I have a 06 GMC 2500HD 4x4 6.0 4L80E 4:10.. 85,500 milage.....
my issue is the torque converter will unlock at random at about 70 mph.
I can pull a trailer with a heavy load and stay under 70 mph and all is great. power is good. however with NO load and level ground the torque converter unlocks and will not lock back up until I slow down to below 65 mph.
NO CODES.....
I have extra parts from other trucks to which I have changed the throttle position sensor, throttle body, ECU on the firewall under the hood, and the torque converter lock up in the trans. also changed the fluid, added lucas all with no change.
it takes it a long time to lock up when its cold, first thing of a morning.
this truck does great under a load or empty as long as I stay below 70 mph.
using the cruise or with out, no change.
any suggestions as to where to look and what at would be great.