Automatics - rev counter question

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S

ssl

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After the little problem we had with the '03 TD4 auto before xmas i have been noticing stuff i'd never looked at before which leads me to a question for you...

when you're cruising in drive at 60mph for example and take your foot off the accelerator what should happen to the rev needle???????? I think it should stay somewhere around 2500-3000rpm in this case but ours seems to drop down to tickover level as if it goes to neutral. is this right???????
 
Just been out in it, 70mph 2500rpm take your foot off accelerator and drops to just over 2000rpm at 60mph and then just under 2000rpm as 50mph comes up, if you brake a bit and get to 45rpm it drops to tickover level until you accelerate again

seems to drive fine but does not seem right to me that it should do this, as it's the wife's car and she ferries our 2 kids about in it I don't want to risk anything

your opinions gratefully received (on the rev thing, not the wife or kids)
 
Cheers Shutty, am starting to feel a bit better! Anyone else confirm what theirs does?!
 
hi there ssl, my auto td4 does the same as yours. The revs drop to idle when I take my foot off the accelerator.

Nick
 
The reason i think it's strange is that it's as though it kicks the car into neutral below a certain speed, if you're in a manual in 5th at 50mph with your foot off the accelerator you will still be showing more than tickover revs as a gear is engaged and the engine spinning faster than tickover, if you then stuck your foot on the clutch you are effectively putting it in neutral and the revs drop. If I switch it into the manual mode (which is feckin rubbish by the way) it shows about 1500rpm at 50mph in 5th with your foot off

Does that make sense?!
 
In a car with manual gearbox you have a solid contact(connection) through the clutch, thus the revs 'must' follow the speed of the wheels according to the gear you selected. In an automatic gearbox there is no solid contact as it goes via something I don't know the name but it's like two halfs of an orange turning into an oilbath but without touching each other. The one half is linked to the engine and the other half is linked to the driveshaft part, but neither of them is 'forced' to follow the revs of the other. When you rev up the engine the one half orange starts to move the oil around and the other half orange starts 'more or less slowly' picking up that speed under influence of the oil. Because of the missing physical connection with an automatic gearbox it is also the reason why it is impossible to give it a "push" in case of a flat battery. I bet a better explanation will follow soon.
 
think i understand, thanks, first thing i've learnt this year, that and spenidng new years day ten pin bowling with 2 kids is not a 'cheap afternoon out'
 
willo said:
an orange turning into an oilbath but without touching each other. The one half is linked to the engine and the other half is linked to the driveshaft part, but neither of them is 'forced' to follow the revs of the other. When you rev up the engine the one half orange starts to move the oil around and the other half orange starts 'more or less slowly' quote]. . . . . . .Titter, you ok Willo ,only yer sound like yer gone er bit banana's. . . . . . . .:p ;)
Click to expand...
 
willo said:
I don't know the name but it's like two halfs of an orange turning into a bloodbath but without touching each other. The one half is linked to the engine and the other half is linked to the driveshaft part, but neither of them is 'forced' to follow the revs of the other. When you seek revenge on the engine... I bet a better explanation will follow soon.
Click to expand...

Lets hope so..
 
ming said:
willo said:
an orange turning into an oilbath but without touching each other. The one half is linked to the engine and the other half is linked to the driveshaft part, but neither of them is 'forced' to follow the revs of the other. When you rev up the engine the one half orange starts to move the oil around and the other half orange starts 'more or less slowly' quote]. . . . . . .Titter, you ok Willo ,only yer sound like yer gone er bit banana's. . . . . . . .:p ;)
Click to expand...

Only 2 remarks on this:

1. The recipe of the fruitsalad I gave was given while still under influence of the drinks you served me at Xmas.......

And 2.
I warned him that better explanations would follow......:D :D :D

Up till now I think only Optimus Prime comes close to that description ???:rolleyes: :) :) :) (I think it's called the torque converter?).

To avoid further risks of confusion of tongues I'd suggest we all start talking Dutch ???
(1st lesson: Torque converter => Koppelomvormer. )
More to come .......:D :D :D :D :D
Click to expand...
 
Hi SSL,
Mine does the same, and here is my definition.
5th gear is an overdrive gear. I am therefore assuming that with most modern auto boxes having an overdrive gear, that the Torque Convertor is only used in 1st to 4th Gear. Once the car reaches top gear at approximately 44mph, the slip drive that you get with a torque convertor in gears 1 to 4 goes, and you get a locked drive in 5th, the same as with a manual car.
Once the car decelerates enough to drop out of overdrive,5th, the revs will then fall away to idle.
You will find that if you select 4th gear,and accelerate up to 60mph+, that the revs will fall away to idle when you come off the throttle, and it's not til you put it into 5th that they will hold the rpm.
This is designed for best economy.

The AOD gearbox I have fitted in my 5.0L V8 Mk1 Cortina uses an extra drive spline for the overdrive, 4th gear.
So, when the car is in 1st to 3rd gear,all be it not very long at all, it uses the slip of the torque convertor through one splined drive shaft. When enough speed has been gained and it goes into overdrive, there is a solid driven splined shaft conected to a non slipping point of the outer body of the torque convertor.Thus giving solid, best economical drive, as a manual would.
 
scott collin said:
Hi SSL,
Mine does the same, and here is my definition.
5th gear is an overdrive gear. I am therefore assuming that with most modern auto boxes having an overdrive gear, that the Torque Convertor is only used in 1st to 4th Gear. Once the car reaches top gear at approximately 44mph, the slip drive that you get with a torque convertor in gears 1 to 4 goes, and you get a locked drive in 5th, the same as with a manual car.
Once the car decelerates enough to drop out of overdrive,5th, the revs will then fall away to idle.
You will find that if you select 4th gear,and accelerate up to 60mph+, that the revs will fall away to idle when you come off the throttle, and it's not til you put it into 5th that they will hold the rpm.
This is designed for best economy.

The AOD gearbox I have fitted in my 5.0L V8 Mk1 Cortina uses an extra drive spline for the overdrive, 4th gear.
So, when the car is in 1st to 3rd gear,all be it not very long at all, it uses the slip of the torque convertor through one splined drive shaft. When enough speed has been gained and it goes into overdrive, there is a solid driven splined shaft conected to a non slipping point of the outer body of the torque convertor.Thus giving solid, best economical drive, as a manual would.
Click to expand...

This is, what I call, an expert explanation. Thanks scott.
 
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