Prius Brake Problem - CONFIRMED BY TECH

And now I'm wondering, if you were to turn off the vsc and traction control functions, via the button sequence I read about in another thread, would the problem go away?
In other words, would that isolate the problem as occurring in software designed to mitigate loss of traction when powering the wheels?
Because I just don't believe this is a problem with the ABS. I believe its a problem that occurs because the traction control software comes into play in a situation it wasn't designed for, namely decelerating.

 

I also believe it is something to do with traction control or stability control, the ABS shouldn't really be active as the friction brakes aren't activated.
The traction control was extreme on the earlier 04-05 cars, and there was reports of the car not being able to be moved in slippy conditions, the TC in my car is still harsh and kills almost all the power.
My dealer had a computer on the car when the hesitation happened and no error code or warning came up. It showed up the skid light came on and that was it. They did say the ABS wasn't activated.
The car doesn't know anything strange is occuring.
I would like to hear if you do manage to switch off the traction control system and test the car on the same bumps if it behaves normally or not.
And again for anyone reading these mails, I can only speak for myself. I am not looking for money, I am not looking for fame; I just want a solution to a possible design flaw that scares me when it happens.

 

I experienced this sensation this weekend. I was actually excited when it happened. It's not a problem when you understand what is going on. This was discussed at length on the podcast so being a new driver I was expecting to experience at some point. When it happened my wife couldn't feel it, but I was glad to see it finally. I was in a construction area and there was about an inch drop in the road. I was coming to a stoplight so I was braking lightly. When front wheels dropped I felt it switch over to the legacy brakes. I think education is the key. There are many things that are different about this car for efficiency sake.

 

So how long would you think it lasted for? Less than a 1 second? Or was it longer.

I get it nearly everyday on bumps, the car feels it hesitates just for a fraction of a second.

I experience the hesitation every single day, but there are times when it goes into a processing loop or something strange happens and lasts longer that a second also. That is when it is scary!

 

The only indication in the Prius that ABS is activated is that the skid light illuminates. If VSC activates the skid light flashes and the beeper is sounded. There is no pulsing of the brake pedal.

 

I can confirm that there is no pedal pulsing happens when the ABS kicks in.
I had an cause to use the ABS on a muddy road at more than 50mph and the ABS and EBD(whatever abbreviation for the stability control). It was a smooth tarmac road plastered with mud from a field.
The hesitation/brakes cutting out is associated with the regenerative brakes in my opinion and not the friction brakes.

 

We've been through this discussion many times before. It's both. You wouldn't have the brake drop out sensation without regenerative brakes *and* friction brakes. Here is what happens:

1) Regenerative braking is under way. One or both of the front wheels lose traction. Note that this happens more easily than loss of traction during friction braking, since friction braking uses all four wheels, where regenerative braking only uses the front wheels.

2) The ECU determines that regenerative braking is no longer viable.

3) The system transitions from regenerative braking to friction braking. During this transition, the retarding field currents are dropped in MG2 and hydraulic pressure is applied to the friction brakes. If everything works as designed, the retarding force from the friction brakes exactly equals the previous retarding force from MG2.

#3 is where the problem occurs. The transition is the problem. Regenerative braking works fine. Friction braking works fine. Getting between them can be an issue. For the transition to be smooth, the braking ECU must guess how well the friction brakes will work and try to match their retarding force to the value previously produced by regeneration.

The root of the problem is guessing how well the friction brakes will work. If the brakes are rusty, the transition will feel grabby. If the brakes are wet or otherwise compromised, the brakes will feel like they cut out, which produces a feeling of acceleration.

On a bumpy or slippery surface, the friction brakes may invoke ABS. ABS helps maintain control at the expense of reduced braking. This means that the very conditions likely to cause a loss of traction during regenerative braking are likely to force the friction brakes into ABS. Using ABS means that the initial frictional braking forces are lower than otherwise expected, making it harder for the ECU to make a good guess during the transition.

A further complication is the amount of time it takes to transition from regenerative braking to friction braking. The original ECU software on the 2010 model produced a noticeable delay in some situations. This delay produced a drop out, as a brief period transpired between the reduction regeneration and the start of friction. Fortunately this software problem has been patched.

Looking at all this we can see that there is more than one factor related to the length and severity of the brake drop out. This leads to comments being posted such as "BRAKE SOFTWARE FIX DID NOT WORK!" The fix did work, but only to reduce the time required for transition. It doesn't change the basic physics behind friction braking, and it doesn't help the ECU better guess how well the friction brakes will work.

Perhaps the ultimate solution is to use an active braking system using accelerometers that monitor braking force. In this manner the ECU could compensate for frictional braking variations. Unfortunately even this would not eliminate all of the complaints, as it could not compensate for basic physics. Slippery conditions and bad roads would still cause a loss or reduction of braking, and some drivers would see this as a failure of the system.

Tom

 

Thanks, Tom.

My understanding was that one of the issues on Gen 3 is/was that an event sufficient to drop from regen into friction braking wouldn't always cause ABS to kick in, and this was addressed in the update by reducing the threshold for ABS.

It does seem to be possible for this to happen - i.e. switching from regen to friction without engaging ABS - on Gen 2, but generally on my car, the friction braking force is about equal to the regen force. I don't have accelerometers or instrumentation to indicate when the regen-to-friction transition occurs - I'm basing it on what I can hear from the inverter. Sometimes, when going over a hard bump, I can hear the characteristic regen whine cut off. That is, it was whining before passing over the bump, and not afterward. As I say, usually there is no difference in force, although sometimes it's weaker.

If your car is always worse, and noticeably worse, when transitioning to friction brakes, there's a strong chance that your friction brakes aren't working properly. As they have so little use in normal service, it can be practically unnoticeable. As I said before, check that the pads are in good condition and they're moving smoothly. If you hear a 'clunk' when pressing the brake pedal, the piston's probably sticking.

I strongly advise only using the OE pads. Particularly you should avoid using semi-metallic or ceramic pads, these will not get up to working temperature.

If someone other than the dealer has serviced your brakes, they may not have bled the air out of the brake fluid properly. If there is air in the system the car may not be able to apply the friction brakes correctly, because the air in the fluid compresses rather than pushing the fluid to move the wheel cylinder. The Gen 2 Prius requires use of the Toyota diagnostic scanner/laptop to correctly bleed the brake system, because there is no way to pump fluid through the rear brake hoses other than asking the brake computer to do it.

 

Anyone wanting to check the difference in braking force between regen and friction braking, I would recommend that in a safe location while lightly braking in regen, pull the shift lever to neutral which will disable regen and enable friction braking.

On my Gen III, there is a slight decrease in braking force. When I do this I find that I have to press the brake pedal just slightly harder to maintain the same braking force and deceleration.

This will also give you some idea of the "sensation" of acceleration which you will realize is only a slight loss of deceleration.

I understand that this is not the exact same response as when you hit road imperfections but it should be very similar.

 

Exactly. Generally the transition is very smooth.

When a transition is forced going over a bump, you have the added effect of poor braking. Obviously this makes the transition more noticeable.

Tom

 

My car always favoured the problem hesitation/loss of braking force when the drivers side (right front) wheel hit a bump or pothole.

The car started squealing intermittently from the same wheel, so I thought a bearing or something. Turns out that the squeal was caused by the pad catching off the lip of the disc/rotor.

The car had done only 96,000km or 60,000miles. But the disc and the drivers side seems to be more worn than the other disc.

I am suspecting that the drivers side caliper maybe sticky and causing more braking force that is overpowering the regenerative brakes and ABS thinking that it needs the ABS.

The dealer played around with the pads and the cars squeal is 95% better and the MPG has improved from 46US/55EUR to 48US/58EUR.

So that would seem to confirm that the brake was dragging and costing more fuel as well.

I don't know if this will explain it for more people but it would appear to have changed the characteristic of the regen loss of braking felt. Now it seems to be just a hesitation rather than an actual perceived loss of braking.