Noise going down a "mountain"...

That WAS the B mode. It will be evoked when in cruise as needed.
You should have just left it alone.......as it was doing what it needed to do for maintaining your set speed.

That downgrade is not really THAT long or steep but switching over to using your brakes in that kind of a situation is NOT the proper thing to do. If the engine compression "braking" is keeping you from going too fast........just leave it alone and let it do what it was intended to do.

P.S. If anyone wants a sample of what "old" mountain roads were like, there is an nice stretch on US 41 near Monteagle.

 

hard to know without hearing the sound. never heard anyone else describe mountain descent like that before though. too flat around here.

 

The first time that happened to me, I was on my way to Florida and it happened going down that same stretch, I just let it keep doing what it was doing. Took me by surprise too and someone on here explained what happened.

 

Sounds like the car was preemptively going into B mode, maybe having cruise on even made it more so?

When descending a mountain I would avoid using cruise, and put it in B right at the top. If you leave it up to the car's computers I believe it will start using B (behind the scenes) but maybe better to put in B sooner, on purpose.

And yeah it revs up the engine, it sounds a little alarming. For sure don't exceed the speed limits, stay in right lane if theres that option.

 

If you keep speed down the engine doesn't need to rev so high. Not a big difference but helps.

 

I'm not preaching, just recalling my last long downhill run: there's a long, steep and unrelenting stretch coming down Burnaby mountain, and if I put in B at the top it starts winding up, but as long as I stay to the right and around the limit with a bit of braking it's not too crazy, only 70 kmh speed limit IIRC. If I foot-off the brake the speed starts climbing, and the revs seem to go up twofold.

 

At least on the Gen 4, and clearly the Gen 3 does it too based on this description, it will automatically use the engine to help braking as needed.

B mode basically exists to satisfy legal requirements that there be a "low gear", and sometimes it is helpful in sustained descents, but it's not often necessary.

 

I'm not sure about it just being legal obligation. Given a long and steep enough descent, you could be in danger of seriously overheated brakes, sometime after the computers decide no more regenerative braking is possible. The Owner's Manuals are strangely terse on the subject though. Makes you wonder how much Toyota has thought it through, and how much of the current use of B mode on long descents is grassroots owners' discovery.

Maybe @Prius Team could shed some light?

 

No, extra engine braking (B mode) will occur automatically when in cruise.
And yes the engine DOES sound like a coffee grinder when that happens and that is perfectly normal.

Like I said, THAT specific mountain grade let you get away with using only the brakes but that is a BAD habit to get into. A steeper grade might have overheated the brakes with disastrous results.

P.S. Manually engaging B mode too soon at the top of a hill often results in you going too SLOW down the grade, thus creating a different kind of danger.

 

Funny. I have actually resorted to opening the rear side windows to add aerodynamic drag (yes, it actually does, but only helpful at supra-legal speeds ). While I've never actually had any problem descending mountain pass' it sure feels like you could. When descending low drag is NOT your friend.

BTW, rear side windows to limit hearing loss. If there are passengers in the rear then I wouldn't.

 

Oddly: I've never seen the displays indicate cessation of charging, even coming down Mount Seymour for example. That's with near-constant use of B, except on the occasionally flat zones where I'd otherwise need to use gas.

 

Hey! A fellow Nashvillian!! I drive Nashville to Atlanta every 3 or for weeks to see my boyfriend. I have had the same experience with my Prius. Actually my 2012 Prius v battery doesn't fill up (still has very good battery capacity) but my old 2005 would fill up pretty quickly going down.

The little 1.5 engine would hit redline without even putting it in B. The engine sounded like it was going to explode, it was especially bad the time I had 4 guys (counting me) and all or luggage. It was a little bit alarming at first. The harder I would plush the break the more it would rev up.

Some people say the Prius will switch to conventional breaks once the battery is full, but in my experience it is still utilizing the electric motor to slow down, at the expense of the ICE revving like crazy.

VERY familiar with this road, any questions, just let me know!

 

Yeah I don't like the traffic or the high speed of other drivers on that route!

I've lived in Middle TN my whole life. Lived in Greenhills near downtown for almost 2 years. I love it here. Only like it more and more! Welcome to TN! You live in a beautiful area.

Where did you move from (sorry for taking it off topic lol)