ABS System Description

ABS Control Unit Inputs and Outputs for 25P Connector


Terminal number Wire color Terminal sign (Terminal name) Description Measurement (Disconnect the ABS control unit
25P connector) 
Terminals Conditions Voltage 
BRN/WHT FL-GND Detects left-front wheel sensor signal 


YEL IG1 Power source for activating the system 4- GND Ignition switch ON (II) Battery Voltage 
RL - GND Detects left-rear wheel sensor signal 


RL + B 
WALP Drives ABS indicator 7 - GND ABS indicator (Ignition switch ON (II)) ON About 6 V 
OFF Below
0.3 V 
BLK GND1 Ground ——— ——— ——— 
FSR +B Power source for the ABS fail-safe relay 9- GND Every time Battery Voltage 
13 GRY DLC Communicates with the Honda PGM Tester 


14 BRN SCS Use for DTC indication and DTC clearing 


15 BLU/
VABS Outputs wheel speed signal ——— ——— ——— 
16 BRN/
EBD Drives brake system indicator 16 - GND Parking brake ON Below
0.3 V 
OFF Battery voltage 

Terminal number Wire color Terminal sign (Terminal name) Description Measurement (Disconnect the ABS control unit 25P connector) 
Terminals Conditions Voltage 
18 WHT/
STOP Detects brake switch signal 18-GND Brake pedal Pressed Battery voltage 
Released Below
0.3 V 
19 BLU FR - GND Detects right-front wheel sensor siganal 


20 GRN/
FR +B 
22 GRN/
RR + B Detects right-front wheel sensor siganal 


23 BLU/
24 BLK GND2 Ground ——— ——— ——— 
25 WHT/
MR + B Power source for the motor relay 25-GND Every time Battery Voltage 


When the brake pedal is pressed during driving, the wheels can lock before the vehicle comes to a stop. In such an event, the maneuverability of the vehicle is reduced if the front wheels are locked, and the stability of the vehicle is reduced if the rear wheels are locked, creating an extremely unstable condition. The ABS precisely controls the slip rate of the wheels to ensure maximum grip force from the tires, thereby ensuring the maneuverability and stability of the vehicle.
The ABS calculates the slip rate of the wheels based on the vehicle speed and the wheel speed, then it controls the brake fluid pressure to reach the target slip rate.

Grip Force of Tire and Road Surface 

Wheel sensor The wheel sensor outputs the speed signal to the ABS control unit according to the pulser's rotation speed. 
Modulator- control unit ABS control unit The ABS control unit processes the signal from the wheel sensor, then outputs the ABS control signal to the modulator unit. 
Modulator unit The modulator unit receives the control signal, then controls brake fluid pressure for each wheel. 
Motor relay (inside of the ABS control unit) The motor relay drives the ABS pump motor. 
ABS fail-safe relay (inside of the ABS control unit) The ABS fail-safe relay cuts the power to the solenoid valve when the problem is detected. 


ABS Control Unit

Main Control

The ABS control unit detects the wheel speed based on the wheel sensor signal it received, then it calculates the vehicle speed based on the detected wheel speed. The control unit detects the vehicle speed during deceleration based on the rate of deceleration.

The ABS control unit calculates the slip rate of each wheel, and transmits the control signal to the modulator unit solenoid valve when the slip rate is high.

The pressure reduction control has three modes: pressure reducing, pressure retaining, and pressure intensifying.


Electronic Brake Distribution (EBD) Control

The electronic brake distribution (EBD) function helps control vehicle braking by adjusting the rear brake force before the ABS operates. Based on wheel sensor signals, the ABS control unit uses the modulator to control the rear brakes individually. When the rear wheel speed is less than the front wheel speed, the ABS control unit retains the current rear brake fluid pressure by closing the inlet valve in the modulator. As the rear wheel speed increases and approaches the front wheel speed, the control unit increases the rear brake fluid pressure by momentarily opening the inlet valve. This whole process is repeated very rapidly. While this is happening, there is kickback at the brake pedal.

During self-diagnosis. if the ABS control unit detects a problem that affects the EBD, it turns on the brake system indicator and the ABS indicator.

Self-diagnosis Function

1.The ABS control unit is equipped with a main CPU and a sub-CPU. Each CPU checks the other for problems.

2.The CPUs check the circuit of the system.

3.The ABS control unit turns on the ABS indicator when the unit detects a problem and the unit stops the system.

4.The self-diagnosis can be classified into these two categories:
  • Initial diagnosis
  • Regular diagnosis

On-board Diagnosis Function

The ABS can be diagnosed with the Honda PGM Tester.
The ALB Checker cannot be used with this system. For air bleeding and checking wheel sensor signals, use the Honda PGM Tester. See the Honda PGM Tester user's manual for specific operating instructions.

ABS Modulator

The ABS modulator consists of the inlet solenoid valve, outlet solenoid valve, reservoir, pump, and the pump motor. The modulator reduces the caliper fluid pressure directly. It is a circulating-type modulator because the brake fluid circulates through the caliper, reservoir, and the master cylinder. The hydraulic control has three modes: pressure intensifying, pressure retaining, and pressure reducing. The hydraulic circuit is an independent four channel type, one channel for each wheel.


Pressure intensifying mode: Inlet valve open, outlet valve closed
Master cylinder fluid is pumped out to the caliper.
Pressure retaining mode: Inlet valve closed, outlet valve closed
Caliper fluid is retained by the inlet valve and outlet valve.
Pressure reducing mode: Inlet valve closed, outlet valve open
Caliper fluid flows through the outlet valve to the reservoir.
Motor operation mode: When starting the pressure reducing mode, the pump motor is ON.
When stopping ABS operation, the pump motor is OFF.
The reservoir fluid is pumped out by the pump, through the damping chamber, to the master cylinder. 

Wheel Sensors

The wheel sensors are the magnetic contactless type. As the gear pulser teeth rotate past the wheel sensor¢s magnetic coil, AC current is generated. The AC frequency changes in accordance with the wheel speed. The ABS control unit detects the wheel sensor signal frequency and thereby detects the wheel speed.


Wheel Speed and Modulator Control


When the wheel speed drops sharply below the vehicle speed, the outlet valve opens momentarily to reduce the caliper fluid pressure. The pump motor starts at this time. As the wheel speed is restored, the inlet valve opens momentarily to increase the caliper fluid pressure.