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Valve block, TCS function

Valve block, TCS function


The valve block, which is an integral part of the hydraulic unit, modulates the brake pressure to the calipers when ABS braking and TCS modulation with braking are activated.

The valve block contains 12 solenoid valves: four inlet valves, four outlet valves, two pressure increase valves and two pressure relief valves. These are arranged as follows: one inlet and one outlet valve for each of the four wheels and one pressure increase and one pressure relief valve for each of the front wheels. A description of the inlet and outlet valves and their accumulator chambers is given under the heading “Valve block, ABS function”. The valves concerning the TCS function are described here.

The pressure increase valve is normally closed but is opened by the control module to supply the return pump with brake fluid for building up pressure in connection with active TCS modulation.

The pressure-relief valve is normally open but will be closed during pressure build-up and pressure maintenance, and opened for reducing pressure to maintain the brake pressure, calculated by the control module, at the wheel concerned in connection with active TCS modulation.

The valve block contains 2 check valves for the TCS function. If the driver brakes while TCS modulation is in progress, the TCS function with braking will be disengaged and the braking function will be chosen instead. During the delay which may arise, the check valve mounted parallel with the pressure-relief valve prevents the braking function from being affected because the check valve leads the brake pressure out to the wheel.

The check valve mounted after the outlet valve prevents the accumulator from being filled with brake fluid.

The return pump is continuously active during TCS modulation.

Diagnostics

The control module performs a valve test each time the ignition is switched on. This test takes place at the same time as the pump-motor test and the valves are actuated for 20 ms.

A fault in the left-hand front inlet/outlet valves will set diagnostic trouble code C2450.

A fault in the right-hand front inlet/outlet valves will set diagnostic trouble code C2455.

A fault in the left-hand rear inlet/outlet valves will set diagnostic trouble code C2415.

A fault in the right-hand front inlet/outlet valves will set diagnostic trouble code C2485.

A fault in Pressure Increase Valve Front Left will set diagnostic trouble code C0141.

A fault in Pressure Relief Valve Front Left will set diagnostic trouble code C0146.

A fault in Pressure Increase Valve Front Right will set diagnostic trouble code C0151.

A fault in Pressure Relief Valve Front Right will set diagnostic trouble code C0156.

In all the above cases, the TCS and ABS functions will be disabled and the warning lamps for ABS, brakes and TCS OFF will light up as well as the central warning lamp.