Originally Posted by BCDave
Its not the trans that is slipping when the T/C is unlocked - it is t/c itself that now slips.
If you shift down out of O/D, it will be unlocked all the time, so moving the lever down to D (from O/D) you get the full "down one gear PLUS unlocked" effect. It will only lock in the highest gear, so unless you are in O/D, it cannot do the 400 or 500 RPM thing.
However - if you are NOT in O/D, and you lightly step on and off the gas pedal a couple of times (without actually changing speed of the vehicle), you will likely get a "bounce' in RPM of the same amount as you put "tension" on it and then back off. Adding gas brings it up to its max slip, while getting off the gas shows what the rpm would be if it was locked.
I don't get that RPM 'bounce', though. So maybe mine is locked up in 4th. We both got a PM from another member who said his Sienna's converter is locked up in 4th as well as 5th.
Thanks for clarifying about what is slipping and what's not. I'm never too old to learn something.