And by turning it off, that’s when the light comes on right? And i didn’t know it keeps the temp down and prolongs the trans life when it’s off (obviously city driving not highway)
It primarily has to do with the torque converter. Automatic TJs are equipped with a locking torque converter. In low gears, the torque converter is always unlocked, though in 3rd or 4th it can be either locked or unlocked.
The torque converter only locks when there is sufficiently low torque demand to prevent lugging the engine. The higher the gear ratio, the lower the torque demand.
When the torque converter is unlocked, a significant amount of mechanical energy is lost as heat, similar to slipping a clutch on a manual. This heat builds up in the transmission and shortens the life of the fluid and of the components.
Lower gearing in the axles and the use of the OD off switch reduces the torque demand on the transmission torque converter, thus allowing it to remain locked for a longer period of time.
In addition, higher transmission RPMs help protect the transmission because the fluid is pumped through the cooler(s) more quickly at higher RPM. An auxiliary transmission cooler is recommended for hot or demanding environments (such as frequent towing).
The RPMs have much less of an effect on a manual, as the clutch is almost always engaged, and the heat of a torque converter is not produced. But lower gearing still helps protect the clutch, and in some cases, the synchros.
As an aside, if you've ever wondered why automatics traditionally got worse mileage than manuals, it is because of the torque converter. Unlike the manual,, the torque converter wastes a significant amount of mechanical energy by converting it to heat when it is unlocked. With modern advances in 6-10 speed transmissions, locking torque converters, and better control systems, new automatics generally outperform manuals in terms of fuel efficiency.
