1995 ACURA TL Service Repair Manual

Piston/valve:

The piston/valve assembly consists of the piston, cut valve, and sleeve. There are four piston/valve assemblies in the mod-

ulator unit to control the brake fluid pressure to each caliper. The piston/valve assemblies for the rear brakes also serve as

proportioning control valves to prevent the rear wheels from locking if the ABS malfunctions or when the ABS is not acti-

vated.

Solenoid valves:

The modulator unit opens and closes the inlet and outlet solenoid valves, and shifts the ABS high-pressure passage

according to the signals from the ABS control unit. There are three solenoid valve assemblies, each containing an inlet

and outlet valve, in the modulator unit; one for each front wheel, and one for both rear wheels. The inlet valves are nor-

mally open (open when the coil is not energized), while the outlet valves are normally closed.

MODULATOR UNIT

RESERVOIR

To ABS CONTROL

UNIT

To RIGHT-FRONT

WHEEL

To LEFT-FRONT

WHEEL

PUMP MOTOR RELAY
From MASTER

CYLINDER

PRIMARY PISTON

From MASTER

CYLINDER

SECONDARY

PISTON

To LEFT-REAR

WHEEL

To RIGHT-

REAR WHEEL
MOTOR
PUMP

PISTON/VALVE
CUT

VALVE
PISTON
SLEEVE
SOLENOID VALVES (3 SETS)

RELIEF

VALVE

ACCUMULATOR

PRESSURE

SWITCHProCarManuals.com

Anti-lock Brake System (ABS)

Features/Construction (cont'd)

Wheel sensor:

The wheel sensor is a contactless type that detects the rotating speed of a wheel. It consists of a permanent magnet and

coil. When the gear pulsers attached to the rotating parts of each wheel turn, the magnetic flux around the coil in the

wheel sensor alternates, generating voltages with frequency in proportion to wheel rotating speed. These pulses are sent

to the ABS control unit, and the ABS control unit identifies the wheel speed.

WHEEL SENSOR

COIL PERMANENT MAGNET

at HIGH SPEED

at LOW SPEED

GEAR PULSER

GEAR PULSER

ABS control unit:

The ABS control unit consists of a main function, which controls the operation of the anti-lock brake system, and sub-func-

tion, which controls the pump motor and "self-diagnosis".

For safety, the main function consists of two systems, and the ABS control unit activates the solenoid valve only when the

outputs of the two systems agree with each other.

The main function section of the ABS control unit performs calculations on the basis of the signals from each wheel

sensor, and controls the operation of the anti-lock brake system by activating the solenoid valves in the modulator unit

for each front brake and for the two rear brakes. The ABS has individual control of the front wheels and common con-

trol ("Select low") for the rear wheels. "Select low" means that the rear wheel that would lock first (the one with the

lowest resistance to lock-up) determines the ABS activation for both rear wheels.

The sub-function section has the fail-safe function that monitors the system operation by inputting the brake switch,

parking brake switch and pressure switch signals, and stops the anti-lock brake system when it detects an abnormality

in the system. It also has a self-diagnosis function and the pump motor control function.

WHEEL SENSORS
ABS CONTROL UNIT

RIGHT-FRONT WHEEL

LEFT-FRONT WHEEL

RIGHT-REAR WHEEL

LEFT-REAR WHEEL

BRAKE SWITCH

PARKING BRAKE SWITCH
MAIN FUNCTION

• BASIC ABS

FUNCTION CONTROL

SUB-FUNCTION

• SELF-DIAGNOSIS FUNCTION

• FAIL-SAFE FUNCTION

• PUMP MOTOR CONTROL

PRESSURE SWITCH

PUMP MOTOR RELAY
MODULATOR UNIT

SOLENOID

SOLENOID

SOLENOID

FRONT FAIL - SAFE RELAY

REAR FAIL - SAFE RELAY

PUMP MOTORProCarManuals.com

Pump motor control:

The ABS control unit monitors the brake fluid pressure in the accumulator by the pressure switch ON/OFF signals. The

ABS control unit turns the pump on when the pressure in the accumulator drops, and stops the pump when the pressure

rises to the specified value.

If the pressure does not reach the specified value after the motor has operated continuously for a specified period, the

ABS control unit stops the motor and activates the ABS indicator light.

Self-diagnosis function:

The self-diagnosis function, provided in the sub-function of the ABS control unit, monitors the main system functions by

constantly transmitting the data between the two Central Processing Units (CPUs). When an abnormality is detected, the

ABS control unit turns the ABS indicator light on and stops the ABS, although the basic brake system continues to operate

normally.

When the ABS control unit detects an abnormality with the ABS and turns the ABS indicator light on, the diagnostic trou-

ble code (DTC), which shows the problem part or unit, is recorded in the control unit. The DTC can be read by the blinking

frequency of the ABS indicator light.

Fail-safe function:

When an abnormality is detected in the ABS control sys-

tem self-diagnosis, the solenoid operations are suspend-

ed by turning off the two fail-safe relays. This discon-

nects the ground circuits of all the solenoid valves to

prevent ABS operation.

Under these conditions, the braking system functions

just as an ordinary one.

Fail-safe relay:

The fail-safe relay's terminal side contact is normally

open. When there is continuity at the relay coil, the fail-

safe relay is closed, thereby connecting the ground cir-

cuit to the solenoid valve.

ABS indicator light:
SOLENOID VALVE

BAT

REAR FAIL-SAFE RELAY

FRONT FAIL-SAFE RELAY

The ABS control unit turns the ABS indicator light on when one or more of the following abnormalities are detected. This

is only a partial list.

When the operating time of the motor in the power unit exceeds the specified period.

When vehicle running time exceeds 30 seconds without releasing the parking brake.

When absence of speed signals from any of the four wheel sensor is detected.

When the activation time of all solenoids exceeds a given time, or an open circuit is detected in the solenoid system.

When solenoid output is not detected in the simulated ABS operation when the engine is started or the vehicle is driven.

To check the indicator light bulb, the light is activated when the ignition switch is first turned on. The light goes off after

the engine is started if there is no abnormality in the system.
ABS CONTROL UNITProCarManuals.com

Anti-lock Brake System (ABS)

Operation

The following description of ABS operation is for one of

the front wheels. The ABS operation for the remaining

wheels is the same.

Ordinary braking function:

In ordinary brake operations when the ABS is not func-

tioning, the solenoid outlet valve is closed and the inlet

valve is open, the brake fluid pressure is transmitted to

the back-pressure chamber between the sleeve and pis-

ton, and the cut valve is pushed by the piston. As the

high-pressure is also transmitted to chamber A between

the sleeve and cylinder, the sleeve pushes the cut valve

seat toward the cut valve, too.

Under there conditions, the cut valve is kept open, and

the hydraulic pressure from the master cylinder is trans-

mitted to the caliper just like an ordinary brake system.
BACK-PRESSURE CHAMBER

MASTER CYLINDER

CUT VALVE (OPEN)

CUT VALVE SEAT

PISTON

SOLENOID VALVE

PUMP

ACCUMULATOR PRESSURE SWITCH

When ABS is functioning:

Control by reducing caliper fluid pressure:

When brake inputs (force exerted on brake pedal) are

excessively large, and a possibility of wheel locking

occurs, the control unit operates the solenoid valve,

closing the inlet valve and opening the outlet valve.

As a result, high pressure in the back-pressure cham-

ber is released to the reservoir, and the piston is

pushed by the caliper fluid pressure toward the back-

pressure chamber. However, the cut valve seat is

kept in the pushed position because high pressure is

transmitted to chamber A. As the piston moves, the

cut valve moves and shuts the flow from the master

cylinder to the caliper, the volume of the pressure

reduction chamber connected to the caliper increas-

es, and the fluid pressure in the caliper declines,

relieving the braking force. The wheel speed is there-

fore restored, preventing the wheel from locking.
RESERVOIR

PRESSURE REDUCTION CHAMBER
CALIPER

CHAMBER A

SLEEVE

PRESSURE DECLINES.

CHAMBER A

CUT VALVE (CLOSE)ProCarManuals.com

Control by increasing caliper fluid pressure:

When the ABS control unit senses that the caliper fluid

pressure declined, and the wheel speed is restored, it

signals the solenoid inlet valve to open and the

solenoid outlet valve to close.

As a result, the high-pressure brake fluid is transmitted

to the back-pressure chamber, and the piston is pushed

toward the pressure reduction chamber, increasing

the caliper fluid pressure, and thereby the braking

force again.

When the master cylinder side's fluid pressure is low,

the cut valve is slightly opened as the piston moves,

and the caliper fluid pressure is transmitted to the

master cylinder. The kickback is felt on the brake

pedal this time. When the force depressing the brake

pedal is relieved while the ABS is functioning, the cut

valve is opened and the pressure in the pressure

reduction chamber is returned to the master cylinder

side. As a result, the caliper fluid pressure is relieved.
MASTER CYLINDER

PRESSURE INCREASES

CUT VALVE

PRESSURE REDUCTION CHAMBER

SOLENOID VALVES

When high-pressure declines:

The ABS control unit monitors the pressure in the high-

pressure passage by means of the pressure switch sig-

nals. The ABS control unit turns the ABS indicator light

on, and stops the ABS when it detects an excessive drop

in pressure in the high-pressure passage.

When the pressure declined due to leakage from the pas-

sage, for example, the pressure in chamber A declines,

too, and the cut valve seat and sleeve return toward

chamber A.

As a result, the valve at the sleeve end closes, which

hydraulically locks the back-pressure chamber and

blocks the piston movement. Because the cut valve

opens as the cut valve seat moves, this connects the

brake fluid passage between the master cylinder and

caliper for ordinary brake operation.ProCarManuals.com

Anti-lock Brake System (ABS)

Operation (cont'd)

Proportioning Control Valve Function:

The modulators for the rear brakes serve as proportioning

control valves to prevent the rear wheels from locking if

the ABS malfunctions or when the ABS is not activated.

When this function is not provided, the hydraulic pressure

from the master cylinder and the hydraulic pressure to

the rear brake system are equal.

If the fluid pressure is transmitted to the rear brakes at

the same rate as the front brakes, the rear wheels will

lock first because the rear axle load becomes lighter

when the brakes are applied.

To prevent the rear wheels from locking, the proportion-

ing control valve function changes the distribution rate

of the fluid pressure to the rear wheels when the pres-

sure in the rear brake system exceeds the given value of

the fluid pressure from the master cylinder. The fluid

pressure point where the distribution rate changes is

called the turning point.
FLUID PRESSURE WITHOUT PCV

FLUID PRESSURE WITH PCV

TURNING POINT

MASTER CYLINDER FLUID PRESSURE

The cut valve seat in the rear brake system has a shoulder between sections A and B. Section A, where pressure from the

master cylinder is applied, has a smaller diameter than section B, where pressure from the rear brake caliper is applied.

This design provides the proportioning control valve function as follows.

1. When the fluid pressure from the master cylinder is below the turning point, the cut valve seat is pushed by the

spring force and the cut valve is open. Therefore, the fluid pressure from the master cylinder is transmitted to the

rear brake caliper side. Under these conditions, fluid pressure from the master cylinder is equal to the pressure to the

rear brake caliper, but because of the diameter difference between sections A and B, the force on the cut valve over-

comes the spring force, moving the cut valve seat toward the cut valve slowly.

2. When the fluid pressure to the rear brake caliper reaches the turning point, the cut valve is closed by the cut valve

seat, blocking the fluid passage between the master cylinder side and rear wheel cylinder side.

3. When the fluid from the master cylinder exceeds the turning point, the fluid pressure from the master cylinder rises,

while the pressure to the rear brake caliper remains at the turning point value. As a result, the cut valve seat moves

away from the cut valve and the cut valve opens. The passage between the master cylinder and caliper opens

momentarily, but it is blocked again because the fluid pressure to the brake caliper rises, and the cut valve seat

moves to close the cut valve. As described above, when the pressure in the master cylinder is above the turning

point, the cut valve seat reduces the pressure in the rear brake caliper to the prescribed amount by repeating this pro-

cess.

From MASTER CYLINDER

To REAR BRAKE CALIPER

SPRING FORCE

SECTION WHERE PRESSURE FROM REAR CALIPER IS APPLIED.

SECTION WHERE PRESSURE FROM MASTER CYLINDER IS APPLIED.
CUT VALVE SEATProCarManuals.com

Component Locations

ABS CONTROL UNIT

UNDER-DASH RELAY BOX

RIGHT-FRONT WHEEL

SENSOR CONNECTOR

UNDER-HOOD

FUSE/RELAY BOX
SERVICE CHECK CONNECTOR (2P)

ABS INSPECTION CONNECTOR (6P)

MODULATOR UNIT

UNDER-HOOD ABS

FUSE/RELAY BOX
RIGHT-REAR WHEEL SENSOR

RIGHT-REAR WHEEL

SENSOR CONNECTOR

LEFT-REAR

WHEEL SENSOR

LEFT-REAR WHEEL

SENSOR CONNECTOR

RIGHT-FRONT

WHEEL SENSOR
LEFT FRONT WHEEL

SENSOR CONNECTOR
PARKING BRAKE SWITCH

UNDER-DASH FUSE/RELAY BOX

LEFT-FRONT WHEEL SENSORProCarManuals.com

GAUGE ASSEMBLY
UNDER-HOOD FUSE/RELAY BOX

ABS INDICATOR LIGHT
BATTERY

(100 A) FUSE

IG (50 A) FUSE

STOP/ HORN

(20 A) FUSE

UNDER-DASH FUSE/RELAY BOX

UNDER-HOOD ABS FUSE/RELAY BOX

HEATER

CONTROL RELAY

(7.5 A) FUSE

BACK-UP LIGHTS/

METER LIGHTS

(SUN ROOF)

(10 A) FUSE
ECU (ECM)/

CRUISE CONTROL/

EAT ECU

(15 A) FUSE
ABS MOTOR (40 A) FUSE

ABS B1 (20 A) FUSE

ABS B2 (15 A) FUSE
ABS PUMP MOTOR RELAY

UNDER-DASH RELAY BOX
ABS UNIT (10 A) FUSE

REAR FAIL-SAFE RELAY

FRONT FAIL-SAFE RELAYProCarManuals.com