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Solenoid valve ST, for system pressure

Solenoid valve ST, for system pressure

Solenoid valve ST is located in the valve housing which is mounted on the transmission casing. The solenoid regulates the automatic transmission's system pressure.

Applies to: B204i/E: The solenoid is not grounded to the gearbox but internally in the control module. It is supplied via control module pin 17 with feedback to control module pin 18. The solenoid is controlled by a variable current from the control module, where the current strength is affected by the throttle position. At high currents, approx. 1 A, the fluid pressure is low and when the current is low or is cut completely, the system pressure is at maximum.

The solenoid is controlled by a 300 Hz PWM voltage via control module pin 17 and is grounded via pin 18. It is controlled by the variable current that results from the present pulse ratio. At a high pulse ratio, i.e. high current (approx. 1 A), the system pressure is low and at a low pulse ratio, i.e. low current, the system pressure will be high. If a break occurs, the system pressure will be at maximum. The valve is then completely open.

Applies to B235R, B205L/R: The solenoid is controlled by a 300 Hz PWM voltage via control module pin 17 and is grounded via pin 18. It is controlled by the variable current that results from the present pulse ratio. At a high pulse ratio, i.e. high current (approx. 1 A), the system pressure is low and at a low pulse ratio, i.e. low current, the system pressure will be high. If a break occurs, the system pressure will be at maximum. The valve is then completely open.

The control module uses bus information on the current “Engine torque” (unit Nm) and the gear presently selected to control the system pressure.

All: When shifting N-R, the control module will reduce the system pressure to reduce the risk of jerky shifting. When the fluid temperature is higher than 10°C and the vehicle speed is above 5 km/h, the control module will increase the system pressure to shorten the shifting times.



The function is blocked in the following situations:


• The fluid temperature exceeds 20°C.

• The vehicle speed exceeds 7 km/h.

• The position of the selector lever changes.


The coil resistance in the solenoid is approx. 4 ohms.

The current strength at idling speed with the handbrake applied and the selector lever in position D is approx. 900 mA and the fluid pressure 3.4-4.6 bar.

Electrical faults (short-circuit to ground or B+ and open circuits) in the solenoid valve or wiring harness will put the transmission in limp-home mode.