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?

 

[xnicke]

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

 

[izzmus]

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!

 

[MikeHalloran]

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

 

[Rob45]

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!

 

[evelrod]

"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

 

[GregLocock]

Great catch Rod.

Cheers

Greg Locock

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

 

[MikeHalloran]

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

 

[Zapster]

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???

 

[MikeHalloran]

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

 

[Zapster]

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.

 

[patprimmer]

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.

 

[MikeHalloran]

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

 

[sbi]

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