By Larry Carley February 6, 2010
According to media reports, Toyota's popular Prius hybrid car reportedly has a brake problem. Under certain operating conditions, as when braking on a rough or slick road, there appears to be a momentary lag in the brakes being applied. To some drivers, it may feel as if the brakes are not working. Toyota says the brakes are working, but may be slower to engage because of a conflict between the car's conventional hydraulic brakes, its antilock brake system and its regenerative braking system.
The brake system in the Prius is very sophisticated and uses regenerative braking to recapture energy when the brakes are applied. Rather than waste the kinetic energy that is normally converted to heat by friction when braking, the hybrid drive system engages the generator to slow the vehicle. In ways, it is similar to downshifting with a manual transmission to slow the car, or using an exhaust "Jake Brake" on a heavy duty truck to slow the truck. The difference is that the hybrid system disengages the engine and turns on the generator to convert the car's momentum into electricity. The current that is produced is pumped back into the hybrid battery to recharge it and extend the range of the hybrid system when it is operating in battery mode.
The Prius also has conventional disc brakes, and antilock brakes that kick in and pulse the conventional brakes when wheel slip is detected while braking on a wet, rough or slick surface. Because the regenerative braking handles much of the braking effort, the conventional brakes normally don't experience much wear.
The problem is, there is an apparent conflict when the car is braking on a rough road. The control system seems to hesitate as if it can't make up its mind which part of the braking system to employ. If the vehicle's wheel speed sensors are telling the ABS system that the wheels are experiencing some slippage, the ABS system wants to take over and cycle the brakes. But at the same time, the hybrid control system is trying to command the generator to apply regenerative braking.
The fix will likely involve rewriting some of the lines of code that control how the hybrid control system and ABS systems interact under rough road driving conditions. It should be no big deal to fix it, but Toyota will have to validate the fix works before they change the programming in all the 2010 Prius models that are on the road.
A recall is expected soon that will include the programming fix. It will likely require Prius owners to bring their cars back to a Toyota dealer so the new calibration instructions can be downloaded into their car's computer system.
This is called "reflashing" the PCM, a process that requires a factory scan tool and software from Toyota. Independent repair shops may also be capable of doing the reflash if they have the proper scan tool and software. But since there is no cost involved to return to the dealer, the reflashes will likely all be done by Toyota dealers. A typical reflash can take about 45 minutes.
We'll keep you posted with further updates on this issue.