Allison Knock

[noobie1]

The truck is a 2002 Silverado 8.1/Allison with 78K. About a year ago I started noticing a loud knocking sound coming from the transmission which is gradually growing worse. The characteristics are as follows:

 

1. It never occurs at start-up.

2. It only happens in a drive gear, forward or reverse.

3. In the beginning it only happened after reaching operating temperature. Now sooner.

4. It's most prominent when the brake is applied at a stop and the throttle is opened a little to raise the RPM's just above idle. Raising the RPM's past that point, it dissappears.

 

If you've ever heard a vehicle whose torque converter bolts have loosened or flex plate has cracked, that's the sound. Suspecting that, I took the truck to a transmission shop who after listening suspected the same. But on inspection they found everything fine.

Any ideas?

 

-David

[Trouttrooper]

Yup, I've heard (and seen ) a cracked flex plate.

 

Have you checked all the U joints?