Coasting - Auto
 

Should be a nice easy one this.
Whichever way I drive from where I live, there's a big long hill. The sort of hills that if you're doing 60 mph at the top, knock it into neutral, you're still doing 60 mph a mile later :)

But...... is coasting at speed in neutral detrimental to the box?

I don't believe it is detrimental to the gearbox to coast at speed in neutral.

It could be very detrimental to the gearbox if you misjudge the "D" to "N" shift and get "R".

FYI on some modern autoboxes the electronics will prevent you from doing this but on the AW55 and TF80 boxes fitted to all diesel Mk 1 XC90 there is a direct cable connection from the selector (as well as electrics and switches) which physically operates the gearbox internals so there is no safety lock-out....

FYI #2 if you have a post 2005 XC90 you will have the later 6 speed box which (depending on software) does have engine braking features. However, with the engine on overrun (i.e. being driven above tick over speed but no throttle commanded) the fuel supply to each cylinder is ceased. But if you are in neutral, the engine will need to use fuel to tick over. That said though you would think that the fuel used to maintain 750rpm would be less than say 2,500rpm which is what it might be spinning at on over run.

I hope that made sense and you didn't think I was saying the obvious?

Aha.. Mexican overdrive. Auto box not really recommended, manual gear box you lose all engine braking. To be honest throwing the gear box into neutral is not a good driving practice. Only my opinion, but with 35 years in heavy haulage I found the gearbox to be a very good braking system on hill descents.:thumbs_up:

The recent aw 8 speed autos do this in ECO mode , you need to think well ahead though to avoid wearing brakes out , however get it right and a 5% fuel consumption reduction .

The earlier 2000 to 2010 ish autos do coasting a lot with the software update from around 2005 ..

I have found, especially when towing, that the car coasts downhill when in D. If the hill is steep, the car will gain speed without the revs increasing. If you want engine braking then you must go into M and select an appropiate gear.

^^That.

For the pennies a year which you might save by doing this (don't forget to net off the cost of increased brake wear) , I can't see that it brings you anything other than increased risk.

For me in normal driving when on the move you should not be touching the gear selector unless it is to move it sideways into manual mode. The risk of mis-shifting into reverse or Park and causing catastrophic damage (and potentially and accident) is not worth the potential minuscule benefit.

Thanks Tannaton, it's a 2006 6 speed that's fitted.
There is a noticeable difference between neutral and overrun in drive along the same stretch of road.
If I leave it in drive, the engine braking means I'm back on the throttle within quarter of a mile.
If I knock it into neutral the lack of engine braking means I'm back on the throttle about a mile from where I started.
Economy wise, that mile at tick-over is much better than a quarter mile at no fuel (assuming mine has the software update), so much so it's noticeable in the Mpg figures. I work my mpg out based on actual miles and fuel used, rather than the trip computer.

Thanks Skippyfox, it's the engine braking I'm trying to avoid. Big question is, is it detrimental to the actual gearbox?

Same background here, from heavy haulage to heavy recovery/wrecker work :)

I remember about a post on a forum, a Volvo owner who mentioned he used to wait at a traffic light with the transmission in N. He mentioned the transmission failed some time later on. Was it due to having it often times in N while the car was idling ? Who knows, but just to be safe I leave in always in D. Maybe in N with the engine idling there is not enough pressure to lubricate all bearings - but this is just a huge assumption of mine. Worth noticing that the transmission is not designed to stay in N with the engine running in any specific situation. The N is there only for some safety reason, so the R and D gears are not close to each other. Also remember the 'neutral stop shifting' problem with the 01-02 transmissions. I prefer to keep it safe when there is no clear answer.

As for 'engine breaking' with these automatic transmissions, in hilly areas I would just shift it in 4, 3 or even 2 if the car doesn't go too fast (but the 2nd gear seems a little 'rough' on these transmissions)