	-
24	-
25	BAT (Valve power supply)
26	CAN High
27	-
28	IG (ignition voltage)
29	RR Wheel speed sensor (-)
30	-
31	RL Wheel speed sensor (+)
32	-
33	-
34	-
35	-
36	-
37	DLC
38	GND (Ground)