Shift solenoids are actuators used by electronically controlled transmissions to control fluid flow. Some of the today's transmissions use (PWM) pulse width modulated solenoids that are controlled by a duty cycle. A 25% duty cycle means that the PWM solenoid is powered on 25% of the time and off 75%. Standard on/off solenoids has only two positions, either on or off. They consist of a spring loaded plunger wrapped with a coil of wire that when energized, draws the plunger inwards, either opening or closing a hydraulic circuit. When power is removed from the solenoid, the plunger is then pushed by a spring to its normal position. A solenoid can be normally open or normally closed depending on the transmission.