2. Relay

The solenoid valve relay  is actuated by the control module (closes) as soon as the control module receives voltage from the generator (D+ / terminal 61).

As soon as the relay has closed (picked up), battery positive voltage (terminal 30) is present at the valve coils of all solenoid valves in the hydraulic unit.

If the ABS or the TC (ABD) begins to control, the control module applies negative voltage to the appropriate valve coil of the valve to be controlled at the appropriate wheel.

The return pump relay  is actuated (with a negative voltage) by the control module if required, and closes.

As soon as the relay has closed / picked up, battery positive voltage (terminal 30) is applied to the return pump, which then runs.