The transmission can be one of the more confusing parts of a vehicle, but also one of the most important. Transmissions - whether manual or automatic - essentially take the power from the engine and apply it to the drive wheels. As such, this is one of the most critical systems in any car.
To better understand how a transmission works and how to troubleshoot it, drivers need to know about one of the key parts: either the flexplate or the flywheel. The flexplate or flywheel is found in automatic or manual transmissions, respectively, and essentially serve as the link that marries the transmission to the engine. If either of these parts aren't working properly, it could lead to big problems.
At first, these two parts look pretty similar. They are both circular and may have a number of holes in them. The key difference is that the flexplate will be thinner and made of a lighter material. A flywheel typically looks heavy and is made out of steel. The flexplate needs to be thinner because at high RPMs, the torque converter in an automatic transmission will expand.
Unfortunately, this means that flexplates tend to break and crack more easily than their flywheel counterparts. It's also very difficult to get access to a flexplate - often you'll have to take out the entire transmission just to reach this critical part. Some automakers have wisened up and installed a window or small opening where drivers will be able to at least check the flexplate to see if it is cracked.
When replacing a flexplate, keep in mind that this is a critical part. You'll probably notice that the piece has a number of machine-cut holes in it - these are specific for the vehicle and the transmission. Flexplates are not a "one-size-fits-all" kind of part. As such, this is an area where you'll want to look into a high-quality original equipment manufacturer (OEM) replacement.
Flywheels are typically heavier and do not break as often, but can still occasionally crack. One of the more common reasons for a malfunction with these parts is when the clutch is replaced. After this is done, the flywheel needs to be resurfaced so that it adapts to the new clutch. Like with flexplates, you should stick with OEM parts if you plan on replacing your flywheel.