Mitsubishi Evolution X. Manual - part 153

CONSTRUCTION DESCRIPTION

TSB Revision

FOUR-WHEEL ANTI-LOCK BRAKE SYSTEM (4ABS)

35B-7

ASC-ECU

M2351003000713

• This ECU incorporates the ABS function, EBD 

function, ASC function, and TCL function.

• By integrating ABS-ECU into the hydraulic unit, 

no more wiring harnesses for sending signal that 
operates the solenoid valve and pump motor is 
required, assuring higher reliability.

• Self-diagnostic and memory functions are inte-

grated into ASC-ECU. If any malfunction is 
detected by the diagnostic function, ABS-ECU 
activates a fail-safe function and illuminates the 
ABS warning light and brake warning light

*

.

NOTE: 

*

: The brake warning light is used as the 

EBD control warning light.

• ASC-ECU detects the vehicle deceleration speed 

using the signals of the steering wheel sensor, 
wheel speed sensor, G and yaw rate sensor, 
master cylinder pressure sensor, and wheel cylin-
der pressure sensor to acquire the wheel rotation 
status. At the same time, ASC-ECU estimates the 
wheel slipping status based on the prepro-
grammed algorithm, and then controls the sole-
noid valve in the hydraulic unit so that the wheels 
do not lock.

ABS HYDRAULIC PRESSURE CONTROL

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ABS CONTROL CYCLE

1. ASC-ECU calculates the speed and deceleration 

of each wheel based on the signals from the 
wheel speed sensors and G and yaw rate sensors 
of four wheels, and estimates the vehicle speed of 
at that time.

AC506830AB

a

b

c

d

A

B

C

D

e

Increase
Hold
Decrease

Wheel speed

Brake pressure

Actual vehicle speed

Estimated vehicle speed

CONSTRUCTION DESCRIPTION

TSB Revision

FOUR-WHEEL ANTI-LOCK BRAKE SYSTEM (4ABS)

35B-8

2. When the brake 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 
exceeds the specified value (Point A), ASC-ECU 
determines that the wheels are about to be 
locked. At this time, ASC-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 to recover, and when the vehicle speed 
reaches the point B, ASC-ECU outputs the 
pressure hold signal to maintain the wheel 
cylinder fluid pressure. (Between b and c)

4. When the wheel speed deceleration recovers 

further and passes the point C, ASC-ECU 
determines that the wheel lock possibility has 
been eliminated, and maintains the brake fluid 
pressure by outputting the pressure increase 
signal again. (Between c and d)

5. Brake fluid pressure is controlled through the 

repetition of pressure increase and pressure hold. 
(Between d and e)

6. When the wheel deceleration exceeds the 

threshold again, ABS-ECU controls the brake fluid 
pressure by repeating the cycle (Steps 2 to 5).

.

EBD FLUID PRESSURE CONTROL

EBD control is activated within the lower slip ratio 
area where ABS is disabled. EBD calculates the 
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 boosts, holds, and 
depressurizes the rear wheel control solenoid valve 
in the hydraulic unit, and then adjusts the rear wheel 
brake fluid pressure fairly close to an ideal distribu-
tion curve.

.

AC208548AB

Ideal distribution curve when 
seated by fixed persons

EBD operating conceptual design

Rear braking force

Ideal distribution curve when seated by one persons

Braking force distribution curve 
by the existing proportioning valve

Braking force distribution 
by EBD control

Front braking force

Braking Force 
Improved

SYSTEM OPERATION

TSB Revision

FOUR-WHEEL ANTI-LOCK BRAKE SYSTEM (4ABS)

35B-9

INITIAL CHECK

ASC-ECU performs the following initial checks using 
the diagnostic functions. The ABS warning light illu-
minates for approximately 3 seconds (including the 
initial check)

*

 after the ignition switch is turned ON. If 

any malfunction is detected, ABS-ECU continues illu-
minating the ABS warning light and disables the ABS 
control.
NOTE: 

*

: ABS warning light may keep illuminating 

after the ignition switch is turned ON until the startup 
vehicle speed reaches approximately 10 km/h. This 
allows, as long as ASC-ECU retains any diagnostic 
trouble code, which is related to the wheel speed 
sensor malfunction or pump motor malfunction 
recorded during the previous ignition ON status, 
ASC-ECU to continue illuminating the ABS warning 
light until it verifies that the malfunction for that code 
is resolved (startup check).

.

STARTUP CHECK

When the startup vehicle speed reaches approxi-
mately 10 km/h, ASC-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, 
ASC-ECU sequentially energizes each solenoid 
valve for a very short period, and checks the valve 
operation.
NOTE: 

*

: Initial startup indicates the first startup 

after the system has started.

2. Wheel speed sensor check

ABS-ECU checks if the wheel speed sensor 
signal is received from all the wheel speed 
sensors.

.

CONSTANT CHECK

ASC-ECU constantly checks the following items.
1. ASC-ECU

(1) Performs a self-diagnosis of ASC-ECU.
(2) Checks for abnormal output voltage of G and 

yaw rate sensor, and detects an open or short 
circuit in the G and yaw rate sensor.

(3) Checks the output voltage for G and yaw rate 

sensor, and determines that a seizure has 
occurred to the G sensor when the output 
voltage exceeding the specified level 
continues for a certain period or more.

2. ECU power supply

Checks if ECU power supply voltage is within the 
operational range.

3. Wheel speed sensor

(1) Monitors the output current of the sensor 

signal wiring harness, and checks for an 
abnormal output current (open/short circuit).

(2) Checks if there is any wheel that does not 

send pulse signal while the vehicle is in 
motion.

(3) Checks for an input of abnormally higher or 

lower wheel speed in relation to the vehicle 
speed.

4. Pump motor, solenoid valve

Checks that the ASC-ECU output signal and the 
operating statuses of pump motor and solenoid 
valve agree with each other.

FAIL-SAFE AND DIAGNOSTIC FUNCTION

If any malfunction is detected by the diagnostic func-
tion, ASC-ECU illuminates the ABS warning light and 
brake warning light, and disables the ABS and EBD 
controls. In this case, the brake is operated as a nor-
mal brake.

Also, ASC-ECU has the following functions for easier 
system checks.

• Diagnostic trouble code set

• Service data output

• Actuator test

SYSTEM OPERATION

M2351004000374

Refer to GROUP 35C 

− System Operation 

P.35C-14

.

NOTES