[echo77]

Found a nice article on torque converters:

http://honda-tech.com/showthread.php?t=2086594

[Rameshdude]

Hello all,

I experience unexpected acceleration in my Corolla Altis AT, especially while reversing. When on R mode, it moves slowly, but the moment I touch the accelerator pedal, the car literally jerks. Presently I use the hand brake to have a control. Do others experience this ? What else can I do ?
Thanks in advance.

[tarunchoudhary]

Hello All,

In my dad's i10 AT, there's a small button on the gear lever to switch off/on OVERDRIVE.
What exactly does this overdrive option mean to an Auto transmission box and in which driving conditions this option should be used?

PS: Tried searching the complete thread for this info, but could not find any such information.

[noopster]

Originally Posted by Rameshdude

Hello all,

I experience unexpected acceleration in my Corolla Altis AT, especially while reversing. When on R mode, it moves slowly, but the moment I touch the accelerator pedal, the car literally jerks. Presently I use the hand brake to have a control. Do others experience this ? What else can I do ?
Thanks in advance.

Reverse should work exactly like Drive except no upshifts. Please get teh car checked. Your ECU/TCU may need to be reset.

Originally Posted by tarunchoudhary

Hello All,

In my dad's i10 AT, there's a small button on the gear lever to switch off/on OVERDRIVE.
What exactly does this overdrive option mean to an Auto transmission box and in which driving conditions this option should be used?

PS: Tried searching the complete thread for this info, but could not find any such information.

There have been multiple discussions on this but I can't remember where! Simply put, an overdrive is a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise, and lower wear (Source: Wikipedia).
One defining characteristic of being in overdrive is that the car doesn't "pull" more. So it isn't advisable to use it while climbing, or with heavy loads etc.

[Jeroen]

Originally Posted by tarunchoudhary

Hello All,

In my dad's i10 AT, there's a small button on the gear lever to switch off/on OVERDRIVE.
What exactly does this overdrive option mean to an Auto transmission box and in which driving conditions this option should be used?

PS: Tried searching the complete thread for this info, but could not find any such information.

Not familiar with your dad's car, but from what I have experienced driving a lot of different cars this seems to work pretty much the same on all cars.

If you drive the car in auto (D) with the overdrive switched off, no overdrive will engage. When you switch it on, it will automatically engage the overdrive. What it means you will just feel/get an extra gear change. So if say you have 4 gear autobox with overdrive, once you get upto speed into fourth gear and a certain RPM the overdrive will engage, lowering the RPM a little more.

Some more elaborate gearboxes, auto or manual have the overdrive on several gears. You'll find it for instance often on 3rd and 4th gear. The purpose of the overdrive is essentially to reduce the RPM of the engine. Thus increasing fuel efficiency and also less noice. Normally when a car is in highest gear the engine rpm equals the rpm of the output shaft of the gearbox. The overdrive subsequently drops the RPM ont he gearbox output shaft further. Technically it can only be called an overdrive if it lower the RPM of the gear box output shaft compared to the input shaft (ie engine rpm). You loose torgue, so accelerating with overdrive engaged will not go as swifly. On a autobox the overdrive will disengage if you press the accelerator sufficiently to allow for more torgue.

When you drive your autobox in manual mode, you can play around 'manually' with the overdrive. The principle on when you should with switch it on remains the same. Once in high gear, steady speed, switch it on. You increase fuel efficieny and lower the engine noise a bit.

Typically you would keep the overdrive disengaged when driving in mountenous terrain. Going up the mountains it usually doesn't make sense, because you lose torgue. Going down hill you want maximum braking power from your engine and that means high rpm, thus no overdrive

Jeroen

[anujatwork]

Originally Posted by Rameshdude

Hello all,

I experience unexpected acceleration in my Corolla Altis AT, especially while reversing. When on R mode, it moves slowly, but the moment I touch the accelerator pedal, the car literally jerks. Presently I use the hand brake to have a control. Do others experience this ? What else can I do ?
Thanks in advance.

I drive an Altis AT too. I don't find any unusual jerk. All ATs have constant inertia if they are in D or R mode and will keep creeping. The moment you press Accelerator, they are bound to leap forward. Its like 2 athletes racing - one is getting off from stand still (manual car) and the other is coming with a run up and then making a sprint! Later is bound to have a spirited acceleration on pressing the pedal.

[Jeroen]

Originally Posted by anujatwork

I drive an Altis AT too. I don't find any unusual jerk. All ATs have constant inertia if they are in D or R mode and will keep creeping. The moment you press Accelerator, they are bound to leap forward. Its like 2 athletes racing - one is getting off from stand still (manual car) and the other is coming with a run up and then making a sprint! Later is bound to have a spirited acceleration on pressing the pedal.

Just to add to the above comment:

Without actual comparison it is difficult to judge of course. Just one thing to bear in mind. Reverse gear on nearly all cars has a lower gearing then your first gear forward. Manual and auto boxes alike.

What it means in practice is that your car will accelerate a little faster going in reverse for the same deflection of the accelerator. Or to put it differently, to get the same feel of smooth acceleration as you get in drive you need to be a little more carefull with your right foot when going in reverse.

On some cars its more noticeable then on others. I own a car that even has two gears in reverse, making this effect potentially even more pronounced!

Jeroen

[NinadJoshi]

Originally Posted by tarunchoudhary

Hello All,

In my dad's i10 AT, there's a small button on the gear lever to switch off/on OVERDRIVE.
What exactly does this overdrive option mean to an Auto transmission box and in which driving conditions this option should be used?

PS: Tried searching the complete thread for this info, but could not find any such information.

Keep the overdrive enabled forever, and forget about it. Rarely will there be an occasion in a normal driving pattern that would demand the overdrive to be disabled.

Automatic transmissions adapt to the load and road by themselves, so this 'disable overdrive on an uphill drive' is a myth. The worst that will happen is your transmission will hunt between two gears on an ascent, which is perfectly fine since it is using its own algorithm to make the proper gear engagement. If the occasional hunting annoys you, you may temporarily disable overdrive, but it is not at all necessary.

You WANT to get into overdrive as much as possible because that is the most efficient area of operation for the engine. In simple terms, the more time the engine remains in the highest gear (overdrive), the better the gas mileage. The higher the gear, the fewer times the engine turns for the same amount of distance traveled, so the longer the engine lasts.

[Jeroen]

Originally Posted by NinadJoshi

The higher the gear, the fewer times the engine turns for the same amount of distance traveled, so the longer the engine lasts.

Sorry, this is simply not true. What is true is that you an overdrive improves fuel efficiency ( and reduces engine noise as you drop the rpm's).

However there is no correlation whatsoever between distance covered, engine turn over and general wear and tear. The idea that less rpm allows for less wear and tear is simply not true.

Now, admittedly, the way engines get designed these days the above difference for all intents and purposes is mostly a hypothetical/academic one. Who cares about wear and tear if your average engine, well maintained, runs perfectly fine for 200.000 + kilometers. No matter how your run it.

Most petrol engines from an overall wear and tear point of view do best when they get loaded up to 50 - 70% of their nominal output. That's not necessarily the most economic way of running them. With Diesels it tends to be a little higher. Aircraft engines, tend to be even higher.

Although I've been in India only about eight months, I've noticed that a lot of Indian drivers really keep the rpm's low and shift gears very quickly. Although most likely great for fuel efficiency, not necessarily great for wear and tear. But as I mentioned before, to a large degree a theoretical issue.

If you're concerned about fuel efficiency by all means keep those revs as low as possible. If you are not or less concerned about fuel efficiency but like to give your cars the beans so to speak don't be concerned about wear and tear. As long as you don't constantly redline it, you'll be fine, if not better, then all these folks that never get above 1500 rpm. At least from a wear and tear perspective that is.

Jeroen

[OmThoke]

Originally Posted by Minardi

One thing I wanted to know...when you stop at the red light, you still keep the gear on 'D' and put your foot on the brake or do you put the gear on 'P'?

You should shift it to P ideally, and put the hand-brakes, rather than stressing yourself by holding the brakes manually.

Secondly I thought the brakes were powerful and I was really careful not to slam them hard. I was scared I'll lock the brakes.

Don't worry about that!

Can the gear change be controlled indirectly through the speed at which you press the accelerate? I though I read somewhere, if you press the accelerator suddenly, then the auto will shift down to give you that 'surge'. Is this true?

Yes, if you press the accelerate hard, it shifts faster, but if you press and hold, then it'll rev harder, and the RPMs will go higher, and when you release, it'll go to the next gear.

On a side-note, when you're driving automatic, you won't even need a hand-brake or manual brakes if your car has the auto hill-assist sort of a feature, which most high end cars do (like Audi A4, and higher segment), though I'm not sure about a City or a C-segment car (I doubt if they 've 'em, in which case, you'll have to learn how to survive with manual work!).

And, once you get used to it, you'll simply start loving it, and won't want to drive a manual.

I've a manual Cedia, and an automatic s80; I enjoy driving automatic in city, and the manual transmission on sparsely crowded streets, and highways.

[NinadJoshi]

Originally Posted by Jeroen

Sorry, this is simply not true. What is true is that you an overdrive improves fuel efficiency ( and reduces engine noise as you drop the rpm's).Jeroen

Sorry, but I absolutely and wholeheartedly stand by what I said earlier.

Everything else remaining the same, 1000 rotations of the engine on 4th gear will most certainly take you a lesser distance than 1000 engine rotations done on the 5th gear.

Gear ratio lowers as you go higher gear.

Not using overdrive is like driving a motorcycle on just 3 gears, keeping the 4th unutilized.

[Jeroen]

Originally Posted by NinadJoshi

Sorry, but I absolutely and wholeheartedly stand by what I said earlier.

Everything else remaining the same, 1000 rotations of the engine on 4th gear will most certainly take you a lesser distance than 1000 engine rotations done on the 5th gear.

Gear ratio lowers as you go higher gear.

Not using overdrive is like driving a motorcycle on just 3 gears, keeping the 4th unutilized.

When talking about fuel efficiency I 100% agree with you. And I have mentioned the same.

But you also stated that the engine would last longer. I.e. less wear and tear. At least that how I read / interpreted your statement "last longer".

And that is not true. When petrol engines run in their most fuel efficient part of their performance envelope they do not necessarily run with minimum wear and tear.

But don't get too worried over this one. I also mentioned with today's modern engine it's a bit of a theoretical debate, as modern engines easily run for 200.000. Whether you never go above 1500 rpm, or whether you constantly run them at 3000 rpm.

Jeroen

[Samurai]

Originally Posted by NinadJoshi

Automatic transmissions adapt to the load and road by themselves, so this 'disable overdrive on an uphill drive' is a myth.

Actually, it is not a myth, at least not for the reasons you are thinking. If you are only going uphill, the AT will rarely go into overdrive, so disabling overdrive is a moot point. However, any experienced hill driver knows that ghat roads are a combination of uphill and downhill. At least that is the case in south India, where we have a downhill after every uphill. At every downhill, the AT car on overdrive tend to almost free fall, which is not safe considering the sharp turns. Therefore, it is safer to disable overdrive, sometimes even switch to 3 (assuming 5 speed AT) in very narrow ghats.

[d_himan]

Originally Posted by tarunchoudhary

Hello All,

In my dad's i10 AT, there's a small button on the gear lever to switch off/on OVERDRIVE.
What exactly does this overdrive option mean to an Auto transmission box and in which driving conditions this option should be used?

PS: Tried searching the complete thread for this info, but could not find any such information.

you'll find the i10s AT always trying to upshift as quickly as possible with the OD turned on (normal).

With the OD off, two things happen:

1. Only 3 forward gears instead of 4.
2. Gear shifts happen at 2.5-3K RPM in normal driving instead of 1.5-2K.

In the real world, I use it occasionally when I want to close gaps in traffic and drive relatively aggressively, something akin to a 'sports' mode. With the OD on, there's no power when you need it the most - downshifts take their own sweet time and the opportunity to overtake/close the gap usually goes away.

Slight reduction in FE, but not much unless you keep revving away to 5K RPM

[anujatwork]

Originally Posted by OmThoke

And, once you get used to it, you'll simply start loving it, and won't want to drive a manual.

I've a manual Cedia, and an automatic s80; I enjoy driving automatic in city, and the manual transmission on sparsely crowded streets, and highways.

I completely agree with this one. Once you get used to squatting your left leg under your right thigh, you just dont feel like touching the manual unless on a pleasure drive at night. What's more! The Auto with a larger engine gives me a better mileage than the manual. Yes, I am sedate driver & mostly commute within the city.