Transmission Shift Solenoids
![The shift solenoid is in the valve body.](graphics/shift-solenoid.png)
Electronically controlled transmissions use shift solenoids and valves to control fluid flow. Linear (standard on/off solenoids) have only two positions, either on or off. The TCM activates these solenoids based on inputs from both engine and transmission sensors.
![Shift Solenoid](graphics/shift-solenoid-a.png)
They consist of a coil of fine wire wrapped around a spring-loaded plunger that, when energized, moves, either opening or closing a hydraulic circuit. When powered down, the plunger is forced by a spring to its at-rest position. Many modern transmissions use PWM (pulse width modulated) solenoids adjusting flow by pulsing the circuit on and off.
Shift Solenoid Operation
![Normally Vented Shift Solenoid](graphics/shift-solenoid-b.png)
A solenoid can be normally open (vented) or normally closed, depending on the application. A normally vented solenoid directs fluid to the sump when it's off and to the clutch or servo when it's powered on.
Sticking Shift Solenoids
![Sticking Shift Solenoid](graphics/shift-solenoid-c.png)
Unchanged and contaminated ATF covers the transmission case and internal components with tarnish and debris. It clogs solenoids and valves, causing them to stick open or closed. A sticking solenoid will cause a vehicle to remain in gear, neutral, or not upshift.
Circuit Checks
![Multimeter](graphics/multimeter.jpg)
Check the circuit and solenoid for electrical problems. A short-to-ground occurs when an unwanted flow of electrons detours to ground before the circuit's desired load. Check for a frayed wire touching the vehicle's dash, frame, or transmission. An open can occur anywhere in an electrical circuit. There is no current when a circuit is open. When the circuit is open, often caused by a bad connection or a broken wire, the solenoid will not function.