transmission slips when car is parked outside and the temperature is hot

I remember reading a post last year when someone was having issues similar to yours... Turns out the battery vent was clogged up with a lot of debris. Cleaning out the debris from the battery vent solved the problem. Worth checking out. Good luck.

 

First, fuel economy in the upper 30s in the midst of an Arizona summer doesn't surprise me if your driving is mostly at non-highway speeds. The AC really pulls fuel economy down at low speeds; it forces the ICE (internal combustion engine) to run considerably more than it would otherwise.

Second, the transmission has nothing to "slip" (I'm not dismissing your symptoms). There is no changing of gears in the Prius' continuously variable transmission.

One of the first things to come to mind is "hot battery." When the battery and the rest of the hybrid system reach a certain high temperature, the car takes steps to reduce battery use (using it heats it more). The battery fan runs, as you've observed. Because the electric motor is doing less work, its normally-available immediate torque is reduced and the ICE, with less instantaneous torque, is doing more of the work. I can see how what seems like the sensation of a slipping transmission can be felt in those conditions.

What I'm puzzled about is this:

So this happens only with high SOC? And how do you know you have no regenerative braking? That is quite abnormal if that is indeed true. Are you seeing green arrows on the Energy monitor during braking or coasting? That indicates regeneration.

As for SOC quickly dropping, I have seen that on first startup on a hot day while running AC. In the first minute or so of warmup the car uses the battery preferentially anyway (in all temperatures) as ICE heat is diverted to warm the catalytic converter. Then the AC will be running its hardest to cool a very hot cabin. On hot summer days with the AC on, especially (again) in slow traffic, I've seen SOC drop very quickly after a day-long sun-baking.

Even in the Mid-Atlantic, I go to great pains to try to park in the shade in the summer. I also have tinted windows, a windshield sunshade, and I keep my windows cracked. The battery is a large thermal mass and it cools slowly. Prevention is the best medicine.

I'll be interested in hearing from others in AZ or similar climates to get their take on this.

 

The battery charge goes from what ever it was when I first parked it down to one bar.

Not during the park, when you first drive it right? Either way I would investigate the HV battery, sounds to me its on its last legs. My battery charge only goes down like that when in EV mode,driving several blocks, without the gas engine running. I'm unsure on your next step, but also wanted to point out about the comment
remove the inside cover from the back seat. Looked pretty clean.
I drive in dusty conditions, so occasionally use an air chuck with an air compressor to blow out dust that settles inside the vents, you could try this in case there is something blocking air flow you can't see. I would guess though it would take awhile for a clogged system to cause the problems you have.

 

The gasoline engine and the motor/generator are mechanically geared together. The gears push against each other and the difference between the two is fed through the driveshaft and moves the car forward.

So if the gasoline engine is revving without the car moving forward, then it means that the motor/generator is freewheeling (little or no power being fed into it and little or no power being taken out of it).

You've been getting poor mileage for a while now and the battery fan is always running. This suggests to me that the HV battery is damaged (there's likely a few cells internally shorted) and the charging system is going into overdrive trying unsuccessfully to bring it up to full charge. This constant charging is both lowering your mileage and causing your battery to overheat, damaging it still further.

So I'm guessing that the HV battery problem has now progressed to the point where it is going intermittently open when it gets hot, causing the motor/generator to freewheel and the gasoline engine to rev, since it has nothing to push against.

I'm also guessing that with a new HV battery your mileage will return to normal.

 

The HV battery may be old but I wonder if it is the inverter or the computer that's causing the jerking. Maybe worth double checking to see if the new inverter coolant pump still works.

Also this thread may be related:

http://priuschat.com/forums/gen-ii-prius-main-forum/65883-my-transmission-going-read.html

Good luck!

 

I was just getting on to write a similar post, as it sounds like I had the same thing happen to me today (and about 3-4 times over the past). This has only happened to me in the summer (again a hot Arizona summer) after the car has been sitting at work. It usually starts and gets going just fine. I've noticed it as I've been leaving the lot (slight uphill after slowing for a speedbump). I'll place my foot on the accelerator and it feels like the car lags. It feels like nothing is happening. and today I noticed that my SOC had dropped to only one bar, and the engine ran all the way home. There was regenerative braking (according to the MFD).

I should add that I could hear that the battery fan was running at the time.

 

It didn't do this in past summers, right? Take it to a dealer. Something in the traction battery system is too hot and from this distance we can only guess why.

 

Hi Jonesie12a:
Hopefully this one year belated reply finds that your problem has been solved. My 06 Prius had a bad HV battery at 'only' 43000 miles. The dash warning lights all lit up with the red triangle very large. This was in January here near Boston, but in the 20's.
I don't understand why your dash warning triangle did not come on. Perhaps your battery was more marginal than mine.
I phoned the dealer early next morning and had AAA flatbed the car to the dealer. By early afternoon their super Prius tech had pronounced the HV as bad. Toyota paid for a rental for the five days it took to ship a spare from somewhere on the east coast by truck. No air shipment allowed. Hmm?
At the time of my battery trouble this was the first such failure in the Boston region. Notably, the HV battery is covered for a long time for cars with CA spec emissions. 10 yrs or 150k miles I believe.
Re the 'brake' problem, I've been the guy who leaves more room between the car in front and me. Thus, I haven't had to slam on the brakes very often. I do notice the loss of power on right turns when accelerating from a stop due to traction control. Thus, I try to avoid that situation. Toyota tried to make the car feel like regular cars, but one does learn of the foibles of a high tech vehicle.
I still like the car, but it takes a bit of learning how to live with it. Having had over a dozen cars in my time, the Prius is still the best overall. Now if I could only find better speakers for the JL 'premium' sound systems. ANYONE HAVE SOME SUGGESTIONS? The factory tweeters in the sail panels are definitely poor.
torrens89

 

Likely a non-issue.

If the battery's really hot, the computer does what it can to:

1) Cool it off (by running the fan).
2) Not heat it up any more (by not trying to pull power out of it).

This means that, from a stand-still, the computer requires ALL of the power to the wheels to come from the engine. In order to supply ALL the power, the engine has to operate at a higher power region of its RPM curve - it needs to spin quicker.

This is just your computer taking care not to overheat the traction battery.