2019 Silverado cabin rumbles when slowing down

[mmmikkke]

On 2/10/2020 at 10:54 AM, lapoolboy said:

It may not even be the transmission.  My testing leads me to believe it is in the driveshafts.

I think so too.  Driveshaft or U-joints or rear differential.

 

I'm speaking specifically about the 60-80 mph rumble, which I now officially designate as the 6080R design defect. 

 

Others have spoken of low speed rumble, which probably has a different cause.  This may also need a shorthand designation (e.g. 302010R).

 

Still others describe a low speed unlock of the torque converter.  This one I am familiar with.

 

GM programs the 6 and 8 speed transmissions to keep the torque converter locked as you decelerate toward a red traffic signal.  In this situation, the torque converter behaves exactly the same as a newbie driving a manual transmission truck (in-gear decelerating) and forgetting to disengage the clutch until almost stopped.  The torque converter remains locked until you get down to 600 rpm, the engine begins to lug just for an instant, and finally the torque converter unlocks and allows the truck to stop without stalling the engine.

 

These transmissions have multiple lookup tables (like individual worksheets in an Excel spreadsheet) to, for example, keep the torque converter locked while decelerating, unlocking at the last possible moment.  There are different tables prescribing different unlock rpm behavior depending on whether the air conditioning compressor clutch is engaged or not.  That is what makes these issues so hard to pin down.  (Go to the hptune website/forum to get additional information about these tables.)

 

I hereby designate this third issue--a programming defect--as the Lazy Unlocker TCM programming defect.  Hptune can fix this issue easily in 2017 and earlier trucks, and maybe also 2018 and 2019 trucks at extra cost (because GM made changing the table values more difficult).   In such cases, the Lazy Unlocker defect can be corrected by just changing a few values in these tables, such as "minimum torque converter lock speed").

 

 

Edited September 24, 2021 by mmmikkke
clarified