Flexplate – What Does It Do ? – What Happens When It Fails

In vehicles with automatic transmissions, a metal disc called a flexplate connects the crankshaft to the torque converter.

A flexplate, as the name implies, serves as a flex-shock device between the torque converter and the engine crankshaft.

Flexplate
Flexplate

A flexplate consists of a steel, circular disk with perforated holes and are balanced to improve smooth running.

Flexing allows it to respond to small differences of alignment between the engine and the transmission.

The flexplate is also ringed by a series of teeth referred to as the ring gear.

The starter motor engages this ring gear to start the engine.

Also, It is balanced and must be replaced in the same position as it was removed.

The bolts are torqued in a crisscross pattern according to the manufacturer’s specifications.

The torque converter used with an automatic transmission acts like a flywheel, smoothing out engine pulses.

Flexplate cracking on various, engine applications is common.

The cracking on these flexplates can be seen either around the crankshaft or torque convert bolt hole patterns. In severe situations, the outer portion is completely separated from the mounting areas.

What Can Cause A Cracked Flexplate:

Cracked Flexplate
Cracked Flexplate
  • Out of balance engine or torque converter.
  • Bad starter drive can cause teeth or ring gear to wear rapidly or break off.
  • Teeth can also break when engine is running and starter is engaged.
  • Failure to torque the flexplate bolts to proper specifications and in proper sequence.
  • Some applications may require starter shims to be used. If these shims are not used when the starter is installed, improper alignment could occur.
  • Poor quality parts.
  • Missing Dowel pins.

A loose bolt on a flexplate or a crack between it’s holes can result in a knocking or rattling sound. The sound is very similar to a bad connecting rod and is often misdiagnosed.

The flexplate can become cracked over time. As a result, This can cause the flexplate to wobble. You might notice rhythmic clunking or grinding noises while the engine is running. A misaligned flex plate can also keep the engine from running smoothly. If you suspect you have a flexplate problem, there are a couple ways to investigate further.

If you listen to the transmission bell housing with a stethoscope, while the engine is idling, any sounds coming from inside the bell housing indicate flexplate problems. Some bell housings have an inspection port. Remove the inspection port, and shine a shop light on the flexplate.

Starting Problems

Damaged Ring Gear
Damaged Ring Gear

A warped flex plate also won’t engage properly with the starter motor. When you are starting your engine listen for rhythmic bogging down of the engine during starting. The teeth of the ring gear can also become damaged over time, causing a grinding sound. Ring gear problems can damage the starter motor as well. So,  If you find you have to replace your flex plate, you might want to check your starter as well.

A cracked flex plate is almost always a sign of other problems. If not properly diagnosed it will occur again and often damage to the transmission may follow. The same things that cause the flex plate to crack can also cause transmission problems. Consequently, Damaged and leaking front seals and front transmission pumps are common symptoms.

Conclusion

A car making noise when starting often indicates either a starter failure or broken teeth on your transmission’s flex plate.

Some cars require several hours to replace the starter and hundreds of dollars for a replacement part. If your car needs a new flex plate, it’s a much bigger issue. The transmission must come out to replace the flex plate. It’s $1,500 and up to replace a flex plate in most vehicles.

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About Danny Bender 175 Articles
I have been into cars for as long as I can remember but the engine is what interested me the most. Now with over 40 years in the engine rebuilding and machine shop industry I have retired. I will be spending my time sharing what I have learned while keeping up with the latest Technology - Helping Solve Engine Problems !

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