coasting in neutral

Coasting with an automatic will not usually offer any gain over just letting it do it's job. This will cause more wear on the selector system any may very well produce additional wear on the forward clutch pack because of the additional application cycles - many late OD transmisions usually keep the forward pack engaged at all times when a forward position is selected. There may also be some reduced lubrication to some parts of the trans because of the engine operating at idle. Also, it is not legal to coast in nuetral in many states because of the lack of engine braking control and additional load that may be placed on the brakes. The best thing to do with an automatic transmission is to leave it alone to do it's job. Engineers have been paid many millions (billions?) of dollars to figure out how to make it work reliably over many tens of thousands of miles. Why second guess them? You may also be interested to know that an automatic transmission has internal sacrificial materials designed to wear out thus eventually requiring replacement. This is the same concept as a clutch in a manual shift vehicle. Either may very well outlast the vehicle or the owner with a little care and skill in driving.

Automatic transmissions have a pump to provide hydraulic pressure to operate the control servos and to circulate the oil through the torque convertor and transmission oil cooler. The pumps in all modern (or almost all?) automatic transmission operate off the input shat only. As long as the engine is running, the pump is still running and oil is being circulated, so there is not likely to be a problem. However, if you coast with the engine off, the pump is not running and oil is not being circulated. This can cause problems because you might not get oil to the places that need it. I suppose it is also possible that if you coast for a very long distance at a relatively high speed with the pump running only at idle, it might not provide enough lubrication to protect the output side of the transmission. Personally I do not see any advantage of shifting into neutral when slowing down. Unless you are able to coast so far that you are going to roll to a stop without the brakes, all you are doing is adding wear to the brake pads for no benefit.

Ed

JMH wrote:

Somneone else has already done a good job of explaining the situation in automatic transmissions, so I'll concentrate on manuals for you.

With a manual, it's pretty simple, really - The lubricating mechanism, whatever it might be, is almost always operated by the countershaft of the transmission, which is always (in my experience) mechanically connected to the output shaft, even when the tranny is in neutral, and therefore, always spinning anytime the drive wheels are turning. Unlike the automatic, where the pump/splasher/whatever is driven by the input shaft, and goes "dead" when there's no input from the engine, the manual transmission's luberication system is active whenever EITHER the input shaft OR the output shaft (or both) is turning. What it boils down to is that with a manual, as long as either or both ends are spinning, the lubrication system is working as designed, and there's no risk of damage. (other than that damage caused by the mere fact that the transmission is operating at all, instead of sitting idle)

I don't think any damage should occur in either case. However, it's rather pointless, as well as potentially dangerous (due to no engine braking) and therefore illegal in many areas.

I appreciate all the excellent replies from everyone. Thanks for all the info, it has been really great.

The reason why I wonder about this (and why it is a benefit to me) is that I drive a 1983 MB 300D-turbodiesel. The car has more or less the aerodynamics of a brick, but it gets me 30MPG in comfort, so all is well, IMO, especially when I use biodiesel and vegoil in it. Anyway, down this one side of a bridge I go over, there is a nice long downgrade, then a short straightaway with slight hill, and then a tollbooth. If I let off of the throttle at the apex of the bridge (at 55mph), and go down with the car in gear, Ill decelerate to 35 MPh by halfway down the bridge, and be at idle speed by the tollbooth. It has that kind of drivetrain losses.

If I shift into neutral at the bridge apex (at 55mph), I coast down staying near the same speed all the way down the bridge. I then slowly approach 35 mph when the speed limit drops from 50 to 35, approaching the toll booths, and am at about 20 at the toll booth. At that point, nearly fully stopped, I shift into drive and go off again.

I am not dragging pads on any of my 4 disc brakes, I use synthetic diff oil and MB ATF. There is just a lot of drivetrain loss, and so for me it is seemingly economical to coast down the downhill, rahter than adding a slight throttle to keep the speed constant going DOWN a rather steep incline.

Thus my question.

I am glad that I most likely didnt harm much by doing it (yet), but I have started leaving it in gear on the way down... I dont want to wear clutch packs or cause insufficient lubrication by the time 1/2-3/4 mile rolls around at the tolbooth.

So any other comments would be great, and otherwise, thanks very much for the info!

JMH

Yeah, you must be saving $0.09 worth of fuel and risking damage to a transmission that will cost $3000 to rebuilt. Why not see if you can drive around with only 3 tires instead of 4 and save some money there too. If you only make left hand (or right hand) turns it might work out for you.

That is a real good place to get busted for doing it too. The cross winds on a bridge will make it sway worse than normal when coasting. Cops can spot that in a second.

I got popped for coasting by a cop sitting at the bottom of a hill. I asked the cop just how he noticed and he laughed and said it's really easy to see the body sway of a vehicle with no engine load on it.

It is big time illegal to coast down a hill in neutral in most places. The cop mentioned charges like dangerous driving and careless driving on top of the ticket for coasting.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

JMH wrote:

Its just common sense .I traverse a certain 12 mile grade about 4 times a year .The gas savngs from coastng is apparent. On the other hand I know of someone that was killed by shutting off the engine down a 1 mile grade and had the steering lock. He was tring to make the gas station at the bottom of the hill.

It should be a law, as most are not likely to understand the need to keep power brake vacuum or the importance of keeping the brakes cool .

"TOLYN9" wrote

I.e. VW Lupo 3L automatically shifts to neutral when gliding without load. Applying the brake (or pushing button) will engage the correct gear again.

Important, also for steering.

Not in this situation. If you have to brake you can still turn the engine through the gear, the electronics shuts off fuel injection. Saves even more gas, because you get "electricity for free".

Thomas

As long as the ENGINE is idling, coasting in neutral is harmless regardless of speed (note that it is illegal according to traffic laws in some jurisdictions, though!)

Most automatic transmissions have engine-driven fluid pumps, so that if the engine is NOT running, they do not have fluid circulating and therefore the clutch packs are not being bathed in cool fluid. If you tow the vehicle at high speed or long distances in neutral, there's enough residual friction in the clutch packs that they can overheat and fail.

A very FEW automatic transmissions- though none that I can think of that are currently being built- have both an engine-driven fluid pump and a driveshaft-driven pump so that towing at any speed in neutral is OK, and its even possible to "push start" these cars because the transmission will go into gear while being pushed (1965 and earlier Chrysler Torqueflite transmissions, for example).

JMH wrote:

"Steve" wrote

I had a car like that. An old 63 Rambler. You could restart the car as long as you were doing at least 20 miles an hour.

Ian

Given that AMC (Rambler) bought transmissions from Chrysler, Borg-Warner, and GM at various times, it may well have had a Chrysler TF in it. But the use of dual pumps was once very common- I think the first generations of Hydramatics had dual pumps too.

Ah, the good ol' VW bus. No power steering, no power brakes, no steering wheel lock. Turn off the key and everything still works. Even the acceleration was about the same. :-)

You owe me a new keyboard!!!! :) :) :)

The 2-speed GM Powerglide had a rear pump. Get it rolling and put it in Drive to bump-start.

Rob

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There are several small and mid-size GM cars available right now(Alero, Malibu, Cavalier, etc.), with automatics that can be "towed-four-down", and they are very popular with the RV crowd.

Rob

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How is this accomplished? A friend claims that certain Saturns have this characteristic. Is the gearset different than a traditional planetary set? Or do they have some sort of output shaft driven oiling system? Or is splash lubrication enough for these transmissions?

Ed