A transmission solenoid is an electrically operated valve present in automatic-transmission vehicles. The valve controls the flow of fluid in and throughout the automatic transmission.
The components of a transmission require the constant circulation of transmission fluid to reduce the friction between the moving parts and to cool the entire system. In an automatic transmission, this circulation of fluid is controlled by solenoids. When it's time to shift between gears, the solenoids receive an electrical signal and redirect the flow of the fluid to the desired gears. In modern vehicles that have an on-board computer to control and monitor the vehicle in its operating state, the solenoids are controlled by the computer.
The purpose of a transmission is to deliver power from the engine to the drive shaft. Every engine has a range of revolutions per minute within which it runs best. The transmission ensures that the engine keeps operating within that suitable rpm range, regardless of the speed of the vehicle. It operates through a number of gear combinations and changes gear ratios to suit the condition of the engine. Automatic transmission is a kind of vehicular transmission system that has the capability to automatically adjust gear ratios within the system, thus freeing the driver from manually changing gears.