Well I’ll start off I purchased a 1999 Silverado 6.0 vortec with 4L80e trans. The next day of owning the truck when getting into high way speeds (55-65) mph the truck would down shift and hold 3rd gear. Never would go back into OD, come to a stop sign or red light and truck would start shifting hard (1-2) and (2-3), I can cut truck off and sit for a few minutes and it will be back to normal until I attempt high way speeds again. I have already changed internal wiring harness due to fluid being in connector, still same issue I took it to a shop and was told loss of communication to TCM and trans slip. They reset codes but only made it maybe 2-3 miles up the road at highways speeds and same issue. Any advice greatly appreciated!!!

 

Welcome from WA! Please post your question in the appropriate section for your vehicle. This is just the new members welcome section and will not get you much help.

 

HELLO! From north Alabama here!!

 

Welcome to the forum.