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62 Brakes
9-5
ESP valve block
The hydraulic unit comprises 12 solenoid valves
that govern the brake pressure to the brake units
during ABS, TCS or ESP regulation.
The pressure-increase and pressure-reducing
valves in the TC/ABS unit are similar in their
design and operation. In order to enable ESP
regulation, the valves are connected to the rear
wheel brakes and the inlet and outlet valves are
also activated during ESP regulation. This
arrangement enables the brake pressure on all
four wheels to be modulated. This solution makes
it possible to build up pressure, maintain
pressure and reduce pressure individually on all
four wheels.
If the driver applies the brakes while ESP
regulation is active, the brake pressure sensor
will inform the control module of the input brake
pressure in the hydraulic unit. With this
information, the brake pressure to the wheels not
under ESP regulation can be controlled to
correspond to the driver's braking force. The
wheel under ESP regulation when the brakes are
applied is controlled according to the ESP
criteria.
The pump is active continuously during ESP
regulation.
Brake application
For ESP regulation, the control module starts the
pump, which continues during the entire
regulating process, at the same time as the
pressure-reducing valve is closed to enable a
build up of pressure. Pressure is built up by
opening the pressure-increase valve to supply
the pump with brake fluid.
The control module closes the pressure-increase
valve when a predetermined pressure has been
attained. The pressure is then modulated by the
control module opening the pressure-increase
valve. It will keep the pressure-increase and
pressure-reducing valves closed to retain
pressure and open the pressure-reducing valve
for pressure reduction. The inlet and outlet valves
are activated to regulate each wheel
independently.
The valves are activated by a pulse train, where
the duration corresponds to a pressure increase
or a pressure reduction. Any excess brake fluid
resulting from the pressure-reduction valve
opening is returned to the master cylinder.
The ESP function will regulate up to two wheels
simultaneously.