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Old 20th September 2006, 19:55   #1
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Hello,
I have heard few telling, engine RPM/Speed can rising automatically without pressing accelerator pedal of an Automatic Transmission vehicle in drive position (gear lever).
Is this true? Can this happen?
If Yes, what may be the possible cause and how to find whether what they say is true.
Will this data be stored in ECU/ECM, what are the possible analysis procedure.
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Old 20th September 2006, 21:37   #2
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This is really wierd but it's true, when i test drove the Civic A/T, the tacho would rise without change in speedo reading.

I've never seen this on any M/T. Haven't driven many A/T though.

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Old 21st September 2006, 15:42   #3
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This doesnt happen . As most of us are not used to AT , we may ourself step on the Acc pedal without our knowledge. Other there is nothing which can make the rpm shoot up!
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Old 21st September 2006, 19:55   #4
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Quote:
This doesnt happen . As most of us are not used to AT , we may ourself step on the Acc pedal without our knowledge. Other there is nothing which can make the rpm shoot up!
I did press the acc pedal but i was refering to the inconsistency between the tacho and speedo. On a M/T the tacho cannot rise without the speed increasing (unless you've got a bad clutch).

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Old 21st September 2006, 20:19   #5
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Quote:
Originally Posted by shaktideep
engine RPM/Speed can rising automatically without pressing accelerator pedal of an Automatic Transmission vehicle in drive position (gear lever).
Can you pls elaborate?

Is your gear lever in 'D' ?
Is your car in motion when this is supposed to happen?

Typically, if your car is at rest [gear lever not in 'D'], then the engine is idling, and tacho would should idling speed.

If the car is in motion, and you get your foot off the acclerator, the engine rpm should start decreasing, till the gears are changed internally, when it may show a slight increase in engine rpm before it continues to fall. This is assuming you have not engaged Cruise control.
[Been a while since I drove an A/T, so guys, pls correct me if I am wrong here .. tacho should continue to show a steady drop]

If Cruise Control is engaged, the engine rpm may drop / rise depending on gradient.

Last edited by condor : 21st September 2006 at 20:20.
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Old 21st September 2006, 22:51   #6
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if the transmission has to shift down, the tacho would rise even if speedo is constant.

I have observed this when:

1. I press accelerator too hard. it shifts down to give that extra punch.
2. uphill enountered. it shifts down to give that extra torque.

in both cases the tacho goes up.


if this was not the case, the mechanism which takes care of these condition can be faulty, and give a false downshift causing ur tacho to rise.

experts on ECU/AT mechanism?

EDIT: to 4th post of shan2nu, I am sure u know it but i have observed that in A/T different cars take different reaction time to upshift/downshift...and hence tacho keeps shifting basd on the conditions i mentioned above. it's like it is not able to make up it's mind on which gear to settle.

Last edited by vivekiny2k : 21st September 2006 at 22:54.
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Old 22nd February 2011, 16:39   #7
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Default Re: Can Engine RPM/Speed shoots up automatically in AT vehicle

Hi All,

even though it is quite an old thread, i just bumped onto it today and thought i'd give my 20cents for the benefit of everyone interested.

I am an engineer working on auto transmissions and i know what you guys are talking about here. The reason that you don't always see a linear corelation between the engine speed and the vehicle speed as in the case of an MT vehicle is that in an AT vehicle, you don't have a clutch between the engine and transmission. There is a torque converter instead. A clutch would give a 1:1 sped ratio (the engine speed and transmission input speed are same) unless the clutch is worn out. The torque converter on the other hand uses fluid to transfer the torque and has a variable speed ratio (the engine speed can be higher or equal to the transmission input shaft). As a result, when you press the accelerator pedal, the engine revvs up first and then the vehicle starts speeding up.

The downside to it is obvious - you feel a lag or feel that the engine is revving up without any acceleration, but the upside is that after that lag is over, you get more torque going into the transmission as that is what the torque converter does - It multiplies the torque going into the transmission.
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