I don't fully understand it. I think it's all automatic. Basically the lock-up is on when you reach a cruising speed condition. It's off during hard acceleration or when slowing down.
The application of the lock-up clutch locks the turbine to the pump, causing all power transmission to be mechanical, eliminating losses that come with a fluid drive. I guess that's why we see the rpm's drop. It's no longer a 100% fluid connection and there is no loss associated with a fluid drive and we cruise at the same speed, but need less rpm's and gas to maintain that speed.
I think it's possible to install a switch to turn the lock-up on/off.
Mark
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