trans cooler
When on the highway, giving a little and letting off a little of the gas, does the tach move up and down a little? If locked it shouldn't, should be rock solid proportional to the speedo.
Several things can cause it to not lock, one of them is "misfires". If your computer thinks there's an engine miss it will not lock the torque converter. Also I understand that if you have a problematic TPS your computer sees the throttle all over the place and never locks (it wants to lock at low throttle/MPH ratio). Several little things can cause this. A mechanic can tell you what it's doing.






