Brake Failure

The brake system is a lot more integrated than you are thinking.

 

the brakes failed on your prime?

 

i think you nailed it.

 

i may need engine replacement too. thinking of just trading though

 

how many miles on her?

 

Seriously let me know how fixing the brakes go. I've calculated it would be cheaper and better to just replace things than buying another turd for $7,500. I've already spent $2,250 on Project Lithium. Just gotta wait for it to get here. Hopefully I can get another year out of the engine. I don't know what will go next, the engine or the brake stuff, or something else.

 

The hydraulic brakes do a lot more than "kick in to stop the car at low speeds". Because regen is limited in the power it can sink, your braking will often start mostly hydraulic at higher speeds, with the regen proportion then increasing as the car slows, and then hydraulic taking over again at the lowest speeds when regen isn't effective, as the second of these graphs from the Toyota manuals will show.



Anyway, the effect that you are probably seeing is just the effect of having brake trouble codes logged. The system does not try to be HAL and figure out exactly what the trouble is and what it still should or shouldn't be able to do. It tells you about the problem, and drops back to one of a few preprogrammed stages of fail-safe, which include not trying to coordinate with regen the way it normally would.

 

Well, it works by turning your kinetic energy into electrical energy, and there's a limit on power (rate of energy over time). The battery can only accept so much power. The mechanical power is your wheel RPM times the braking torque, so the same torque at twice the speed involves twice the power, and at higher speeds the battery can't handle that much. So the regen has to limit its torque at higher speeds (take the max power the battery can accept, divide by the wheel RPM, use the right conversion factors, that's the most torque it can use).

So as the car's speed comes down, regen can do more of the braking torque, without exceeding the battery's limits.

But then as you get down very close to zero speed, the effectiveness of a slowly-turning motor being used as a generator starts to fall off. To fully stop you then, the car would have to switch to putting power into the motor in reverse, until you came to a stop. But that would kind of defeat the purpose of regen.

I sometimes use that trick myself on purpose, when I'm testing brakes for drag. I'll drive for a while all without touching the brakes (takes some careful route planning) and then turn into a big parking lot where I can stop without touching them either. I'll slow down first with B, and then shift to R below 7 MPH or so where the car lets you do it, and just stab the brakes when the speed crosses zero. That way the brakes should be stone cold when I get out and feel them, if they haven't been dragging.

 

What would you use your temp gun to look for?

Brakes that are dragging hard will heat your pant legs when you stand by the wheel. Nobody needs a temp gun to see that.

In a Prius, if you really like to keep your brakes dragging as little as possible, you should be able to drive 15 minutes, stop without using the brakes, put your hands on the rotors and find them absolutely stone cold.

You'll find them warmer than that if you even just used them to stop the car at the end of the drive, defeating the point of the test. Whether you use your hand or a temp gun.

 

I wouldn’t trust anything but matching part numbers