7102Operating conditions

Foot brake applied (Fig. 7.43(a)) With the foot pedal released brake fluid is able to flow freely between the master cylinder and both brake calipers via the open traction boost unit. The boost piston will be in its outer-most position thereby holding the poppet valve in its fully open position. When the foot pedal is pushed down, brake fluid pressure will be transmitted though the fluid via the traction boost poppet valve to both of the wheel calipers thus causing the brakes to be applied.

Traction control system applied (off-side slipping wheel braked) (Fig. 7.43(b)) When one of the driving wheels begins to spin (off-side wheel in this example) the adjacent speed sensor voltage change signals the ECU, immediately it computes

Pump outlet valves

Master

_ Brake disc

Speed sensor

Accumulator

Pump

Master

Speed sensor

Pump outlet valves

Accumulator

Pump

_ Brake disc

Traction control system Electronic control unit (ECU)

Traction solenoid valve unit

Traction control system Electronic control unit (ECU)

Traction solenoid valve unit

(a) Conventional foot brakes applied

Fig. 7.43 (a and b) Traction control system

Wheel tending to spin

Off-side wheel

Wheel tending to spin

Off-side wheel

(b) Off-side spinning wheel braked

Near-side wheel

(b) Off-side spinning wheel braked

Near-side wheel

Fig. 7.43 contd

Fig. 7.44 Combined antilock brake system/traction control system (brakes applied)

Fig. 7.44 Combined antilock brake system/traction control system (brakes applied)

and relays current to the relevant traction solenoid valve unit to energize both valves, this closes the return valve and opens the outlet valve. Fluid pressure from the accumulator now flows through the open outlet valve and passes into the traction boost cylinder, the upward movement of the piston will instantly snap closed the poppet valve thereby trapping fluid between the upper side of the cylinder and piston chamber and the off-side caliper. The fluid pressure build up underneath the piston will pressurize the fluid above the piston so that the pressure increase is able to clamp the caliper pads against the brake disc.

As the wheel spin speed reduces to a predetermined value the monitoring speed sensor signals the ECU to release the wheel brake, immediately the solenoid outlet and return valves will be de-energized, thus causing them to close and open respectively.

Fluid pressure previously reaching the boost piston will now be blocked and the fluid underneath the piston will be able to return to the reservoir tank. The same cycle of events will take place for the near-side wheel if it happens to move over a slippery surface.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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