Transmission Glitchy After Descending Big Hill

How long of a steep hill? Post a google map route, end-to-end?

for the steepness: have you tried shifting to B, and is braking still needed?

 

I drive though the Sierras all the time and haven't had that issue.
Has the ATF ever been changed or checked?
Are your sure it isn't the ICE kicking-on because of low traction battery? This car will rev a bit higher while it's trying to provide power & recharge the traction battery at the same time. It's does act a bit sluggish while doing so in Eco mode. Switching to normal or sport mode will usually fix this.

FWIW: Placing the car into neutral on the highway isn't a good idea - If you need to jump on it, to avoid something - you'll simply engage and rev your engine - if you forget to put it back into gear; before jumping on it..........

 

A low capacity hybrid battery will fill up quickly on long descents and automatically go into an engine braking mode where it is spinning the engine by the wheel torque.

 

I've found, at least on mine; there is no engine braking unless I have my cruise control set and active. The ECU will fire-up engine braking around 8-10 mph over set limit; otherwise it'll just keep gaining speed with the engine-off. If I cover the brake pedal, engage the switch but not really brake, I can get full regen bars vs a partial. Release the brake pedal; system will go back to partial regen. This is under Eco mode; that's my normal setting.
The other way to engage engine braking is to place the car into B-mode. I only do that on very slow, steep down hill descents <35 mph.

That's been my experience....

 

Because our transmission doesn't have any gears to change, just a fully variable ratio between the engine and wheels controlled by a computer, glitching and bumping into gear aren't things. It's just varying the engine-to-wheels RPM ratio the way the computer is telling it to, and the computer has various reasons for the choices it makes, including the battery temperature and state of charge. There's no need to do anything to "clear" it. While that's a way you can get it to forget what it was doing and why, and so return to doing something more like what you expected, there's no benefit other than being more like what you expected. What it was doing was what its normal rules tell it to do.

 

The problem is most likely in the overheated high-voltage battery, which first helps to climb the hill, then quickly absorbs energy during the descent. If the battery has a couple of weak modules, the computer will limit the use of the battery after such stressful ascents and descents.

 

Yes; check your traction battery cooling system. Clean it out and use your AC system.
I've forgotten how hot and humid it is there and how someone would get used to it after a while....

 

The degrees are greatly exaggerated.
The steepest section, from the "Downhill" sign to 18th Avenue, is 1,050 feet long, the difference in height between the top point and 18th Avenue is 120 feet.
The slope of the slope is α=6.7° or 11.905%

 

Beat me to it lol.

 

You really shouldn't need B-mode for that bunny hill. I only use it on something like long 10% grades.

Here's a video I found. After the first minute, it shows the decent into South Lake Tahoe. Turns are 25-30 mph down that grade. If you use your brakes the entire way down, you'll barely have anything left when you get to the bottom. The was a stop light down there that was replaced with a large roundabout - looks like it was cut from that video. There was horrific accidents there, when flat lander couldn't stop at that light. I still see skid marks, on occasions, going straight through the center of the roundabout.The last three minutes of the video was the decent from Tahoe to Carson City. Since we're talking highway speeds (65+ mph), best to lock-down the cruise control and let the ECU control and manage everything, if you've got adaptive cruise control. I believe B-mode in that situation would place undue stress on the power-train.

 

It's hard for me to imagine B mode putting undue stress on anything. The engine is burning no fuel, the cylinder pressures are very low because the throttle is closed, and the torques involved are a fraction of what they are when you're using the engine to accelerate.

The car is just spinning a bunch of nice oily gears and moving some nice oily pistons. It could do that all day.

A tip for those big descents: there will be a steepness of grade, and a speed of the car, for which even the maximum available engine braking won't hold your speed, and you will be going gradually faster and faster even with the engine singing away at 4900 RPM to hold you back.

In those situations, you can either take a less steep descent, or take it more slowly. Usually there's not the option of changing the slope, so that leaves slowing down. You can (nearly) always find a slower speed that the car will be able to hold. (Sometimes, though, it might be slower than the drivers behind you want to go.)

 

Call me old school, but I just don't like hearing the engine whine like that - especially at those speeds. Adaptive cruise will max out regeneration braking before engaging engine braking - it helps that the Prime has a larger battery to fill. It'll get to about 70% when I've made that run in the past.
If you need to use your brakes; use them. Pulse them and gave them a chance to cool down - DO NOT DRAG them!!!

FWIW; There's a stop lamp at the base of that run too.

 

Unlike downshifting in a conventional car, where it's your job to make sure you don't overrev the engine, the Prius ECUs know exactly the maximum RPM that they like hearing, and will never exceed that while engine braking. If the resistance at that RPM isn't enough to hold your road speed, they'll just do their best without exceeding that RPM and you'll pick up a bit of speed and have to snub with the friction brakes from time to time.

Once I recognized that, I decided to just let that sound become music to my ears.

 

And if you do manually apply the brakes; the CC system will disengage.
You can re-engage using the resume, toggle up.

 

I did not find any sections with more than 7 degrees.
At each section 15-16, 16-17, 17-18 the elevation drops 36-46 feet, the distance between the intersections of the road with the streets is 340-360 feet.
From 18th Avenue the section becomes more gentle and at 22nd it is almost flat.
Measurements "by eye" and practical measurements are quite different things.

I believe that your problem lies in the high-voltage battery. The battery has deteriorated its characteristics and cannot store all the energy that can be obtained when descending this hill without taking into account the braking of the brake pads.
After the battery is charged, the internal combustion engine begins to assist with braking.

Try monitoring the battery charge, battery temperature and the current that comes during the descent from the hill.
The free Hybrid Assistant app for Android phone will help you in this process very well.

 

My angle perception is off in the same way. I think it's pretty common. I've come home after a bike ride and looked up some hill on the map and thought "no way is that only α degrees!".

Again, at the risk of repeating post 6, don't do that. The car is doing what it's programmed to do, and interfering with that doesn't help. Descending the hill in D instead of B meant the balance of regen braking vs. engine braking was more in favor of regen, where the balance in B is more in favor of engine braking. The heavier regen made the battery hotter on the way down. The car had to bring in engine braking anyway because of the size of the hill, and may have chosen to continue spinning off energy at the bottom to get the battery into a more comfortable range.

Again repeating post 6, there are no gear changes, only a computer calculating what the ratio of the continuously-variable transmission should be at each given moment. When it chooses a higher ratio (which might make you think "sluggish to upshift"), it calculates that based on real needs at the time, such as the temperature and charge state of the battery.

Don't try to outthink the car's programming or trick it into acting the way you think it should instead of how it thinks it should. It knows more about its current operating conditions and needs than you do.

Yes, selecting B is a good idea on that hill, as it reduces the heavy charging forced on the battery. Using your brakes (sparingly, as needed) does the same.

MAX2 may be right that your battery is in a worse condition than a new one, and so you'll be more likely to see behavior like this than you would if the battery was new. But in any case, the car is simply managing the needs of the battery; it would do the same with a spanking new battery on a big-enough hill. You could choose to replace your battery now, or you could choose to wait. The car will do its best to operate within the parameters of the battery it's got, to prolong the life of that battery—as long as you quit trying to second-guess it and stop it from doing so.

 

Looks can be deceiving; that's how magic works....
Enter the scientific method, calibrated, measurement test equipment. No guessing involved.