Antilock Brakes / Traction Control Systems: Description and Operation
ABS/TCS Control Unit Inputs and Outputs for Connector A (26P)
Part 1 of 2
Part 2 of 2
ABS/TCS Control Unit Inputs and Outputs for Connector B (16P)
ABS/TCS Control Unit Inputs and Outputs for Connector C (12P)
Outline
This system consists of the ABS/TCS control unit, the modulator unit, four wheel sensors, and the PCM. The system integrates the Anti-lock Brake System (ABS) and the Traction Control System (TCS) and controls both systems using the brakes.
ABS Features
When the brake pedal is pressed while 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, and it thereby ensures 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 attain the target slip rate.
TCS Features
The TCS provides low-speed traction. When a drive wheel loses traction on a slippery road surface and starts to spin, the ABS/TCS control unit sends a brake signal to the modulator unit, which applies brake pressure to slow the spinning wheel. At that time, the ABS/TCS control unit sends an engine retard signal to the PCM to prevent damage to the transmission.
Electronic Brake Distribution (EBD) Features
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 used 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.
ABS Main Control
The ABS/TCS 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/TCS control unit calculates the slip rate of each wheel, and it transmits the control signal to the modulator unit solenoid valve when the slip rate is high.
The pressure reduction control has three modes: Pressure intensifying, pressure retaining, and pressure reducing.
TCS Main Control
The ABS/TCS control unit controls the TCS based on signal inputs from the throttle position sensor, engine speed sensor, shift position sensor, and the wheel sensors. In addition, the ABS/TCS control unit sends an engine retard request signal to the PCM if necessary. The ABS/TCS control unit uses the modulator unit to control the TCS. The pressure intensifying control has three modes: pressure intensifying, pressure retaining, and pressure reducing.
ABS Self-Diagnosis
- The ABS/TCS control unit is equipped with a main CPU and a sub CPU. Each CPU checks the other for problems.
- The CPUs check the circuit of the system.
- Self-diagnosis can be classified into two categories:
- Initial diagnosis: Done right after the engine starts and until the ABS or TCS indicator goes off.
- Regular diagnosis: Done right after the initial diagnosis until the ignition switch is turned OFF.
- When a problem is detected by self-diagnosis, the ABS/TCS control unit shifts to fail-safe mode.
ABS Self-diagnosis Table
NOTE:
- At the DTCs with "*" mark, there are cases that the ABS/TCS control unit shifts to fail-safe mode A1,L1 when the ABS/TCS control unit detects more than one DTC and for different wheel.
- If the ABS/TCS control unit detects the DTCs, which have different fail-safe mode individually, the ABS/TCS control unit take priority the large number mode.
ABS Operation Mode Table
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 manuals for specific operating instructions.
TCS Self-diagnosis
- The ABS/TCS control unit is equipped with a main CPU and a sub CPU. Each CPU checks the other for problems.
- The CPUs check the circuit of the system.
- Self-diagnosis can be classified into two categories:
- Initial diagnosis: Done right after the engine starts and until the ABS or TCS indicator goes off.
- Regular diagnosis: Done right after the initial diagnosis until the ignition switch is turned OFF.
- When a problem is detected by self-diagnosis, the ABS/TCS control unit shifts to fail-safe mode.
TCS Operation Mode Table
Part 1 of 2
Part 2 of 2
On-board Diagnosis Function
The TCS 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 Tester User's manuals for specific operating instructions.
Modulator Unit
The modulator unit consists of the inlet solenoid valve, outlet solenoid valve, TCS normally open (NO) solenoid valve, TCS Normally Closed (NC) solenoid valve, reservoir, pump, pump motor, and the damping chamber.
The modulator controls the caliper fluid pressure directly. It is a circulating-type modulator because the brake fluid circulates through the caliper, the 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.
ABS Control
Pressure intensifying mode
TCS NO valve open, TCS NC valve closed, inlet valve open, outlet valve closed.
Master cylinder fluid is pumped out to the caliper.
Pump Motor
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.
Pressure retaining mode
TCS NO valve open, TCS NC valve closed, inlet valve closed, outlet valve closed.
Caliper fluid is retained by the inlet valve and outlet valve.
Pressure reducing mode
TCS NO valve open, TCS NC valve closed, inlet valve closed, outlet valve open. Caliper fluid flows through the outlet valve to the reservoir.
TCS Control
Pressure intensifying mode
TCS NO valve closed, TCS NC valve open, inlet valve open, outlet valve closed, pump motor ON.
The reservoir and master cylinder fluid is pumped out by the pump, through the dampening chamber to the front caliper.
Pressure retaining mode
TCS NO valve open, TCS NC valve open, inlet valve closed, outlet valve closed, pump motor ON.
Pressure reducing mode
TCS NO valve closed, TCS NC valve open, front inlet valve closed, front outlet valve open, pump motor ON. Caliper fluid flows through the outlet valve to the reservoir.