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?
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
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
Your friend should pay for a transmission rebuild.
Mike Halloran
Pembroke Pines, FL, USA
RE: Coasting in N with engine off
RE: Coasting in N with engine off
RE: Coasting in N with engine off
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 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
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 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 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
RE: Coasting in N with engine off
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 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
RE: Coasting in N with engine off
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
Mike Halloran
Pembroke Pines, FL, USA
RE: Coasting in N with engine off
RE: Coasting in N with engine off
RE: Coasting in N with engine off
RE: Coasting in N with engine off
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
RE: Coasting in N with engine off
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
Mike Halloran
Pembroke Pines, FL, USA
RE: Coasting in N with engine off
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
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
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
Mike Halloran
Pembroke Pines, FL, USA
RE: Coasting in N with engine off
RE: Coasting in N with engine off
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
RE: Coasting in N with engine off
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
I honestly forgot about the t.o. bearing and the engine.
Mike Halloran
Pembroke Pines, FL, USA
RE: Coasting in N with engine off
With the exception of pulling the 4x4 out of the mud with the tractor and getting them to hard ground.
SBI
Central Ne.,USA