Hi everyone. I have a 18’ Silverado 1500 High Country with the 8 speed automatic transmission in it. It has 92,000 KM and never had TC shutter issues.

Today, when shifting into drive or reverse, the truck would clunk and almost push forward in a sense.

I put it in park and got my fiancé to step on the gas, the motor didn’t move anymore than it should as I thought it might’ve been the motor mounts. However, what I did notice is when I was under the truck, and she’d put it in drive or reverse, the driveshaft and transmission would move more than it should, I think anyways…

It does not shift hard or clunk when putting into park, neutral or L (manual gears).

Where should I start looking to repair?

 

Welcome from Florida. I would first check the transmission mount and the U Joints.

 

Can also check the transmissions control module for fault codes. The 8L trans had issues with the fluid temp sensor setting P0711.

 

I doubt it is a TFT issue. The TSB for that was for MY2015. Here it is - https://static.nhtsa.gov/odi/tsbs/20...89896-2280.pdf

 

When you say the driveshaft moves more than it should did you mean rotate? If so, then I agree with @oldchevy - you should check your U-joints. You already checked engine mounts.

If you have a bad U-joint it won't "clunk" when going to P, N because there is no torque reversal on the driveshaft - the torque is just going to zero. Going to L doesn't cause a torque reversal either because you were in D. But, going from D->R or R->D does cause a torque reversal - the driveshaft will rotate through the excessive lash in the system and give you a "clunk."

 

2015 wasn't the only year that had TFT sensor issues. I have replaced a lot of internal harness on later M/Y for this reason. It's a suggestion to check for trans fault codes. If you should have an issue with the internal harness #PIP5700D: 8L45 8L90 Internal Wiring Harness Part Numbers By Application - (Jan 14, 2021) may help you get the correct one.