The ABS VCS etc mystery

Yeah I already thought about that when I had to do three break actuators on the three of the four cars that I own Believe you me All four of my batteries are in top notch and I have an endless supply of 12 volts if I want them I've never had a 12 volt problem yet they get changed before they become a problem so far every time no dying in the street in a parking lot none of it I figured this break box next to the battery or something to run the brakes in case of an eminent some sort of power failure but not a fire that took out the wiring or something LDOs and all of that that's a wonderful thing but I'm sure Toyota designed it so that it's got some capacitors or something in it that take power from the battery and keep those up and then when 12 volt supply fails are as faulty or whatever the capacitors can take over enough to stop the car when my break actuators all of them have gone out I never had any problem stopping the car it just takes a little longer to do so I never noticed my regenerative breaking not working It was my friction breaks I thought that were slow to be brought into action by the movement of fluid by the actuator that takes over when I step on the pedal. Oh well batteries didn't play a role in my actuator failures that I know of two of the cars still have the same 12 volts that were in them when the actuators failed. The audible beeping is just I believe telling me that I have no pressure that a sensors looking for in the accumulator so the brake pump continuously runs to try and satisfy that I could feel this in one of my cars the brakes would work fine for several stops and then the beef would come on while I was driving around town or wherever I was driving. And then would go off while I'm still driving anytime the beep was on I had the brakes of the $69 Dodge truck with four-wheel drum brakes and no brake booster essentially when the beep is off I have full breaking power that the car is always had change the actuator all this went away including the lights and the 12 volt didn't seem to make any difference in these three cars.

 

But that would be great if we could just change the 12 volt and that would solve the problem

 

@ChapmanF. Can you please explain to the OP what the brake circuit that directly connects to the 12v battery does. You can explain it much better than I can since I never owned a Gen 2 Prius. Can you also supply the link to TIS so that the OP can look at the wiring schematic for the car if he chooses to do so.

@Dr M. You need to learn about the 12v system on your Prius. After the car is put into "ready", the 12v supply for the car comes from the hybrid battery by way of the DC /DC converter, which is located inside the inverter assembly. The 12v battery is also charged the same way when the car is in "ready". The 12v battery is NOT supplying power to the car when it is "ready".

Since you seem to have an electrical background, use a DVOM to measure the 12v at the jump point in the fuse box that is under the hood when the car is off. You are measuring the voltage of the 12v battery that is in the rear of the vehicle. Now turn the car on (ready) and measure the voltage at the jump point. You are now measuring the voltage supplied by the DC/DC converter, which will be much higher than what you measured for the 12v battery. If you do not measure a higher voltage than the 12v battery, you have a problem in the DC/DC converter circuit.

This is the very beginning of how the 12v system works in your car. Do some more reading on this forum to learn more. Hopefully, ChapmanF will stop by and explain about the brake circuit that is connected directly to the 12v battery.

 

Last first: rather than just linking to TIS, it's my preference to link to PriusChat's own wiki page on the subject, either with a direct link Toyota Service Information and Where To Find It | PriusChat or just look for "Wiki" up at the top of PriusChat and find it there. (There is no rule says that link can only be posted by me. )

Our wiki page explains TIS, and how to access TIS, and how to make effective use of TIS when you access it, and how to find out if TIS access could be available through a partner like your local library, and so on.

Right, well, first about that brake box in the rear, don't be fooled just because it sits next to the battery and its wires come out of the same tape-wrapped wire harness as the battery cable. It is not directly connected to the 12-volt battery. It's just another of the car's many electrical gizmos stuck in whatever suitable locations Toyota could find, and that happens to be where they physically stuck it, and those wires run in the wire harness forward to the fusebox and the brake actuator under the hood and brake ECU under the dash, as you would kind of expect.

That box is a backup power supply. It is full of capacitors and a control circuit that keeps the capacitors charged when 12-volt power is available. If some malfunction takes out 12-volt power to the rest of the car, that box will keep the brake ECU and brake actuator powered for as long as the charge on the capacitors holds up.

Only the gen 2 Prius (well, I'm not real up to speed on gen 5 yet, but of the ones I know, only gen 2) has an electrical brake power backup like that. Every generation has some kind of fail-safe braking mode if the power is lost. Most generations have a non-electric power assist to the brakes, where pressurized brake fluid enters a "booster" chamber behind the master cylinder piston and pushes in sync with your foot, giving you more stopping power.

They dropped that for gen 2 and went much more brake-by-wire. The ECU senses how hard you press with your foot, and calculates how high a "power assisted" fluid pressure should be at the wheels, and works the actuator valves to just send that much pressure out to the wheels. Because this "power assist" effect depends on the ECU making calculations and the actuator valves directing the fluid, there's a backup box to keep those things online.

Of course gen 2 does have a further, non-electric fail-safe mode for when the capacitor backup's used up, but that has no power assist, and only works the front brakes. Really a system of last resort.

As soon as gen 3 came out, the whole capacitor backup thingy was gone again, and the booster chamber in the master cylinder was back again. Go figure. Gen 2 was the furthest Prius venture into strict brake-by-wire.


Now as for those dash lights.

There is no one reason they come on. Not even just a few possible reasons.

Those lights come on to indicate specific problems off a list of about two hundred, and it isn't necessary to engage in original research to find out which reason(s) they came on for. You just ask the car.

The easiest way to ask the car is with a scan tool that can read the brake system trouble codes. But if you don't have one of those, you can do this, at least in generations where it works: Blink (a/k/a Flash) Codes – How to. | PriusChat

Once you have the trouble codes telling you why those lights came on in your car, you look those codes up in the repair manual (see Toyota Service Information and Where To Find It above), where you find troubleshooting steps to isolate the cause, and repair steps to correct it.