Very basic question: what’s the difference between “D”, “1” and “2” on an automatic shift? More power? More torque? Higher speeds? Hyperspace? The reason I ask is that in 25 years of driving I’ve only ever driven a manual; automatics are easy enough, but I scarcely ever drive them - work cars, very ocasionally - and then only use “D”. Should I be shifting up or down or whatever it is?
Automatics have gears just like manuals. Putting the selector on 1 or 2 locks the gearbox in 1st or 2nd gear respectively and it won’t upshift or downshift no matter what.
These positions are used when you want to descend a steep hill or start the car on a loose surface. In everyday driving conditions you simply put it on D and forget about gear changes.
EDIT: or towing something. When towing you start the car in 1, accelerate, upshift to 2, accelerate and then put it on D. If you put it directly on D the gearbox will quickly upshift to 3rd gear and overheat the torque converter.
On some cars, putting it in 2 will allow it to shift between 1 and 2, but won’t allow it to upshift into 3rd. This doesn’t make too much difference in the way that you use it though.
Another reason to use 1 and 2 is if you live in a very mountainous area you can downshift to put some of the braking load on the engine and transmission, which can prevent your brakes from overheating on very large hills. If you have a manual transmission you’ll want to also downshift under those same conditions. You don’t want to use the same technique to slow an automatic down under normal flat conditions though, like you might do with a manual.
Unless you are towing something (like Dog80 said) or you live in the mountains I can’t think of too many reasons why you’d want to shift out of D.
Aw, I wanted hyperspace.
That’s the “H” position on the shifter. I guess the cars your company buys don’t have that option. Shame; it’s a lot of fun.
Note that in certain RWD vehicles, “2” doesn’t let you shift between 1 and 2; rather, 2 locks you in second gear, period.
My '96 Chevy Caprice Classic was like this.
God, I miss the torque…
IME, this is the more common arrangement.
Sorry, I have to chime in here:
Selecting “2” on some auto-trans will start it out in second gear, essentially locking it in second (On many, it won’t). When helpful: When surface is loose and the shorter (higher numeric gear/tooth count) gearing of 1 (first gear) sends too much power to the drive wheels. Snow, ice…maybe gravel, MIGHT be candidates for such a technique if your auto-trans actually does this. Many do not.
When towing something, you** might **not want to select D (drive, highest gear), or particularly OD (Overdrive) as the trans might jump in and out of top gear too frequently if you are constantly flirting with highway speeds only to dip below forcing upshift/downshit, etc. On some auto trans, you deactivate OD by pressing a button andd the dash lights warn you that OD is off. Switch it back on once cruising speed is consistent and you will be well served.
Coming down steep grades, you will want to know the usable speed and RPM for each gear selection. Downshifting into 2 is nice, but you might over-rev the engine if you do it at 50 mph. In a manual trans car, you would be more likely to know the speeds for each gear, but in an auto trans car, you might want to check the manual.
I have never towed anything but why would this be? Wouldn’t the heavier load on the engine prevent it from upshifting prematurely, the same way that giving it the gas causes it to downshift? Well, apparently it doesn’t, but I’m trying to understand why.
Well, yeah, that’s the idea but if you’ve ever stomped on the gas pedal of an automatic car and had it go nowhere, you’ll see why forcing the car to downshift is preferable.
Automatics respond to RPM, not load. There’s that “kickdown” notch at the end of gas pedal travel for this reason, too.
Automatics respond to loads. Under heavy load, they will downshift.
Locking out certain gears in an automatic will prevent it from ‘hunting’ between gears as loads and speeds change due to towing, hills, traffic and a variety of factors, including driver behavior.
I can’t recall an FWD vehicle with this arrangement, actually.
Most modern automatics also now have a lock-up torque converter, anybody knows if switching off OD also prevents the lock-up torque converter from engaging, in order words, does the lock-up torque converter only engages once you are in OD, seems to be the case with my 2003 Camry, does it vary by manufacturer.Just to make things clear, engine load has nothing to do with the vehicle load. Engine load in automotive literature is usually given as a percentage and is simply how much you press the gas pedal, 0% being foot off the gas pedal and 100% being pedal to the metal.
Automatics do respond to engine load. Like you said, giving some gas (increasing engine load) causes the gearbox to downshift. Another example, at 100% engine load the car will accelerate almost to the redline before upshifting or when you go down a steep hill with 0% load it will downshift or at least it wont upshift even if the RPM go up.
But the car has no way to know the vehicle load. When towing something heavy you are more likely to go easy on the gas pedal (small engine load) so the car will upshift, which is quite the opposite of what is needed and puts strain on the torque converter. Very few cars take into account vehicle load when shifting. The only one I know for sure is the latest Mercedes M Class SUV (W164).
My sister’s Citroen C4 has a 4 gear automatic transmission with lock-up feature but no overdrive. It certainly locks when in 4th gear. When it locks the RPM fall slightly and it feels like an extra gear change. I don’t know if it does lock in the other gears though.
Actually, the vehicle DOES know load, and Load is a variable available to vehicle OBDII scanners.
It consists of, IIRC, torque generated vs maximum torque generated available at a given RPM.
Half throttle at 2000 RPM will generate a different Load output than half throttle at 3300 RPM.
Modern shifting algorithms certainly pay attention to Load.
On edit:
Check out page 8 of this lengthy PDF for a definition of LOD.
http://www.google.com/url?sa=t&source=web&ct=res&cd=1&url=http%3A%2F%2Fwww.scangauge.com%2Fsupport%2Fpdfs%2FSGIIManual.pdf&ei=bQHRSbmJIpnglQfhkNXnCQ&usg=AFQjCNF0VrIjCBrkRpWxR0PuBNzV-zW0Nw&sig2=D_OF95Gz-PD1Kr66jdUFZw
Actually engine load is calculated from manifold pressure and air mass flow , but at steady states it is closely related to the throttle position.
Folks, having the auto-trans know the current state (speed, load, gear) is only half the battle. E.G., You are towing a boat, and you reach 50 MPH on a nice, flat surface and little load (little accel or none) …The tranny has no way of knowing that you just reached an incline when it shifts to OD…and that this incline just 25 yeard away is going to increase load, so as soon as this incline happens, it down shifts and also the TQ unlocks. Now your 5-spd auto is in fourth with the TQ unlocked, and it was in 5th with the TQ locked for 5 seconds. You slowly build speed, and now you are doing 50-55 again, and the tranny upshifts, the TQ locks and the tranny has no idea that you are now about to slow for traffic down to 35 mph…and you can repeat this cycle over and over due to inclines and traffic, and the tranny is going to hunt.
You are the other part of the thinking that is needed. You can drop the selector and/or choose the button that locks out OD, because you can anticipate hills, traffic, variations in speed, etc.
That’s what selecting a gear is all about. Auto-trans are good, but they don’t know what lies ahead. If you know you have varying speeds and hills and other factors affecting the load on the engine and you sense it’s hunting, drop it down to one gear below OD. Heck, sometimes in the city, I drop a four-speed auto into 2, and as I go stop sign to stop sign, it doesn’t thud into 3rd as I am just about to start breaking.
It’s OK to supplement a great auto-tranny with your human ability to see/sense what lies ahead.
Our 2006 Honda Accord is one; per the owner’s manual the “2” selection locks the transmission into 2nd gear.