I tried to stay away from this one as long as I could…but now I just can’t resist any longer.
I used to design and calibrate automatic transmissions for one of the Big 2 (Daimler Chrysler? No longer can refer to them as one of the Bigs).
Darqangelle I used to hate automatic cars too, until I worked in the field…and developed respect for them. I still prefer driving cars and trucks with M/T’s though.
Okay to answer the OP. The reason you can select 1st gear on the PRNDL selector is for those occasions where you might want to pull a load slow, or move your vehicle with high wheel torque through whatever situation you feel warrants it, w/o going to any higher gears…Descending a hill while offroading comes to my mind as an ideal situation where you would want to use the 1st gear selection. If you don’t ever need to do this, just leave it in Drive.
Side note:
I’ve always known “Granny Gear” as the term that is used for the lowest gear on some of the older model pick-up trucks and even some of the newer pick-up trucks with manual transmissions. When you don’t have a load on your truck, that lowest gear ratio is pretty much overkill. Most of the time you just start out in 2nd gear to avoid having to shift at 5mph at every stop light etc. When you are pulling a trailer or have your bed loaded down, then you might need the extra wheel torque to get going. I don’t know much about semi-tractors, but I know they have a multitude of gear ratios to choose from, and I’m sure they just use the gear that will keep them moving without sputtering when climbing hills…etc.
In an automatic transmission, starting up under load or w/o load does not matter…the torque converter takes care of variations in wheel torque requirements…in a way.
Then, based on throttle position and vehicle speed, the vehicle determines when it should shift 1-2, 2-3, 3-4, and if it’s a five speed auto, 4-5…and if conditions are right…the torque converter will lock together the input and output to form a “solid” connection between engine and tranny…on downshifts, it depends on how much you “step-in” to the throttle. If you go heavy on the throttle, like in a passing situation, you might downshift 4-2, or 3-1…skipping the gear in between. If you’re light on the throttle you might just get a 4-3 or just a torque converter unlock…Oh, and another thing, the only time an A/T shifts based on RPM is at wide open throttle…that’s to protect the engine from over revving.
We have tried to make A/T’s better for climbing hills so they will not shift cycle between a higher and lower gear every time a car deccelerates due to the inclination of the hill. The vehicle measures acceleration as well as speed and based on this, will determine whether or not it’s a good idea to upshift, even though throttle position and speed say it should. If it’s not accelerating enough, we assume that a downshift will need to occur soonafter, so no upshift is allowed until acceleration improves.
And yes, to save your brakes when decending mountain passes etc, A/T’s should be downshifted out of overdrive into plain old drive and sometimes down to 2 to employ the help of your engine to slow down the vehicle and stay under control. The “2” selection on most A/T’s will keep the car in 2 no matter what the conditions…even if you stop and start going again. Drive as opposed to Overdrive will use all the gears below the overdrive gear as if the overdrive gear does not exist. Other than that, the shifting occurs normally.
I hope this is not too jumbled up to make sense.