Coasting with an Auto

[atomicradish]

I've done D to N and N to D coasting in my automatic for over a year now with no adverse effects. I didn't rev match either, mainly because I misunderstood what it was. I had thought that autos rev matched on their own since it always finds the apropriate gear when I shift back to D. I assumed rev matching was for driving a manual and then simply finding the apropriate gear to put it after gaining speed.

Does anyone really know whether or not it is bad to coast off of a long sloping hill in N in an automatic? Some people say yes, others say no.

Also, what are the adverse effects of engine-off coasting in N for autos? From my limited knowledge on transmissions, how would it be any different than in a manual (which I gather is fine to do)? When in N aren't the engine and transmission disengaged in both an auto and a manual? If so, what makes it 'wrong' to do so in an auto but not a manual?

Thanks to anyone who can answer this.

.. unfortunately the conflicting opinions really don't help someone who is confused to begin with.

[Snax]

The primary issue on many automatic transmissions with engine-off coasting is the functioning of the lubrication pump and shift pressure system. Without the engine turning, the transmission pump is not. This can result in odd or catastrophic wear and engagement of gears in the transmission. (The specifics of which I don't fully understand, but having felt the effect of it once while coasting, it does not sound or feel good for the transmission.)

Some cars by contrast, typically compacts, don't operate this way and are perfectly happy engine-off coasting in neutral. You just have to figure out which camp your car is in.

The best acid test is to check the manufacturers recommendations on towing the vehicle. If they give the ok for flat towing without disconnecting the driveline, coasting in neutral is fine, engine off or on. If they say don't flat tow, then don't coast with the engine off. Engine on coasting is likely no problem for any vehicle, but that's just an educated guess.

[atomicradish]

Originally Posted by Snax

The primary issue on many automatic transmissions with engine-off coasting is the functioning of the lubrication pump and shift pressure system. Without the engine turning, the transmission pump is not. This can result in odd or catastrophic wear and engagement of gears in the transmission. (The specifics of which I don't fully understand, but having felt the effect of it once while coasting, it does not sound or feel good for the transmission.)

Some cars by contrast, typically compacts, don't operate this way and are perfectly happy engine-off coasting in neutral. You just have to figure out which camp your car is in.

The best acid test is to check the manufacturers recommendations on towing the vehicle. If they give the ok for flat towing without disconnecting the driveline, coasting in neutral is fine, engine off or on. If they say don't flat tow, then don't coast with the engine off. Engine on coasting is likely no problem for any vehicle, but that's just an educated guess.

Thanks for the advice Snax. Appreciated.

The Metro is definetly a compact. I'll check for that right now.

[bbgobie]

Got my scanguage a couple days ago. The scangauge reports fuel consumption is definately lower while coasting in N. I would expect the scanguage to report 0 fuel consumption if the engine on my car was being turned over by the tranny. More testing of coasting from 100-0 will be needed, but I'll need to find space to do this.

[bbgobie]

Originally Posted by brucepick

As written elsewhere in this thread, I also think it varies with different cars. I put a tach into my car so I could see better what was going on with engine rpms.

My car has auto trans without lockup converter, so ymmv, as always.

Leavng it in D on a nice long downhill coast will see the rpms drop to 1500 and stay there.

Putting it in N for a similar nice long coast will see the rpms drop to about 800. Idling in N when not moving, rpms = 700-800.

My take on it is, coasting in D has the car's momentum spinning the engine via the tranny. It's not fully "engaged" as when your foot is on the gas, but those rpms have to be coming from somewhere. My foot is off the gas so I think it must be the car that's driving the engine. Those coasts seem to lose speed faster than the ones where tranny is in N. That always gives me the best coast. Oh yeah - electric rad fan, so the airflow is NOT driving the engine!

I drive Rte 9 in CT daily, it has loads of long up and down grades where I pulse to about 70-75 going down, and then glide the max possible till I need to help it get up the next hill.

Your describing engine braking. Yes your car is driving the engine, but that doesnt mean your using fuel.

I'm no expert, but I've never heard that coasting in D your engine will cut off fuel. Your engine is forced into an rpm to match that of the tranny or else it wouldnt be a gd thing for either, to keep that rpm fuel must be injected. If coasting in D meant no fuel going to the engine hybrid cars wouldn't have to shut off the engine when coasting.

Another thing to consider, if you think your car shuts off fuel while coasting in D, try this and this will be easy to measure. Drive up to a decent speed, downshift to like 2nd and coast. You should be able to quickly figure out if you are using fuel. It'd be the equivalent to a few hard accelerations.

[ma4t]

You know, I tried coasting in neutral a few times. I noticed that the RPMs stayed the same. There was no shock when I shifted back into drive.

From all that I'm reading about damaging the transmission, I am not going to keep doing it.

M

[Raccoonjoe]

Originally Posted by Snax

The best acid test is to check the manufacturers recommendations on towing the vehicle. If they give the ok for flat towing without disconnecting the driveline, coasting in neutral is fine, engine off or on. If they say don't flat tow, then don't coast with the engine off. Engine on coasting is likely no problem for any vehicle, but that's just an educated guess.

If you're worried about damaging your transmission, this is the advice I would follow. However, if you are like most folks out there, you don't *want* to kill the tranny, but are aware of the risks and such associated.

In the Jeep, I coast in N with the engine on. Not only because of the fact that she's hard to start sometimes (and the wear on the starter), but because of the lack of lubrication in the transmission with the engine off. I've torn apart a few trannies in my time, and can tell you I would *not* recommend it, unless the manufacturer says OK to straight flat tow. If they require driveline dis-assembly, then leave the motor on. (personally?? I would never flat-tow anything w/o disconnecting the driveshaft.....possible exception being a Jeep (or other vehicle with manual t-case), where you can effectively disconnect the driveline by shifting the t-case (and tranny) into Neutral)

I have never rev-matched my engine when shifting from N to D. The tranny is smart enough to find the correct gear...and if you keep your foot out of the throttle, the car will shift into "freewheel mode", similar to coasting in D. Autos are notoriously difficult to drive in the mountains, as they do *not* have any real capacity for engine braking. You remove throttle input = transmission slips and free-wheels. You take it out of gear = motor idles like it was sitting in your driveway in N.

This is what I would do if you have questions. Find a friend with a manual (or drive one yourself). Take it out on the freeway, wind it up to ~70. Push in the clutch and coast along with the clutch in for a few seconds.....then dump the clutch out. You should feel the car throw you forward, as the motor/tranny mate, and try to match RPM. (also a good reason for hard shifts, 'specially with inexperienced drivers). Your auto tranny does a lot of things for you.....auto shifting + auto clutching. If your auto tranny slams into gear (acting like the manual in the above example) when shifting from N -> D at highway speeds (and *absolutely no* throttle input), your tranny is most likely on it's way out.

If all else fails.....stick a few of the HDD magnets (those extremely powerful "rare earth magnets" to the bottom of the tranny pan. Drive for a while, then change your tranny fluid. If your causing damage, you will see the metal shavings/crap in the fluid or stuck to the magnets.....


*edit*

Sorry, this post got waaayy longer than I thought. Stepping off my soapbox now....

[ffvben]

the automatic trans fluid is the link from engine torque to wheel motion. this process heats up the fluid from all that friction. this fluid must be cooled down, so shutting your eng. off in a auto while coasting, would stop the pump from pushing fluid to the radiator/cooler. this might fry your fluid which in-turns cooks your clutches/plates inside trans. it could warp/crack clutches/plates/bearings/servos if the temp is to high. now coasting in neutral with engine running seems fine, the fluid is being pumped through the cooler.
as SNAX states , check your owners manual for towing your veh. on 4 wheels some you can tow others you have to disconnect drive line.( mainly awd or 4x4)
just a note : I have seen a heavy duty manual trans (f350-f450 trucks) with cooler lines also. they might need them for the heavy work load to keep gears cooler.

[omgwtfbyobbq]

Originally Posted by Raccoonjoe

If all else fails.....stick a few of the HDD magnets (those extremely powerful "rare earth magnets" to the bottom of the tranny pan. Drive for a while, then change your tranny fluid. If your causing damage, you will see the metal shavings/crap in the fluid or stuck to the magnets.....


*edit*

Sorry, this post got waaayy longer than I thought. Stepping off my soapbox now....

Good idea!

[ma4t]

We have a couple of hills that allow that. If the light at the bottom is red, I'll EOC if I'm sure I'm going to be sitting for a while.

I don't plan on keeping the vehicle for more than another year, so I won't know for sure if it causes any long-term issues.

m