Lexus ES300 (1997 year). Service manual - part 15

58

CHASSIS — BRAKES

3) Operation

The hydraulic system of the TRAC system consists of the following 2 circuits: the front right and front
left. Although the hydraulic circuit described below is one circuit, it is applicable to other circuits as well.

a. During Normal Operation (TRAC System not Activated)

When the TRAC system is inactive, the ABS & TRAC ECU does not output any control signals. Thus,
all valves are OFF.
Accordingly, pressing on the brake pedal causes the brake fluid to flow from the master cylinder to
port (A), port (B), No. I check valve, port (C), and port (D), thus applying fluid pressure to the wheel
cylinder.
Also, releasing the brake pedal causes the brake fluid to flow from the wheel cylinder to port (D), port
(C), No. 2 check valve, port (B), and port (A), thus returning to the master cylinder.
Thus, the brakes operate in the normal brake mode.

 Hydraulic Circuit 

 Condition of Each Valve and Pump Motor 

Part Name

Signal from ABS & TRAC ECU

Operation

Master Cut Solenoid Valve

OFF

Port (B)

Open

Reservoir Cut Solenoid Valve

OFF

Port (H)

Closed

Pressure Holding Valve

OFF

Port (C)

Open

Pressure Reduction Valve

OFF

Port (F)

Closed

Pump

OFF

Stop

59

CHASSIS — BRAKES

b. During Vehicle Acceleration (TRAC System Activated)

When a front wheel slips during acceleration, the TRAC system controls the engine output and braking
of the front wheels to help prevent wheel slippage.
The brake fluid pressure applied to the right and left front wheels is controlled separately according
to 3 control modes (pressure increase, holding, and pressure reduction) as explained below.

i) Pressure Increase Mode

During sudden acceleration or driving on a slippery surface, if the front wheels starts to slip, a control
signal from the ABS & TRAC ECU causes the ABS & TRAC actuator to control the valves and pumps
as described below, to effect the pressure increase mode.
When the master cut and reservoir cut solenoid valves turn ON, port (B) close, port (H) opens, and
the pump operates at the same time.
Accordingly, the brake fluid will be suctioned up from the master cylinder through port (G) and (H).
Then the brake fluid pressurized by the pump will be applied to the wheel cylinder through port (C)
and (D).
Also, the brake fluid that is pressurized by the pump is regulated to a constant pressure by the pressure
regulator valve.

 Hydraulic Circuit 

 Condition of Each Valve and Pump Motor 

Part Name

Signal from ABS & TRAC ECU

Operation

Master Cut Solenoid Valve

ON

Port (A)

Closed

Reservoir Cut Solenoid Valve

ON

Port (H)

Open

Pressure Holding Valve

OFF

Port (C)

Open

Pressure Reduction Valve

OFF

Port (F)

Closed

Pump

ON

Rotating

60

CHASSIS — BRAKES

ii) Pressure Holding Mode

When the fluid pressure in the wheel cylinder is increased or decreased to attain the required pressure,
the ABS & TRAC ECU sends a control signal to tuna ON the pressure holding valve.
Then, the fluid pressure that is applied to the wheel cylinder is cut off to hold the fluid pressure in the
wheel cylinder.

 Hydraulic Circuit 

 Condition of Each Valve and Pump Motor 

Part Name

Signal from ABS & TRAC ECU

Operation

Master Cut Solenoid Valve

ON

Port (B)

Closed

Reservoir Cut Solenoid Valve

OFF

Port (H)

Closed

Pressure Holding Valve

ON

Port (C)

Closed

Pressure Reduction Valve

OFF

Port (F)

Closed

Pump

ON

Rotating

61

CHASSIS — BRAKES

iii) Pressure Reduction Mode

When the fluid pressure in the wheel cylinder needs to be decreased, the ABS & TRAC ECU sends
a control signal to turn the pressure reduction valve ON, causing port (F) to open.
Accordingly, the brake fluid in the wheel cylinder flows to ports (E) and (F), reservoir, and pump, to
reduce the fluid pressure in the wheel cylinder.
The fluid pressure reduction rate is controlled by repetition of the pressure reduction and holding
mode.

 Hydraulic Circuit 

 Condition of Each Valve and Pump Motor 

Part Name

Signal from ABS & TRAC ECU

Operation

Master Cut Solenoid Valve

ON

Port (B)

Closed

Reservoir Cut Solenoid Valve

OFF

Port (H)

Closed

Pressure Holding Valve

ON

Port (C)

Closed

Pressure Reduction Valve

ON

Port (F)

Open

Pump

ON

Rotating