If a vehicle's transmission system runs out of fluid and it is not replaced, the moving parts within the transmission and engine can grind against each other and cause irreparable damage to the system. Transmission fluid should also be changed on a regular basis to ensure proper working conditions.
A car's transmission system works to convey the raw power produced by the engine to the wheels, causing the car to move. Transmission systems are found in two main varieties: manual and automatic. These terms refer to the ways in which the internal gears of the transmission system are controlled and adjusted for various levels of function. Manual transmissions require the driver to shift the gears herself based on the speed of the car and the direction of movement. Automatic transmissions contain internal systems that change the gears in line with the speed, requiring only minimal input from the driver.
The transmission fluid is integral to the proper function of the transmission system because it ensures that the moving parts pass easily around each other. Over time, tiny metal shavings from the gears, along with other debris, get mixed into the fluid and can wear down the system if not replaced.