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Neutral vs. drive when waiting on a light

I am having a debate with my dad over what you should do when driving a automatic transmission car. My dad says that when you come to a light, it is better for the transmission to shift to neutral while you wait for the light to change. I say that puts unessary shifting on the transmission and just to leave it in drive. Which one of us is correct?

You’re more correct, but not worth much discussion. Neither way will cause problems (if one is careful).

I’m with you.

Also, ask your dad what happens when you’re stopped at a light and you suddenly notice a car coming up behind you too fast to stop (probably on a cellphone). I’d much rather be in gear.

Why ask the transmission to shift into gear thousands of extra times? That’s FAR more wear than just leaving it in gear and let the torque converter provide the “neutral” like it was designed to do…

The biggest problem with that is those times when you’ve stepped on the gas first… only then to put it in gear …BAM

Then there’s the time delay which may not seem like much to you, but when the tenth and eleventh car in line don’t make it through a light they know dang well they should have, your actions are now rude and inconsiderate to the big picture of the most effecient and smooth use of traffic timing for everyone on the road.

Leave it in drive. If it’s going to be an extended halt i.e. a slow moving train, then park is appropriate.

Leaving it in drive is better. However, it may be too late to change your father’s mind on this. Let him do what he wants with his car(s), and ask that he respect you and let you do what you want in yours.

Technically you’re correct.

The engine is connected to the tranny via a fluid coupling device called a “torque converter”. It’s sort of like a hollowed out bagel cut in half with vanes inside each half, and filled with fluid. As the front half spins, it drags the fluid and the fluid drags the back half. In new cars, the design has evolved to almost totally disconnect the halves of the torque converter at idle, the goal being to eliminate all load on the engine and increase gas mileage a bit. In short, the car becomes “free rolling”, virtually disconnected from the engine. Why put added wear and tear on the linkage and the tranny internals when no load exists leaving the car in D?

But, it’s your dad’s car and if he feels more comfortable putting it in neutral, then that’s his choice.

In hot weather when the AC is running and the light seems long or if stopped for a train, etc., I shift to neutral. Doing so takes a great deal of stress off the cooling system and the AC system and keeps the cabin temperature from rising. If the temperature is moderate there doesn’t seem to be any reason to shift out of Drive.

I would think that advice would vary by the car being used. None of mine have had any trouble idling in gear with the AC on in a 100+ degree environment. However, when waiting on a train I do shift to neutral and put the parking brake on. This adds a slight amount of extra wear to the car, but it reduces wear on my foot that was on the brake.

There may be a small bit of extra heat generated when the AC is on, as the ECU does bump the idle up a bit to compensate for the extra load on the crank, and that would generate a bit of extra heat from the engine itself and an immeasurably tiny added but from the Torque Converter, but I would submit that the amount of added heat is extremely small.

But if that action makes you feel more comfortable, I support your doing it. I really don’t believe the difference is large enough to even measure.

The AC itself being on is a definite load and a definite difference, but that’s true whether the tranny is shifted to neutral or not.

[b] But, it's your dad's car and if he feels more comfortable putting it in neutral, then that's his choice.[/b] 

Worth repeating.

All the vehicles that I drive have belt driven fans and the drop of pressure on the high side of the AC is significant when shifting out of gear at idle and the radiator also makes a significant temperature drop.