Torque convertors in the newer trucks have 2 stages. A fluid coupling occurs first when you are starting, accelerating or pulling up a big hill. When you are cruising at a constant speed, the lock up portion of the converter is engaged and 100% of each rpm is transferred through the converter, just like a standard clutch.
The first position of the TorqLoc switch manually locks up the convertor clutch just like a standard transmission, this results in lower transmission temperature and excellent transfer of retarding hp when slowing down.
The second position returns the truck to a stock application.
The third position reverts to AutoLoc function (please review our AutoLoc page)and is good to drive in this mode 100% of the time.
The computer will automatically disengage the lock up clutch at 25 mph to prevent the engine from stalling if you forget to switch the lock up off.
BD Diesel Convertor, Duramax Allison 1000