what happens to our automatic transmission when you coast down hill around 50-75mph in neutral to save fuel.I know it is not good,but why?I am trying to explain it to my friend because he does it all the time.

 

Are you SURE that you KNOW it's not good for the transmission?

 

from what i heard

 

I can't believe it is good for the internals of the tranny...if you are in neutral, what is turning and how fast? Now you start down a grade and let's say you throw it into neutral and at the bottom of the grade you are doing 70. Now you slip the tranny back into drive. How fast does everything begin turning now? From idle (700 rpm) to 3000 rpm instantaneously, transmitted to the input shaft...I'll pass. Now I know I am not expert in trannies, but this seems double-dumb.

EDIT: I just read that the input shaft does not turn in neutral BUT the output shaft is free to turn...meaning I guess that it would be driven by road speed.

Here's Click & Clack's take on it

 

It's totally fine if you do it correctly...When your ready to put the tranny in to drive raise the RPM's to about 2,000 RPM's(depending on how fast your going) and put it in drive. I do it all the time with my Truck. 55,000 miles and hauling.

 

if I'm not mistaken when coasting the front pump is not turning and the tranny internals are. the ??????? what is keeping all of that fluid that is not being pumped through the cooler cool. I coasted all the way down the smokie mtn parkway into Cherokee and at the bottom at the first stop light the tranny was puking and would not go into gear. my opinion coasting in nuetral is a bad Idea for any amount of time. just my .02 worth

 

BAD idea. A friend of mine (i hate i when people say that) told me on manual transmissions that its better to leave it in gear until you make the complete stop. He had too many drivers burn up the trannys in his rigs. Imagine the difference in an automatic, which I figured doing so would be ok in a standard. pressure has a lot to do with the cooling and I would not risk it. I guess thats the same reason you unhook the drive line when towing a car over 30mph. No circulation= no cooling.

 

I'm no professional, but I don't think that that saves any fuel anyway. When you are coasting in neutral, the engine uses enough fuel to idle. When you are coasting in gear, don't the injectors shut off all together?

Joe

 

Haha im glad someone else said that, Im pretty certain your correct. Ive watched my egts on a looooooooooooong downgrade in drive, and it goes to the 100-150 range, on a long enough downgrade, but when upto temp and at idle its around 200-250. PLUS you can notice as soon as you touch the pedal the injectors make noise vs before then they are pretty silent. This leads me to believe if you put it in neutral and idle it would infact use more fuel. Not that they use much fuel at idle anyways, what is it 1 gallon to 3 hours as 700rpms? IIRC

 

I"am trying to explain to my friend that if you leave it neutral at 70mph the transmission pump is only pumping enough fluid for the trans what it would require at idle and that means it wouldent be lubricated enough.does that sound right???

 

sounds good too me..and your not saving fuel any ways..the fuel you save you loose getting the motor back up too speed...

 

I always noticed the noise too, especially on slow coasts, the noise of the injectors going off and on is very obvious, especially on my early 99! My first truck was an '86 Bronco, with the 302, and I know for a fact that it shut off the injectors. I think I remember seeing somewhere that any multiport system does that.

Joe