I have an '09 Prius with about 19k. The cc on relatively level roads works perfectly. However, we took a trip last week on rural highway that was very hilly with speeds averaging 55-60 mph...the cc worked well maintaining speed going up the hills but lost control going down hill...accelerating with gravity as the car went downhill. I had to brake on 3 different occassions to slow the car. Finally, I disengaged the cruise control. Anybody have similar experience? What was/is the fix?
If hills are steep enough you will gain speed going downhill simply by coasting; CC doesn't regulate speed by actively slowing the car. The Prius' aerodynamic drag is less than that of most other cars, so it may gain speed coasting down hills where other cars you may be accustomed to driving don't. On the other hand, if CC is applying throttle on a steep downhill, that is a problem. Watch the energy monitor during these events. Assuming it is coasting (and behaving normally), you should see only green arrows, showing energy flow into the battery with no ICE power. Be aware of CC's aggressiveness in trying to maintain speed. As you approach a hill crest CC often overshoots the set point before backing off the throttle. This is normal though not ideal.
i have had this problem in both my pri and hycam. i don't know if it's a toyota thing or difficulty controlling a hybrid. i even have trouble on windy up and down roads like going thru jacksonville fla., it wants to speed up too much trying to catch up.
In our 2010 I've noticed significant variations while entering Portland from the North on I295 where the highway undulates and twists. I'd have the CC set to PSL of 50 and the Prius will accelerate uphill to maintain speed but give it too much oomph and continue to accelerate downhill.
Two points of clarification: The 2010 model's CC does slow the car, by increasing regeneration far above the normal foot-off-the-gas-pedal drag. But this won't help OP with a 2009. Double whammy here -- Prius has not only less aerodynamic drag, but also much less engine compression drag (pumping loss) than other cars.
I have a 2005 and have noticed similar conditions with my CC. There is a sequence of three short, steep rises approaching my home with a 40mph limit. The CC has a delayed response and over-corrects on each rise. That is to say, the car is nearly at the top of the rise when acceleration starts, then is almost at the bottom before deceleration starts. The result is the car's speed quickly cycles between 37 on the upslope and 43 on the down. It's just a matter that the CC (Gen-II) software has a response time delay of more than a second, thus is over-correcting to rapid changes in road geometry. I solved the problem by taking the car out of CC when I approach that area. As soon as Toyota develops a 3D look-ahead camera and software to support it, then the CC will be able to anticipate what it's supposed to do and we won't have a problem......R I G H T ....
The Prius doesn't use engine compression to slow the car either. Unless you have selected "B" mode. This is because the engine is NOT connected directly to the drive wheels in the HSD. RTFM. "Cruise control should not be used in hilly terrain." Also, "cruise control should not be used in rain or snow or on loose surfaces." Yes, the Prius will take off on downhills, cruise control engaged or not. It's designed for maximum rolling efficiency. The design worked well! Depending on how your brake switch is set, you can depress the brake pedal a bit to engage regen braking without canceling cruise (I do this with Pearl in the mountains). I will also sometimes switch to "B" mode when descending -AND- open the windows to maximize the drag, but I usually still have to use friction braking. But these are long descents. I don't use cruise on them.
When traveling above 41/46 mph (depending on generation), the ICE must spin, and my ScanGauge shows that the throttle plate is not set wide open, so it still displays some compression or pumping loss. But this is very small in a Prius, unlike other cars where it is a major source of drag to slow the car. With the CC changes in the 2010, a number of us have found it useful as a speed-controlling automatic brake on the downhills -- and very efficient, as it uses only regen when the traction battery is not filled. But again, this does not help owners of earlier versions.
Yes I've noticed exactly the same behavour SG. It appears that CC will try to slow the car with only the standard "regenerative simulated engine-brake" that you normally get with your foot completely off the accelerator. If that's not enough then yes the speed will climb.
