Coasting with an Auto

[Telco]

I wouldn't worry about it, as was posted earlier this is a little manufacturer's butt covering. This is put in so when Little Johnny floors it in neutral, then shifts into drive, and drops the trans, they can deny the warranty claim as abuse. Just shift into drive, then apply the gas. Don't apply gas then shift in.

Now you don't want to coast with an auto with the engine off, almost all conventional transmissions have their fluid pumps driven by the input shaft. If the fluid isn't being pumped then bearings go dry and the transmission will burn itself up. If towed, the trans won't survive a 20 mile ride, so it might take 40-50 miles cumulative of engine off coasting to cause damage. If the trans pump is driven off the output shaft then there is no problem with engine off coasting because the rear end will be driving the pump. Sometime in the 1950s (I don't know exactly when) the makers started going to an input shaft pump so anything newer than that, with a couple of exceptions, are not going to be good for engine off coasting.

[Snax]

Popimp, and anybody else who is wondering:

An automatic transmission is essentially no different when it comes to the physical connection of gears to other gears and reductions and such in the gearbox. Aside from the obvious automatic nature of it, the only difference is that the gears always remain in constant mesh with either the output or input shaft depending on design, and each gear has it's own clutch and is selected by engaging the hydraulic pump that tightens it's clutch band or plate. No two of the gear clutches are engaged at any one time, and lockup bands or converters can only engage when one of the gears is actually engaged.

So essentially an automatic transmission, just a like a manual, shifts in and out of neutral numerous times between each gear selection as you drive. Selecting neutral on the shifter just tells the computer not to select a gear. Placing it back in drive, due to the pumping and valving action will in virtually every case, prompt the system to reselect the appropriate gear to re-engage.

The bottom line is that unless your transmission has an obvious issue with reselecting the proper gear, engine on coasting in neutral is no different to it in terms of wear except as it relates to the number of shifts required to regain your target speed. In fact, leaving it in gear while coasting down to a stop can actually cause MORE wear as the system selects each successively lower gear.

[bfg9000d]

well coasting in N to a stop will without a doubt not hurt the trans. Still I would believe the more shifting that occurs the more wear on the trans. If you feel a jolt when shifting to N to D I wonder how much stress that puts on the trans?

[popimp]

The manual says not to tow on the ground.

[Snax]

That just means the hydraulic pump is input shaft driven and that towing with the engine off could do serious damage to the internals.

If on the other had you are feeling significant jolts when shifting from N to D, then it's probably not a good thing to be doing. The stress on the gearbox, if the control system is operating correctly, should produce less shock on the driveline than the average manual shift however, as the torque converter should absorb much of the difference in rpm.

[popimp]

The Jolts are only felt at lower speeds around 30mph. At highway speeds, there is no jolt, just a smooth shift.

[Hockey4mnhs]

so dont rev match becase its alot smoother when i rev match at to 2k at 55mph?

[popimp]

Hockey4mnhs:

Rev match? At 55mph my shifting is smooth. It's a lower speeds where I feel a small jerk.

[2TonJellyBean]

Originally Posted by popimp

Hockey4mnhs:
Rev match? At 55mph my shifting is smooth. It's a lower speeds where I feel a small jerk.

Hrmmm... perhaps the actual engagement force is the same regardless of speed but the momentum at higher speeds makes it hard for you to notice?

[omgwtfbyobbq]

Originally Posted by popimp

Hockey4mnhs:

Rev match? At 55mph my shifting is smooth. It's a lower speeds where I feel a small jerk.

I think that's a feature my auto has too. It'll shift into a higher gear then a lower gear at speeds below say, 45-50mph, in order to minimize shock and wear compared to just dropping it into 1st-3rd. It goes away at highway speeds since it just drops it into 4th and that's that.