If you have a second generation Toyota Prius that sometimes seems like it's slowing down too much when you take your foot off the gas pedal, you may find the following information helpful. You may also notice that the "energy" screen shows the car is in regen mode the second the throttle is released. So far, we've only had one customer with this complaint, but I expect we'll hear it again.
Our customer had a 2006 Prius with about 20K miles on it, so it was still under warranty, but she didn't want to go back to the dealer. We thought we'd offer to check it out, just for our own edification. Her complaint was that the car was acting as if it were in regen mode when it should be coasting along on the freeway. As soon as the throttle was released, the car started to slow down too much. She also noticed that the energy screen indicated the car was in regen mode while this was happening.
We first checked for TSBs (Technical Service Bulletins) issued by Toyota, but found none matching this complaint. We checked for trouble codes in all 16 ECUs (electronic control units). We found history codes B1200, B1207, and B1271 in the Gateway ECU, but these were likely caused by turning the power on with the dome light fuse out. This can happen at the dealer when the car is new because it is shipped without the dome light fuse to reduce parasitic drain on the battery during transit.
We then drove the car on the freeway with a scanner attached while monitoring the HV ECU REGEN REQ (regen request) and the REGEN EXEC (regen executed). One of the first things we noticed is that the energy screen indicated regen as soon as the gas pedal was released, but the scanner indicated that there was no regen occurring. It didn't feel like the car was in regen mode, since it was coasting freely. We checked this on another Prius and found the inaccuracy in the energy screen was normal. We continued to drive the car, but never experienced the rapid slowing when the gas was released.
When I got back to the shop, I was talking to Brad (our resident smart guy) about the problem, and he said, "maybe it has a bad shift selector switch and it thinks it's in Braking Mode". Braking mode... What's braking mode? Oh yeah, it's that's the shifter position most people never use. As soon as I climbed back in to the driver's seat, I had a pretty good idea what had happened....
The second generation Prius has a 4 position shifter switch. Anyone can tell how to make a Prius go forward or backward with a quick glance at the shifter, but what's that "B" all about? "B" stands for "braking" as in engine braking. Braking mode is intended to mimic the feel of downshifting in a conventional car, as might be done to spare the brake pads while driving downhill. I originally thought the same thing could be accomplished by stepping lightly on the brake pedal. However, I later found out that the ICE is used for engine braking when the battery pack is fully charged, and can not take any more charge from regen.
You may remember the big media hoopla about second generation Priuses seemingly stalling and coasting to a stop for no reason. All the hybrid naysayers gleefully said, "I told you so!" to anyone who would listen. It turned out that the "stalling for no reason" was in fact caused by drivers accidentally bumping the "Park" button while fumbling for the radio knob. When you told a Prius to park while driving at 60 MPH, it would obey and coast to a stop. Toyota issued updated software that prevents the park button from having any effect while you're moving. Anyway, you may notice that the shift selector would be as easy, if not easier, to bump while reaching for the radio.
