How to Know if a Flexplate Is Warpedby Chris Stevenson
Flexplates serve a very important function in vehicles equipped with an automatic transmission. A flexplate consists of a steel, circular disk with perforated holes and, depending on the model, teeth on the outside ring which engage the starter. A flexplate bolts directly to the crankshaft flange, and connects to the torque converter with large mounting bolts. A flexplate, as the name implies, serves as a flex-shock device between the torque converter and the engine crankshaft, and many flexplates have balancing weights to provide smooth engine RPM. Diagnosing a warped flexplate requires some keen observation and process of elimination.
Start your vehicle as normal, but pay particular attention to the noise of the starter while it spins and engages the flexplate, or torque converter if your vehicle does not have a toothed flex plate. Notice if the starter has a low-, medium- or high-pitched whine while spinning. Be aware of any starter spin that bogs, then picks up again, and bogs again in a rhythmic sequence. Such characteristics denote a warped flexplate, provided the starter functions properly and can be ruled out.
Listen for any rhythmic clunking or grinding noise while the engine idles in park or neutral, or idling in gear with a foot on the brake pedal. Broken or loose bolts on the crankshaft end of the flexplate, or on the torque converter side, miss-align the flexplate, causing a high side of the plate to strike or rub against internal components. A warped flexplate can make metal-to-metal contact with the inspection plate cover, or the starter drive mechanism, and can also make a knocking noise once the vehicle is put in forward or reverse gear.
Drive the vehicle at normal speed, letting the transmission shift automatically from the lowest to the highest gear. You can also manually shift from drive 1 to drive 2, then normal drive. Feel for any unusual vibration that travels up through the chassis and into your driver's seat. The vibration will be heavy, although not quite as heavy as a failing U-joint. A warped flexplate causes a vibration through all speed ranges, especially if the flexplate has thrown a balance weight.
Set the vehicle in park and apply the emergency brake. Raise the vehicle at both ends and place two jack stands under the rear frame, and two jack stands under the front frame. Start the engine and let it idle. Slide under the vehicle with a stethoscope, and place the stethoscope probe against the underside of the bell housing. Any noise emanating from the this region indicates a problem with the flexplate and/or torque converter-flexplate combination.
Locate the inspection cover on the underside of the bell housing case. It will look like a half-moon piece of sheet steel, connected to the bell housing with two or three bolts. Use a socket and wrench to remove the bolts, then remove the inspection cover. With the engine idling, use a shop light to illuminate the flexplate and watch the rotation of the teeth end of the plate. A good flexplate does not wobble. A warped flexplate wobbles around the outside ring teeth, even if the warp is slight.
Determine from memory, or any past repair orders, if the flexplate has been replaced. One of the most common causes for flexplate warpage involves the wrong torque pressure and sequence on the flexplate. Even if the flexplate was old and re-used, the manufacture's specifications call for an exact torque pressure and tightening sequence. Over-torquing the flexplate stretches the metal, causing high and low spots. Under-torquing can cause a premature separation, causing the bolts to slam and break off, or jam the flexplate teeth into the starter gear teeth.
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