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Coasting in N with engine off

Coasting in N with engine off

Coasting in N with engine off

(OP)
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?

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

Quote:

"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.

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

Coupla miles, no problem.  

Coupla hundred miles, big problem.

Ropes and towbars are mostly illegal here now anyway.


Mike Halloran
Pembroke Pines, FL, USA

RE: Coasting in N with engine off

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).

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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

Mike Halloran
Pembroke Pines, FL, USA

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

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.

RE: Coasting in N with engine off

The shafts uses ball-bearings and the gears plain bearings if i'm not misstaken.

RE: Coasting in N with engine off

The only problem is that, for most manual transmissions, the output shaft is the shaft with the freewheeling gears and shift sliders.  The countershaft is solely driven by the engine, and is also what slings the lubricant around.  With the engine off, the output shaft doesn't get any lubricant.  (As mentioned earlier)

Front wheel drives can be much different.  Saturn transmissions, manual and automatic, are designed such that they can be flat-towed indefinitely.  Quite handy for the RV crowd!

RE: Coasting in N with engine off

The very first generation of automatics had two pumps, one driven by the tailshaft, so they could be used to bump- start the engine, and coincidentally would probably survive towing.  The second pump went out of fashion around 1958.

Mike Halloran
Pembroke Pines, FL, USA

RE: Coasting in N with engine off

Mike:
You wrote
"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.

You're right,  that's not obvious,  but a very good thing to know!

RE: Coasting in N with engine off


Quote:

"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.

Again, this will work for the SHORT HAUL...for any longer than an emergency tow this could lead to all sorts of expensive and dangerous problems with the clutch linkage, pressure plate and/or, release bearing, especially in some of the older autos that use a solid realease bearing (if any of these awful little things are still being used).

Rod

RE: Coasting in N with engine off

Great catch Rod.

Cheers

Greg Locock

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

RE: Coasting in N with engine off

I'm used to old cars that are still new enough to have ball throwout bearings.  In which case a 200 mile tow went without a problem.

Mike Halloran
Pembroke Pines, FL, USA

RE: Coasting in N with engine off

Not sure what you guys are talking about.  If you are towing with the clutch pushed in, are you recommending running the engine at the same time?????    Otherwise, the throw-out-bearing and pressure plate do not turn.  Why would you even want to keep the engine running???  

RE: Coasting in N with engine off

I recommended towing in high gear with the clutch pushed in and the engine off.  This has the disadvantage of rotating the throwout bearing and the pilot bearing for longer periods of time than they are ordinarily used.  It has the advantage of rotating the countershaft, so the transmission bearings survive the tow.

Yes, ordinarily you would drop the driveshaft so none of this is an issue.  Somtimes that's not practical.  E.g. on a Corvair, you can remove the driveshaft with your fingers ... but only after removing the engine.

Mike Halloran
Pembroke Pines, FL, USA

RE: Coasting in N with engine off

The throw out bearing will not turn unless the engine is turning over.  Flywheel turns the pressure plate.  The pressure plate turns the throw-out-bearing.  You are correct in that the transmission input shaft will roll in the pilot bearing if towing with clutch pedal pushed in and trans in gear.

RE: Coasting in N with engine off

If the Corvair box is like an air cooled VW box, they have half the main shaft and half the cluster gear on each shaft, so the bottom shaft turns even in neutral.

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.

RE: Coasting in N with engine off

A Corvair box is just like a regular Chevy gearbox, except for a hole running the length of the output shaft.  The engine sits behind the axle, and drives the input gear at the forward end of the gearbox by means of an amazingly tiny spline and an amazingly skinny and long torsion bar- like shaft extending from the pilot bushing forward through the axle pinion and the tranmission output shaft.

I honestly forgot about the t.o. bearing and the engine.


Mike Halloran
Pembroke Pines, FL, USA

RE: Coasting in N with engine off

As a farm kid from Ne. I aways thought we were a bit strange up here,  towing a 4x4 you kick the transfer case in neutral and go, anything else GO'S on the trailer!
With the exception of pulling the 4x4 out of the mud with the tractor and getting them to hard ground.

SBI
Central Ne.,USA

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