I have a 97 GMC Sierra with 5.7L - 4L60E tranny. On rare occasions the tranny shifts very hard (almost a power shift). During this time the torque converter will not lock up. The only way to get it to stop is pull the battery or pull the ECM fuses for 20 minutes or so. After it's reset, it works like new.
It has happened four times since I have owned the vehicle (purchased it new in 97).
Any thoughts as to why this happens?
Thanks in advance. Dan

 

After some research , I found out that hard shifting is the transmission default. When the torque converter (TC) senses a zero torque (when the tires free wheel similiar to slipping on painted lines or metal from a toll plaza) the transmission goes into default mode (full pressure hard shifts) to protect itself.
The transmission will reset itself after it senses 3 or more TC lockups (removing power also resets it).
I'm sure there are other reasons such as tranny age, but this diagnosis fits my particular situation.
Any thoughts about this?