Coasting in N with engine off 1

[Tomfh]

Hi,

My friend recently borrowed a 4 year old Automatic Mitsubishi Magna. He was driving on the highway and was running out of fuel. He was stupidly turning the engine off and coasting down hills in neutral to save fuel. He accidently put the transmission back into Drive at high speed with the ignition still off. After this the transmission wouldn't shift out of 3rd gear when in Drive. You could manually select 1,2,3. After several hours the transmission started changing as normal. The dashboard 'D' indicator still flashes to indicate an error.

How bad is it to do this to an automatic? Can you do serious damage?

 

[patprimmer]

This is bad for transmission life and road safety.

With the engine off, the power boost to the steering and brakes is also disabled.

In extreme circumstances, like if the key is removed, or in some cars, if the key is turned one notch to far, the steering might lock in a fixed position.

If the engine is not running, the oil pump in the gearbox is not running, and so the gearbox is not lubricated.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

Engine- off coasting would trash the bearings in a manual box. In any automatic made since ~1960, it would trash pretty much everything.

Your friend should pay for a transmission rebuild.




Mike Halloran
Pembroke Pines, FL, USA

 

[swall]

Why would coasting trash the bearings in a manual box? BTW, one of my friends, goofing off, did the key removal thing while driving and yes, the steering wheel locked. He quickly got the key back in, fortunately.

 

[crysta1c1ear]

You normally learn that business about the key when young, doing the Starsky and Hutch stops: handbrake gently on, key out, and get out of the car while it is still rolling.

 

[NickE]

"You normally learn that business about the key when young, doing the Starsky and Hutch stops: handbrake gently on, key out, and get out of the car while it is still rolling"

Yeah but kids today drive automatics, and dont know anything about starsky and hutch except that it was a bad movie with snoop dog being the best actor in the whole thing.

 

[DAVIDSTECKER]

The real fun was in a rental truck with a manual trans.
The key did not lock the steering and you could shut it off while still in gear for a couple of seconds, then turn the key back on to get a huge bang. This was especially fun during rush hour under a bridge. Mufflers didn't care for it much.

 

[MikeHalloran]

In nearly all manual boxes, the countershaft is on the bottom. It's the one driven by the engine when the clutch is engaged, and the one with the gears spinning in the oil sump.

When you turn off the engine with the box in neutral, only the output shaft spins. The gears do not spin with it, so the bearings don't get splashed with oil.

Maybe not so obvious related hint: If you have to tow a car, and can't drop the driveshaft, leave the box in high gear and block the clutch pedal down.




Mike Halloran
Pembroke Pines, FL, USA

 

[xnicke]

I thought the gears (on both shafts, are they not always in contact!?) always spun whenever any of the shaft were turned.
I admit i haven't seen many boxes, and english is not my nativ language, so maby i am mistaken.

 

[patprimmer]

The power flow in most manual transmissions is from flywheel to clutch plate, then to first motion shaft which is the input to the gearbox.

The first motion shaft is a short shaft at the front of the gearbox. It connects to the clutch and just inside the gearbox, it has a gear solidly attached to it. This gear turns the lay shaft.

The lay-shaft is a shaft normally in the bottom half of a gear box. It has a number of gears solidly fixed to it. When the clutch is engaged, and the engine is running the lay-shaft turns as it is driven by the first motion shaft gear.

Up till this point, all gears are firmly attached to their shafts, and cannot turn on the shaft.

One lay-shaft gear is driven by the first motion shaft gear, but the rest of them drive the mainshaft gears.

All mainshaft gears are free to spin on the mainshaft, so the lay-shaft gears are constantly in mesh and spinning all, or all but one of the mainshaft gears at speeds different to the mainshaft.

The gear selectors are hubs that are on splines on the mainshaft. All gear selectors turn at mainshaft speed due to being attached via a spline.

To select a gear, one gear selector is pushed along the spline until it hits a mainshaft gear. The selector and the gear have notches that can lock together. They may also have a clutch between them to synchronise the speeds.
Once the notches engage, that mainshaft gear is locked to the the selector and they turn as a unit at mainshaft speed.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Pat and Mike, thanks for the explanations. I did the coasting thing one time on a BMW R80 motorcycle. These have a driveline layout and transmission similar to a car and it could be expected that the same lack of oiling to the bearings would result. I'll check my service manual to see what the oil level would be relative to the mainshaft gears, although it is a moot point, as I no longer have the bike. The coasting was downhill, intermittent for about 30 miles going into Valdeze Alaska. Was low and fuel and wouldn't have made it otherwise.

 

[GregLocock]

Hmm, I'm really struggling with this as applied to manual boxes (incidentally I agree on autos, the driver should pay for a complete overhaul of the box).

When most people tow manuals on a rope they knock them into neutral. I've never heard of this damaging a box. I can't remmeber seeing a warning in the manual about doing this.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Coupla miles, no problem.

Coupla hundred miles, big problem.

Ropes and towbars are mostly illegal here now anyway.




Mike Halloran
Pembroke Pines, FL, USA

 

[ivymike]

towbars seem to be legal in the west and midwest. I've towed from CA to CO and from IL to TX with towbars, been pulled over speeding both times, and never been bothered about the tow setup. Disconnected the driveshaft both times (auto trans).

 

[patprimmer]

Is the OP from Florida USA ?

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

If he was coasting down hills, he probably wasn't doing it in Florida.



Mike Halloran
Pembroke Pines, FL, USA

 

[carnage1]

The worst manual boxes I have ever removed (never tried to repair) have come out of small light vehicles pulled behind motorhomes. The people in the motorhome have no idea the horrible sounds the little car they are pulling is making. I have seen boxes that appeared to be stuckin fourth in nutral and fourth and wouldn't move otherwise. The entire shaft got hot enough to melt the brass synchro rings and braze the topshaft solid. This being said ALL of these boxes were completely out of oil. The sight plug for filling manual trans fluid is usually about 1/2 way up so at propper fill levels the output shaft turning may fling some oil arround, and while in neutral the trans only has to disipate the heat generated by the friction of the ouput shaft bearings so shouldn't need to shead much energy.

 

[Tomfh]

I'm Australian. My friend was driving through the NSW South Coast

 

[icrman]

The major difference between and automatic transmission and a manual. Is the type of bearings used. Most manuals use frictionless bearings, ie ball bearings generally. And they will live much better with minimal lubrication. In the automatic, it has mostly plain type bearings or bushings. And need a more generous supply of lubrication to stay happy. For extended towing I think there are transmission lube pump kits, and or disconnects.

 

[GregLocock]

Not really. The major difference is that autos are designed to have oil pumps, manuals aren't. Consequently when the oil pump stops working on an auto all the bearings that were expecting ot get a pressure feed of oil, don't. In a manual the bearings are designed to receive splash lubrication, which will still work to some extent even if the input shaft is not driven.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.