7103 Combined Abstcs arrangement

Normally a traction control system (TCS) is incorporated with the antilock braking system (ABS) so that it can share common components such as the electric motor, pump, accumulator, wheel brake sensors and high pressure piping. As can be seen in Fig. 7.44 a conventional ABS system described in section 11.7.2 has been added to. This illustration shows when the brakes are applied fluid pressure is transmitted indirectly through the antilock solenoid control valve to the front brake calipers; however, assuming a rear wheel drive, fluid pressure also is transmitted to the rear brakes via the antilock solenoid control valve and then through the traction boost unit to the wheel brake calipers thus applying the brakes.

ABS operating (Fig. 7.44) When the wheel brake speed sensor signals that a particular wheel is tending towards wheel lock, the appropriate antilock solenoid control valve will be energized so that fluid pressure to that individual wheel brake is blocked and the entrapped fluid pressure is released to the pressure reducing accumulator (note Fig. 7.44 only shows the system in the foot brake applied position).

TCS operating (Fig. 7.44) If one of the wheel speed sensors signals that a wheel is moving towards slip and spin the respective traction solenoid control valve closes its return valve and opens its outlet valve; fluid pressure from the pump now provides the corresponding boost piston with an outward thrust thereby causing the poppet valve to close (note Fig. 7.44 only shows the system in the foot brake applied position). Further fluid pressure acting on the head of the piston now raises the pressure of the trapped fluid in the pipe line between the boost piston and the wheel caliper. Accordingly the relevant drive wheel is braked to a level that transmits a reaction torque to the opposite driving wheel which still retains traction.

One limitation of a brake type traction control is that a continuous application of the TCS when driving over a prolonged slippery terrain will cause the brake pads and disc to become excessively hot; it thus may lead to brake fade and a very high wear rate of the pads and disc.

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