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Overdrive
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marky
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Joined: 12 Jul 2004
Posts: 2418
Location: Subang Jaya

PostPosted: Mon Sep 05, 2005 3:54 pm    Post subject: Reply with quote

Thats actually the way an A/T system works. It sense the accelerator before it decides if it has to downshift or not. The other will be speed sensor which makes it to change when below a certain speed. A/T is a more complicated gear shifting systen in our car.
KCKS wrote:
I myself also feel its not neccessary to play with the OD switch. I can easily shift gears by just playing with the accelerator pedal. Press half to downshift one gear, Press futher to 3/4 downshift another gear. That time the engine rpm should rise quite high then made a loud sound like a sport car. Evil or Very Mad Other car heard that sound then give way for you to overtake, its very powerful sound from engine. Twisted Evil I came across this when im going to Banting yesterday. Crying or Very sad Not like others protons just got loud sound (Muffler exhaust) but the car accerelate slightly faster to overtake ppl. Laughing

slbeh wrote:
KCKS wrote:
I think SlBeh means OD on, last time he told me that Accent not require to play around with the OD like others car did, just let the OD always on. Its because accent A/T got Skip Shift, Grade & Fuzzy logic Fuction, so only need to press accelerator harder to shift to lower gear. Wink
If a guy always off the OD all the time, i think the guy should think that the petrol is very cheap for him Laughing Just joking. Smile


yes, u are right....very seldom off the OD Wink

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noname
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Joined: 07 Oct 2005
Posts: 1548
Location: Petaling Jaya

PostPosted: Thu May 17, 2007 12:29 pm    Post subject: Reply with quote

sean76 wrote:
Hi everyone...

Can someone explain to me in plain lay-man english what O/D does and when is to use it. Maybe it has been discussed before in this forum but i could not locate it. If it has been discussed, please admin or those with the capabilities direct me to it...so far from what i know it gives extra boost when overtaking other cars...correct me if I am wrong. also the indicator on the panel.....saying 'O/D OFF' should be left on or off...Haiyaa...confusinglah...help appreciated...thanks.


pls continue your discussion here...
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keam66
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Joined: 26 Mar 2007
Posts: 70
Location: Serdang,Selangor

PostPosted: Thu May 17, 2007 2:37 pm    Post subject: Reply with quote

OD only apply when you need to overtake another car(only if you are going at 90 -100 km). I seldom use d 'OD' because it uses a lot of petrol as compare to normal drive. I only use 'OD' when there is a speed trap. Once activate it will slow down your car considerable if you happen to be going down hillor above 100 km. It will set your speedo meter to around 80 km per hour. Immedietly after the speed trap, i will release d 'OD' when i had the chance.
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HighCurrent
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Joined: 15 Dec 2004
Posts: 135
Location: kulim, kedah

PostPosted: Sun May 20, 2007 12:52 pm    Post subject: Reply with quote

keam66,

Your understanding of OD is entirely misleading. OD ON versus OD OFF is not the same as NORMAL versus SPORT mode. It is NOT a speed limiter also.

OD = OverDrive = Engine speed is at lower speed compared to drive speed = 4th gear in Accent AT gearbox system.

The default setting on our car is OD 'ON', meaning that 4th gear is 'ENABLED', meaning that the gear will shift according to the speed, the pressure you put on the accelerator, from 1 to 4.

Turn off the button, or 'OD off' (displayed on the meter cluster), will limit the shift until 3rd gear only. If youre speeding above 120 km/h, then suddenly you OFF the OD, you basically FORCE the gear to down shift. DONT DO THIS Exclamation You may damage your AT quickly.

If there's a speed trap, just press the brake slowly. You dont need to 'skillfully' OFF the OD.

Driving uphill or downhill may require you to turn it off (making it 'OD OFF'). Our gearbox is not as smart as European car which downshifts if you release the accelerator. Our's will either go to higher gear or retain the speed downhill without any engine brake if you leave it on. That's dangerous. Turn OFF will make you have some engine brake effects.

During uphill, the AT is not as smart, sometimes it upshift early when you still need the power. So you need to turn it OFF to limit the gear until 3rd gear only. Or if the hill is too slanted, you can drive with D1 (limit to 1st gear) or D2 (limit to 2nd gear). Either two, you need to carefully do it at low speed. Dont shift to those two during high speed.
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david
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Joined: 09 Jan 2005
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PostPosted: Sun May 20, 2007 2:47 pm    Post subject: Reply with quote

well explain, bravo!
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eugene jk
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Joined: 20 Feb 2006
Posts: 132
Location: Kajang

PostPosted: Mon May 21, 2007 12:42 am    Post subject: Reply with quote

Wikipedia~ a bit lengthy but for your reading pleasure

Overdrive can refer to two different things. An overdrive is a device which was commonly used on rear-wheel drive automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque.

It also refers to a combination of gearing inside a transmission or transaxle which results in the output speed being greater than the input speed. In the latter case, "overdrive" does not refer to a physically separate identifiable part/assembly.

History
Early manual automotive transmissions were limited to three or sometimes four speeds, with only the most sophisticated being five speeds. This left an unfulfilled need for a higher gear ratio for highway cruising, which was filled by the addition of separate overdrive units. However, automotive manual transmissions manufactured since the 1980s tend to include a larger selection of gear ratios than before, the highest of which is usually greater than 1:1. This trend has rendered overdrives a complex and obsolete solution to economy gearing in automobiles, and very few cars are fitted with them today, although they still appear on large trucks, where more gear ratios are always in demand.


Usage
Generally speaking, overdrive (OD) is the highest gear in the transmission. Most automatic transmissions have three speeds and overdrive (fourth speed). Overdrive allows the engine to operate at a lower rpm for a given road speed. This allows the vehicle to realize better fuel efficiency, and often quieter operation on the highway. When you switch it on, you allow the transmission to shift into overdrive mode after the certain speed is reached (usually 70+ km/h ~ 45+ mph) depending on the load). When it is off, the transmission shifting is limited to the lower gears. For normal driving conditions, operation of the overdrive should be enabled only if the average speed is above 70 km/h.

It may be necessary to switch it off if the vehicle is being operated in a mountainous area.

The automatic transmission automatically shifts from OD to third gear when more load is present. When less load is present, it shifts back to OD. Under certain conditions, e.g.: driving uphill or towing a trailer, the transmission may "hunt" between OD and the third gear, shifting back and forth. In this case, switching it off can help the transmission to 'decide'. It may also be advantageous to switch it off if engine braking is desired, for example, driving downhill. The vehicle's owner's manual has information and procedures regarding such situations..


How an overdrive unit works
The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit. It can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets which are integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit. A number of such transmissions and transaxles are manufactured by Aisin, for use in vehicles produced by many different manufacturers. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load.


Overdrive in Europe
The vast majority of overdrives in European cars were manufactured by an English company called Laycock de Normanville (later GKN Laycock), which is now defunct, though an uprated hp version of this product is produced in the U.S. by Gear Vendors, and remanufactured in the UK by an ex Laycock de Normanville employee trading as Overdrive Repair Services. De Normanville overdrives were found in vehicles manufactured by Ford, British Leyland, Jaguar, Rootes and Volvo to name but a few. Another British company, the former aircraft builder Fairey, built a successful all-mechanical unit for the Land Rover, which is still in production in America today.


Fuel economy and drivetrain wear
Using overdrive gearing, the car's engine RPM goes down. This reduces engine wear and normally saves fuel. One should refer to the car's owner's manual for the proper speed to run at overdrive. However, all engines have a range of peak efficiency; it is possible for the use of overdrive to keep the engine out of this range, thus cutting into any fuel savings from the lower engine speed.

There is some debate on the overall efficiency of overdrive gearing, as it requires more moving parts than direct 1:1 drive, but most will agree that within the transmission, this effect is minimal. The other difficulty can be in the drive shaft rotation speed.

Overall drivetrain reduction comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes into effect when the differential gearing is high ratio and an overdrive is used to compensate. This may create unpleasant vibrations at high speeds and possible destruction of the driveshaft due to the centrifugal forces or uneven balance.

The driveshaft is usually a hollow metal tube that requires balancing to reduce vibration and contains no internal bracing.

The higher speeds on the driveshaft and related parts can cause heat and wear problems if an overdrive and high differential gearing (or even very small tires) are combined, and create unnecessary friction. This is especially important because the differential gears are bathed in heavy oil and seldom provided with any cooling besides air blowing over the housing.

The impetus is then to minimize overdrive use and provide a higher ratio first gear, which means more gears between the first and the last to keep the engine at its most efficient speed. This is part of the reason that automobiles have been coming with more gears in their transmissions. Also, it is why more than one overdrive gear is seldom seen in a vehicle except in special circumstances.
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keam66
General Member


Joined: 26 Mar 2007
Posts: 70
Location: Serdang,Selangor

PostPosted: Mon May 21, 2007 10:23 am    Post subject: Reply with quote

Thank you, High Current for valuable input.... Embarassed
HighCurrent wrote:
keam66,

Your understanding of OD is entirely misleading. OD ON versus OD OFF is not the same as NORMAL versus SPORT mode. It is NOT a speed limiter also.

OD = OverDrive = Engine speed is at lower speed compared to drive speed = 4th gear in Accent AT gearbox system.

The default setting on our car is OD 'ON', meaning that 4th gear is 'ENABLED', meaning that the gear will shift according to the speed, the pressure you put on the accelerator, from 1 to 4.

Turn off the button, or 'OD off' (displayed on the meter cluster), will limit the shift until 3rd gear only. If youre speeding above 120 km/h, then suddenly you OFF the OD, you basically FORCE the gear to down shift. DONT DO THIS Exclamation You may damage your AT quickly.

If there's a speed trap, just press the brake slowly. You dont need to 'skillfully' OFF the OD.

Driving uphill or downhill may require you to turn it off (making it 'OD OFF'). Our gearbox is not as smart as European car which downshifts if you release the accelerator. Our's will either go to higher gear or retain the speed downhill without any engine brake if you leave it on. That's dangerous. Turn OFF will make you have some engine brake effects.

During uphill, the AT is not as smart, sometimes it upshift early when you still need the power. So you need to turn it OFF to limit the gear until 3rd gear only. Or if the hill is too slanted, you can drive with D1 (limit to 1st gear) or D2 (limit to 2nd gear). Either two, you need to carefully do it at low speed. Dont shift to those two during high speed.
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noname
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Joined: 07 Oct 2005
Posts: 1548
Location: Petaling Jaya

PostPosted: Mon May 21, 2007 9:13 pm    Post subject: Reply with quote

one advise, if not sure about OD, just leave it OD ON ... save the trouble.

our OD work differently from other brand of car. our car OD OFF will display on panel, while other brand will not display on panel.... Wink
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