The transmission is a complex mechanical system that controls the application of power from the engine to the driveshaft. It experiences more wear and tear over time than most other parts of your vehicle due to the heat and friction produced by their many moving and interacting components.
You may start to notice that the vehicle starts to hesitate when it goes into gear. The moment you shift from park to drive, the vehicle should immediately go into the proper gear. For automatic transmissions, you might notice that when shifting into drive or park that there is a delay before you feel the gear engage. This is usually a transmission-based concern. Manual transmissions can have the same lacking response issue, but after shifting into gear the engine’s RPMs will surge, but the vehicle will not move as fast as the engine sounds like it is going. This is usually caused by a clutch that needs to be replaced, but might sometimes point to a more severe problem.
Manual transmissions will emit sounds that can be described as being slightly more mechanical, louder and abrupt sounding. A clunking sounds when you shift gears almost always lies within a transmission, while constant velocity joins or the differential may be the source if the clunking is coming from the underside of your vehicle.
A leak is probably the most recognizable symptom and should be repaired as soon as possible. Letting the fluid leak is one of the most common causes transmission break down. Automatic transmission fluid is the life-blood of a transmission as it lubricates, cleans and conditions the seals and acts as hydraulic fluid. Without it, or even if it gets too low, the engine will seize up and stop working completely. The fluid is bright red in color, clear and smells somewhat sweet if everything is working correctly. If the fluid is dark and/or has a burnt smell then it is time to get the fluid changed. To check if you are running low on fluid, take your vehicle for a short drive to warm it up and then lift the hood and read the dipstick. Unlike motor oil, transmission fluid is not burned off or consumed by a vehicle so if the level is low then there is a leak somewhere that must be patched. It is recommended to top up the fluid anyway even if the leak still exists to make sure there is enough fluid for the transmission to function properly until you get it fixed.