A cracked or broken flex plate commonly causes a knocking or clunking noise from the area where the engine meets the transmission, often loudest at idle, when shifting into gear or when the engine starts and stops. Other signs are a grinding or whirring starter, a starter that will not engage because the ring gear teeth are damaged, and vibration. A flex plate knock is often mistaken for a rod knock, so check it before condemning the engine.
Bad flex plate symptoms
- Knocking or clunking that changes with load or when shifting into gear.
- A rattle at idle that may quieten at higher rpm.
- Grinding or whirring when starting, or a starter that spins without cranking; see bad starter symptoms.
- Vibration felt through the floor.
Flex plate vs rod knock
A rod knock usually rises with rpm and load and is heard from the engine block. A flex plate knock often comes from the bell housing area and changes when the transmission is shifted into gear. A mechanic’s stethoscope, or watching the flex plate through the inspection cover while someone shifts gears, helps tell them apart. See engine ticking noise.
What cracks a flex plate
- A torque converter not fully seated into the pump, then pulled forward by the bolts, which stresses the plate; see how to install a transmission.
- Loose or missing converter bolts.
- Misalignment between engine and transmission, including missing dowels.
- Wrong spacer or flex plate for the converter on swaps.
- Worn engine or transmission mounts that let the drivetrain move; see transmission mount symptoms.
How to check it
With the engine off and the battery disconnected, remove the inspection cover and turn the engine by hand, looking for cracks around the bolt holes and center, loose converter bolts, and damaged ring gear teeth.
Can you drive with a cracked flex plate?
It is not a good idea. A crack grows, and a flex plate that breaks completely leaves the vehicle stranded and can damage the starter, the torque converter and the bell housing. If you hear a new knock from the bell housing area, inspect it soon rather than waiting for the noise to get worse.
Replacing a flex plate
The transmission must come out to replace the flex plate, so if the transmission is also worn, it makes sense to address both at once. Use the correct flex plate for the engine and converter, torque the bolts to specification with thread locker as the manual specifies, and confirm the converter is fully seated before bolting it up.
If the transmission itself is worn out, our listed units include a torque converter; see transmissions by code and transmission replacement cost.
More guides: bad transmission pump symptoms.
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