The modern automatic transmission is perhaps the most complicated component found in the car.  Mechanical, hydraulic and electrical systems as well as computer controls work in tandem to send power from the engine to the wheels. 

Automatic transmission uses planetary gear sets to create all of the different gear ratios that the transmission can produce. The basic planetary gear set consists of three main components: a sun gear, the planet gears and the ring gear. The planet gears are connected to each other through a common carrier.

Each of these three components can be the input, the output or can be held stationary. By deciding the role each gear plays, the different gear ratios are generated.

An automatic transmission contains two complete planetary gearsets folded together into one component. In addition to the various forward gears, a transmission also has a neutral position which disconnects the engine from the drive wheels, and reverse, which causes the drive wheels to turn in the opposite direction. In the park position, the drive wheels are locked to prevent them from moving.

The torque converter takes the place of the clutch found in manual vehicles.  It is a large doughnut-shaped device that is mounted between the engine and the transmission to allow the engine to continue running when the vehicle comes to a stop. 

If there is a persistent red oil leak coming from your car, it could come from the transmission or power steering system. Most power steering systems also use transmission fluid and leaks can appear on the ground in roughly the same areas as transmission leaks.  If transmission fluid levels go down below the minimum level, serious damage can occur.

Most manufacturers require the car to be running and on level ground when checking the transmission fluid level. Pull the transmission dipstick out and check the fluid for colour and odour. Transmission fluid is a transparent red oil that looks like cherry cough syrup.  If the fluid looks cloudy or it has a burned odour, have it checked by your mechanic.

A modern transmission should shift smoothly and quietly under light acceleration.  Heavier acceleration should produce firmer shifts at higher speeds.  If shift points are erratic or noisy, have it checked out immediately. Whining noises coming from the floorboard are also a cause for concern. 

Transmission fluid should be changed periodically according to the manufacturer’s recommended intervals. The manufacturer will also state the correct fluid to use.