Automatic TC unlock question

I've searched and not found this particular issue discussed anywhere. My torque converted appears to lock and unlock normally (passes Offroad's tests). However, when driving with the TC locked (say 2400 RPM at 65 MPH in 4th), if I let off on the gas altogether, the RPM will drop 400 to 500 RPM (down to 2000 immediately). I know that the TC is locked initially because I can observe the drop in RPM from the unlocked (2700) to the locked state (2400). It just appears to "unlock" again if I let off on the gas. I had expected that the TC would just stay locked (less tranny heating and better engine braking). It'll do it in 3rd as well. Is this normal?

Sorry for the long post. Thanks for your help, Steven

AFAIK, that's normal. Most auto trannys do that (even on my toyota camry). My RPM drops as well when I let off on the gas. Everything is electronically controlled now. The ECM decides when to lock and unlock.

Just thinking out loud, maybe the reason for TC unlocking when you release the gas pedal is that in the pre-ABS days, you might slam and lock the wheels in an emergency situation. And if your TC is lazy or sluggish, it may not unlock in time and you could stall the engine making you lose power steering, and make the truck harder to control. So maybe releasing early (between letting go of gas and slamming the brakes), this can be avoided. Nowadays with ABS, though possible, wheels are harder to lockup. Nissan may have decided to leave this design in there just for safety reasons since all other auto manufacturers left it in as well.

XCFreak, thanks for the reply. I figured it was probably normal, but since this is my first automatic, I thought I'd ask the experts. I had thought that it might be unlocking in order to reduce the braking effort (only car momentum instead of car and engine rotation), so that the brake pads might wear longer. Your reason sounds plausible to me too...definitely would stink to kill the engine in an emergency. Thanks again.

TC will unlock when you let off the gas, it's normal.

Brent