Lack of Response
Transmissions are designed to go into the correct gear every time, so when they hesitate or refuse to go, it's a sure sign there's something wrong. With manual transmission problems, you may notice after shifting into a gear that the car's engine will rev up, but the car won't be moving as quickly as the engine is running. In this case, a worn-out clutch or more serious transmission problem may be occurring [source: AAMCO].
Automatic transmissions have the same lack-of-response problem, but will usually manifest the issue while engaging the "Park" or "Drive" selection. The car should shift quickly into either of these modes, but if your transmission hesitates to go into either one, then it's likely there's an issue with the transmission.
Whining, Clunking & Humming
It's difficult to nail down exactly how your car may sound if there's transmission trouble, but one thing's pretty certain, you'll probably get a that-doesn't-sound-right feeling when you hear it. Every car is built differently, so the sounds they produce can vary greatly, but if you have an automatic transmission, there's a good chance you may hear a whining, humming or even a slight buzzing sound [source: Lee Myles Transmissions and Auto Care].
Grinding & Shaking
Depending on whether you have a manual or automatic transmission, your car may respond differently when your transmission isn't working correctly...with a manual transmission, a common sign of trouble is a grinding sound or feeling when you shift into a new gear... Grinding gears can be caused by a number of different factors.
Check Engine Light
The check engine light can be a great early indicator that something is starting to go wrong with your transmission. The check engine light can come on for any number of reasons not related to your transmission as well, but don't overlook this clear warning sign.
Leaking transmission fluid is probably one of the easiest ways to identify that your transmission needs attention. Automatic transmission fluid is vital to your car's shifting capabilities, so a little fluid on your driveway can quickly turn into a major problem. Automatic transmission fluid is bright red, clear and a little sweet-smelling when everything's working correctly [source: AAMCO]. When you check your automatic transmission fluid, make sure it's not a dark color and that it doesn't have a burnt smell. If it is, you'll need to take it to a mechanic and have it replaced. Unlike your car's motor oil, the transmission doesn't really consume or burn up any fluid during use, so if you notice you're running low on fluid, then it's definitely leaking out somewhere.
In a normally functioning transmission, the car stays in the gear you designate, or that the computer designates for a given RPM range, until you or the computer initiate a gear shift. But on a transmission in which the gears slip, the car can spontaneously pop out of the gear it's in while driving and (in a manual) force the stick back into neutral [source: Procarcare.com].
If you get a whiff of burning transmission fluid, be advised it is definitely not the sweet smell of success. That's because it may indicate your transmission is overheating. Transmission fluid not only keeps the transmission's many moving parts properly lubricated, but it prevents the unit from burning itself up, by providing much-needed cooling.
Refuses to Go into Gear
Believe it or not, there are still quite a few people out there who practice the fading art of shifting manually, with a foot pedal and a "stick" gearshift, and who do so willingly. Despite their somewhat simpler operation, manual transmissions nonetheless have their share of things that can go wrong. One potential problem is that the transmission refuses to budge when you depress the clutch pedal and attempt to move the stick shifter.
excepted in part from "How Stuff Works."