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

.

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