What would happen if?

[portzy]

I've half had the answer to this one but I dare'nt try the full experiment

What would happen if, with an auto (Mercedes of course ), you were in drive on a steep incline and, the engine could not hold station on idle and, the whole thing rolled backwards without driver intervention for a significant distance?. Would this ruin the gearbox?

Second. I'm cruising at say 70/80 in drive and somehow ended up in neutral. Do I just go back into drive?. What would I expect to happen and what would be the attitude of the engine and gearbox to this exercise/practice/abuse?, would it complain vocally and expensively?

Edit: I suppose what I'm trying to fathom with the second is, is it o.k. to coast up to lights and junctions and such then, snick it back into drive expecting the gearbox to 'know' what gear to select.

Portzy

 

[PJH]

Did the 2nd with a Vauxhall Calton a few times, no noises or jerks.

 

[GRAV888]

If you coast in neutral and then put back in gear, you will probably find that it will select a lower gear than usual if you are still moving.
You should not coast however, due to the oil not circulating in the box quickly enough.
Silly question, but if you have an auto, why cruise anyway?

 

[andy_k]

Number 1,

no reason why it should, it's only a fluid link so you wouldn't be forcing components to turn the wrong way - the incline doesn't need to be that steep for it not to hold.

Number 2,

Shouldn't really be done as Grav explained.

Andy

 

[uumode]

1. I think my Audi handbook it advised to tow from the NON driving wheels, as it pulls unburnt fuel through the system and destroys the cat. Quattros can't be towed at all and need a trailer. Not sure if it is Audi specific. But your scenerio is with the car engine running so umm.. dunno. Must be one hell of a steep incline for the car to roll backwards? I don't think I have rolled backwards.

2. Tried that on my Polo, it's OK as long as you don't shove it into an inappropriate gear for the speed (same as you would do in a manual) i.e. don't shove it into first when you are doing 60mph! - I only put it in neutral as I had a cycling metallic noise, and wanted to eliminate the gearbox from the equation.

 

[uumode]

Edit: I suppose what I'm trying to fathom with the second is, is it o.k. to coast up to lights and junctions and such then, snick it back into drive expecting the gearbox to 'know' what gear to select.

Portzy

Ermm.. the reason being? to save some fuel by coasting?


I'd just leave it D mate

 

[Benzowner]

Ermm.. the reason being? to save some fuel by coasting?


I'd just leave it D mate

You in fact waste fuel by coasting. In a manual gearbox, if you approach stop in gear, with most modern cars there is a fuel cut off valve when decelerating in gear. If you knock it into nuteral, the fuel will flow through the system to maintain tickover. Not quite so sure though on auto boxes

 

[portzy]

I suppose there is no particular reason for the coasting reference 'cept it was, and still is, a very bad habit I had from my 'manual' days . I still have a Polo for nipping to the shops and that . It was just to highlight the 'suddenly' finding myself in 'N' and worrying about getting back to 'D' without reaching for my wallet

Methinks, again, I'll leave well alone.

 

[Stratman]

And the ultimate silly question.............. Given that the 5-speed boxes have an electric microbrain telling them what to do, if a chap were to be cruising along the motorway at 70 and then, just to see what would happen, moved the selector to first.............

What would win? The sensible microbrain or the nut behind the wheel?

 

[Benzowner]

And the ultimate silly question.............. Given that the 5-speed boxes have an electric microbrain telling them what to do, if a chap were to be cruising along the motorway at 70 and then, just to see what would happen, moved the selector to first.............

What would win? The sensible microbrain or the nut behind the wheel?

Obvious, the sensible microbrain would win and the other microbrain would loose, and pay for it

 

[uumode]

folks at Top Gear and such like seem to dislike the microbrains

they turn off the ESP, turn off the traction control, turn off the ABS (if it's an option) and drive like a complete maniac burning huge amounts of rubber in the process, skiding, sliding, and doughnuting that generates more smoke than an SAS team storming a building or a bonfire can throw at it

They finally emerge from the cabin with a big on their face.

 

[andy_k]

folks at Top Gear and such like seem to dislike the microbrains

they turn off the ESP, turn off the traction control, turn off the ABS (if it's an option) and drive like a complete maniac burning huge amounts of rubber in the process, skiding, sliding, and doughnuting that generates more smoke than an SAS team storming a building or a bonfire can throw at it

They finally emerge from the cabin with a big on their face.

but they are not paying for tyres, drive train components and any other damage that may occur in the process. I'm willing to test anybody's car to destruction under similar circumstances - any takers?

Plus most of them are doing it in the middle of deserted airfields where ABS is less important as there is little chance of finding granny driving along the wrong side of the road in her Micra with the "wrong glasses" on

Andy

 

[guydewdney]

the brain wont allow you to select a gear thats too low for the speed..

i think it also upshifts if you hold it on the rev limiter for 5 seconds or so even if stick is in 1st

it wont roll back - theres a sprag clutch to stop it.

 

[portzy]

it wont roll back - theres a sprag clutch to stop it.

Guy, whats a 'Sprag clutch'?. How far back does it have to roll before it 'Sprags'

A very confused Portzy

 

[guydewdney]

its a one way clutch mechanism - the shaft can only turn in one direction, it should only roll a foot at max to take up all the slack.

see http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/downloadable/dft_avsafety_502256.jpg

have you never heard of google image search

 

[portzy]

its a one way clutch mechanism - the shaft can only turn in one direction, it should only roll a foot at max to take up all the slack.

see http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/downloadable/dft_avsafety_502256.jpg

have you never heard of google image search

Err, no, actually. As you know I'm a relationship expert really. In fact, since I read your eeniee, meenie, mynie, mo, thread I've gone on to write a column for the 'Daily Sport' , such is the height of esteem in which my human interaction experience is held. The next time I have a letter on, a'hem, 'problems with rolling off' , I will advise the purchase of a 'sprag clutch' to ease their difficulties Yeah, right!!!!

Portzy

 

[Clive Gregory]

re: coasting in an auto.
when you back off the accelerator the fuel will be cut off and the torque convertor keeps the engine running until you get down to about 5mph (or when the engine gets down to about idle speed), then the fuel is turned on again to prevent it stalling.

Clive