The "Shift L" button is intended to lock the transmission in first gear for creeper work, or engine braking on a downhill.
While modern automatic transmissions are complex, they’ve become slightly less complex with the advent of computers, which have replaced all or most of the valve body.
Automatic transmissions are generally quite reliable, but when they break, it’s not cheap. Therefore, if there’s anything you can do to prolong the life of your transmission, you should do it.
Here’s what you can do to make your Tundra transmission last as long as possible shared by Tundraheadquarters.com.