Braking problems?

<div class='quotetop'>QUOTE(MGBGT\";p=\"93362)</div>

There might be a third possibility... The Prius might not think there is a fault at all, and might not be applying the brake by wire systems for some reason (ABS or the "traction control system that couldn't"). I would wager that is what's happening to everyone here who's describing having the same issue..

 

When it happened to me, I was on a gravel road. I was coasting down a very gentle grade, maybe 800 yards long. My foot was off the accelerator, the ICE was off, the MFD was showing regenerative action, and the speedometer was slowly increasing from 50 km/h to 55 km/h.

When I braked for the deer, the car had just gone over the washboardy section of gravel, and braking was almost non-existent. I don't recall the ABS cycling as the washboardy texture of the gravel was too noisy and rough to feel if the ABS was active.

However, with the same brake pedal pressure still applied, once the car was past the washboardy section, it strongly braked. There were no warning lights on the dash or the MFD.

With my old Ford truck, thanks to the James Duff variable-rate front coil springs and Rancho 9000 adjustable shocks, I don't really feel the washboardy section or bumps anyway. The suspension is actually very compliant with the shocks set to "1" or Very Soft. It just floats over the washboardy texture.

Tromping the brake pedal with the old Ford all 4 wheels lock up and the truck will quickly dig in and stop.

So I'm wondering if the ABS algorithm isn't too sensitive?

I really don't see how Trac has anything to do with this, as Trac is only applied during strong acceleration with wheelspin. IMHO the Trac is also far too sensitive.

Anyway, I'm leaving for the hobby farm very soon and will be back Monday night. I'll try the braking test again, as I'm sure if anything the gravel road will be even more washboardy since my last visit. If my parents visit I'll take a spin in their Buick LeSabre to see if the ABS in that car is just as sensitive.

 

<div class='quotetop'>QUOTE(Jonnycat26\";p=\"93365)</div>

True, if the computer does not think there is a fault, then these two solenoids should be on and the valves closed. Then however, there would be a discrepancy between the pressure at the master cylinder pressure sensor (PMC 1 and 2), vs wheel cylinder pressure sensor. When this discrepancy is greater than a given value - which I would expect if you 'step on it', this should set fault conditions (you are trying to break but no pressure is getting to the wheel cylinders), which should open the SMC2 valves. Afterall, this is one of the ways the fail safe operation is invoked. However, note that the legend to the second diagram I posted above that shows the fail safe mode, states that the fail safe mode is invoked by the skid control (or TC) ECU. It could be as others have suggested that the TC ECU thinks we are having wheel lockup, and is reducing wheel cylinder brake pressure. To do this, I would assume the TC ECU has to re-set the threshol dfor the pressure differential between master and wheel cylinder sensors, which are the ones used to invoke failsafe. It could be that this threshold is improperly reset during certain conditions?

 

Gee those diagrams look familiar, esp the second one.

The fault condition that would open SMC valves would be complete (including backup) power loss, or accumulator pressure failure due to a leak.
I believe the pressure sensors are there to detect panic stop requests, but that would not be a failsafe condition; the brakes are still working. Instead, the brakes would be operated more aggressively, using ABS to control wheel lock, and VSC (if equipped) to control spin skids.

Remember, in normal conditions YOU DO NOT operate the hydraulics directly. By normally, I mean there is sufficient power to the brake system, and the accumulator pressure is normal. The skid ECU (no such thing as a TC ECU) detects your requests, and operates the wheel cylinders appropriately for you.
If for some reason, the skid ECU lost the ability to apply the brakes by itself, THEN it will open SMC valves so that you could operate the front wheel cylinders yourself using the master cylinder pressure.

 

Diagnosis: 50 klicks on a washboardy gravel road is too fast. Resolution: drive slower :_>

 

bramblerose,

does your Prius have VSC?

DanMan, did you make those diagrams?
If so, I apologize for not giving you credit! I can't remember where I got them from, and thought they were part of a Toyota manual. Not even sure if it's technically ok to post them, but I thought within the context of analyzing and understanding a potential safety issue it would be permissible.

M

 

Yup, posted them under Brakes Q&A as a brake discussion thread.

Well, actually I did get them from NCF, but I did the coloring on the second one. It was the 2nd diagram that clued me in on where it came from. I started that thread because I was having a hot debate with a nay-sayer in PriusOnline about the reliability and safety of the brake system. Guy believed that if the accumulator failed (pump or reservoir), then you would have NO hydraulics at all. I stand by my understanding that the master cylinder will operate the front wheels directly if CPU control failed.

 

I agree that if you *stomped* on the brakes, the mechanical brakes should be engaged, and so far, this has been my experience as well...

What I'm talking about with the traction control is when you're lightly braking (ie, in regen region), and you hit a bump or a rut, and all of a sudden, you feel the car lurch forward before recovering.. On my Classic, I can see this behaviour with my Miniscanner as charging current at around 55 amps all of a sudden dropping to zero before "recovering" to about +5-7 amps about half a second later- this has got to be the traction control cutting off regen, and if you're not expecting it, it could certainly feel like the brakes had failed, and it could certainly cause an accident.. IMO, TC should be turned off completely if the brake pedal is applied.. There should be no instance where braking force (whether it be mechanical via the pads, or electrical via the generator) is cut off completely: extremely fast modulating of braking force a la ABS is OK, but completely turning off is an accident waiting to happen.. At the very least, if the regen is cut off, the system should switch in an equivalent level of mechanical braking to compensate, although I'm not sure how this could be accomplished with the complex electro-hydraulic arrangement...

 

Richard:

Well, that *is* the posted speed limit.

I got back late yesterday from the hobby farm, and I spent a lot of time driving up and down that washboardy 5km gravel sideroad, to see if I could figure what the h*** is up with the brakes.

My folks also spent the weekend, so I had a chance to drive their 2003 Buick LeSabre over the same road and do the same braking test. The Buick had no trouble at all even over the severe washboardy sections.

On the smooth gravel, the Prius will brake quite well. About the only time ABS gives you longer stopping distances is on gravel, as without ABS you can actually dig in a bit and stop quicker. That's why my old Ford F-150 stops well on gravel.

Driving the Prius over the washboardy gravel road, I had to reduce the speed below 30 km/h to get any sort of decent braking out of it. I think I finally figured out why this is so.

The Prius has a buckboard stiff ride, the suspension doesn't appear to have much compliance. So over potholes, expansion cracks, washboardy gravel, instead of the suspension absorbing the bumps, the tires have to bounce above and lose contact with the surface.

As an experiment, considering I would not be going faster than 50 km/h, I aired all the tires down to 26 psi. The braking was much better over the washboardy section. However tire pressure that low isn't practical or even safe for average driving.

I was able to repeat this behavior with my old Ford truck. I have Rancho 9000 adjustable shocks on it, so I quickly and easily cranked the shocks up to Maximum. I was able to replicate the bone-jarring ride and over the washboardy section I also had no braking. Even better, the back end slid out and I went sideways.

Stopping and turning the shocks back to 1 or Very Soft I was able to drive over the same section with no problems. The contrast was very apparent. So at least on gravel washboardy roads, the Prius suspension is far too harsh.

I know some of you folks actually like a hard bone-jarring ride, but if you had to drive on rough pavement or gravel roads, this isn't the car for it.

I'm sure this doesn't explain all the reasons why the Prius brakes may appear to stop working over a pothole or expansion joint (Or washboardy gravel road), but it was something I could easily verify. Asides from the poor compliance of the suspension, perhaps the ABS algorithm is too sensitive.

Jay

 

First off, traction control is for ACCELERATION, not braking. ABS is the traction control eqivalent when braking.
2nd: the classic does not solely apply regeneration as does Gen2. That was the big enhancement with Gen2, so friction brakes are always applicable on the classic. The question is, how much friction brake is applied? Considering the miniscanner only works on the classic, you have to be taking in classic conditions, and not Gen2.

So, instead of just reading regen current, also include friction brake power, if that is available with the miniscanner. You would also need to know the speed and brake force of each individual wheel to know if there indeed is a glitch in the brake system.

 

I know TC is *supposed* to be for acceleration, but it appears to be functioning during braking as well.. ABS is a purely hydraulic operation as far as I understand, it does not extend to modulating (or turning off, as the case may be) the regeneration, therefore what I'm experiencing must be due to the traction control algorithm.. Anyways, call it what you will, TC or ABS, it's all semantics and I don't want to argue terminology- the point is that there is obviously a problem occuring here, and I believe that it's due to the regen being cut off by some entity in the car's electronics..

The fact that the HSD Prius seems to have more of an issue is quite likely due to braking changes- in the Classic, there is still some degree of friction braking, so even with the sudden loss of regen, you still continue to slow down (albeit at a lower rate); with the HSD Prius, if regen shuts off, if you aren't in the friction braking range, you have *NO* braking at all!!! In either case, it's still a problem, but as I said in my original post, more so with the HSD Prius than the Classic..

I don't believe the miniscanner logs any braking parameters except indirectly with the regeneration current; I don't think the ABS ECU is connected to the DLC3 in any case (ie, ABS ECU data will not be available via OBDII), so to get this data you'd either have to install external sensors on the brake lines or somehow open up and hack the ABS ECU..

 

Braking system works as follows:

You request braking amount to skid ECU. Skid ECU asks HV ECU for regenerative braking force. HV ECU returns braking force possible. Skid ECU applies difference to hydraulic brakes.
Skid ECU can withdraw its request for regenerative braking, and will do so for ABS conditions. ABS's goal is to keep all wheels spinning, which ensures all wheels have kinetic friction to the ground. It does this by withdrawing excees brake force. It can't do this if regenerative braking may override individual wheel control, so it withdraws regen. This is NOT repeat NOT TC.

If memory serves, TC applies brakes, as well as withdraws acceleration.

 

TC is NOT the issue. Rather, when regen is active and suspension hop occurs, the regen brake torque decelerates a nearly airborne wheel which invokes ABS. At that time, the ABS algorithm requests regen termination so that the decelerating wheel can accelerate (terminating regen removes the wheel brake torque present). When the wheel accelerates due to it regaining ground contact, ABS exits and regen is once again invoked. On a washboard road, this may occur several times, and does occur with other vehicles (minus regen of course) depending on the suspension stiffness, tire pressures, and other factors. Bottom line is some cars/trucks are better than others on this type of surface. Even tire size (diameter) plays a large role in this activity.

As Dan has correctly stated, this is not a TC issue, rather it is a powertrain control and ABS interaction issue. And its not a problem, rather a characteristic of a vehicle with P185 15 inch tires and 4.9 inches of ground clearance. To your previous posts, TC does exit upon either the application of the brake (switch input) or the loss of positive throttle position (throttle less than about 5% yields no TC activity). There are three things happening when you hit the brake: regen braking via powertrain control, regen braking via brake control, and ABS. Upon brake apply, powertrain regen control quickly transitions to brake regen control. If ABS is required, regen terminates and ABS uses a combination of hydraulic and powertrain control actions (terminate regen, terminate low speed motor torque) to manage wheel slip. Once hydraulic braking is no longer required, ABS will exit, and brake regen control returns. Remove your foot from the brake and powertrain regen control returns. To simplify the system, think powertrain control (engine, motor acceleration, motor deceleration, TRAC) and brake control (motor deceleration, hydraulic brakes, ABS, VSC, EBD, BA).