The solenoid is made up of fine copper wire wound around a thin-walled cylindrical form. The fluid control needle (the plunger) is a smooth sliding fit in the cylinder and, with no current applied to the coil, is held firmly against the pressure relief port by the return spring.
Applying current to the solenoid pulls the plunger into the cylinder against the spring pressure and fluid passed through the relief passage. This reduces the fluid pressure in the system.
In an automatic transmissions using electronic controls, the fluid pressure is controlled by pulsing the solenoid on and off rapidly using PWM (pulse width modulation). Testing the plunger involves monitoring the output of pressure to shift valves as the result of supplying a PWM signal to the plunger solenoid.
The Yokogawa Model DL708E(Eight-Channel)/DL716(16-Channel) provides the following solution for this application :