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Author Topic: Toyota: -Something smells fishy to me...  (Read 11938 times)

compasspnt

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Re: Toyota: -Something smells fishy to me...
« Reply #15 on: January 28, 2010, 10:22:32 pm »

ScotcH wrote on Thu, 28 January 2010 21:33

The point is that the brakes are mechanical.  Even if what you say is true, it will be the accel pedal that will not function, unless of course the computer controlled ABS is somehow preventing you from appying the brakes ... which would be ridiculous, and indeed very scary.

Brakes will ALWAYS overpower an engine ... even at full throttle, a functional brake system will stop the car.  Simply standing on the brakes will do it.  I'm not sure how these poeple are dyning (very tragic), but there are always options.  brakes is one, neutral is another, even if the engine cannot be shut off.



Does this post not address that situation?

ssltech wrote on Thu, 28 January 2010 08:31

One problem however, is that the servo in power-assisted braking is vacuum-driven, and (in most engines) is generated by the engine when the throttle is CLOSED. SOME vehicles (such as the newer Infiniti G37) don't even HAVE a throttle... they determine how much air goes into the cylinders by varying the time that the valves are open (no intake camshaft) and/or the distance to which they open. In these cars (and diesel vehicles, which also have no throttle) there is therefore NO throttle-generated vacuum, and so there's usually a mechanical vacuum pump run off one of the main motor shafts, and an electric pump for when the engine is not running.

In a 'traditional' system, the vacuum is generated when the throttle is partially open or essentially 'closed'. The downward 'pulling' of the pistons pulls air past the open intake valves, and draws a significant vacuum (typically 20+ inches of mercury at warm idle, even more at running speed with throttle-lift) behind the (closed or partially-closed) throttle plate, usually in a plenum which is connected to the vacuum chamber in the brake servo.

When the 'gas pedal' is 'mashed to the mat' the throttling-plate opens and this vacuum disappears, so the vacuum chamber is fed from a line with a "check-valve" in it (which is analogous to a 'diode' in an electrical circuit and the servo chamber is like a 'capacitor') -the check valve lets air flow one way (AWAY from the servo) but not the other, and the servo 'stores' the vacuum in the vacuum chamber.

You can witness an example of this in a traditional servo-brake system, by running your car at idle while parked, and testing what it feels like to press the brakes firmly. -Now stop the engine and press the pedal firmly again. -IT should feel about the same. Now try a third time... then a fourth -Keep going.

The vacuum chamber usually has enough for about three full application/release cycles, and then you should probably notice that the brake pedal suddenly feels VERY 'hard'. -At this point the vacuum is exhausted, and you now have no more servo-assistance. Your braking power is now about one-tenth to one-fifth of what it was before, and at 70MPH (or in fact any other speed) your stopping distance will be extended accordingly.

For a traditional throttle-body-equipped-intake vehicle, the intake generates no vacuum when the gas pedal is "in the mat", and so the brakes would act as if the car had the engine stopped... after a few cycles, the driver would have a 'hard-pedal', the required braking effort would be enormous, and it's pretty much certain that a human driver would not have enough strength even then to overcome a 'screaming' motor.

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ScotcH

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Re: Toyota: -Something smells fishy to me...
« Reply #16 on: January 28, 2010, 10:57:09 pm »

compasspnt wrote on Thu, 28 January 2010 22:22


Does this post not address that situation?



Somewhat ... this is the part you're referring to I guess:

ssltech wrote on Thu, 28 January 2010 08:31

and so the brakes would act as if the car had the engine stopped... after a few cycles, the driver would have a 'hard-pedal', the required braking effort would be enormous, and it's pretty much certain that a human driver would not have enough strength even then to overcome a 'screaming' motor.



Basically you're dealing with unboosted brakes at that point like many old cars, and plenty of broken cars ... my wife's Fiat spider has no vac assist on the brakes (I diconnected it cause it was leaking), and it stops just fine ... I can lock up the whels no sweat.  While the effort is certainly MUCH greater, it is still perfectly effective to use the brakes while unboosted.  Yes, the pedal will feel rock hard, but there is leverage at work, and you're still appliying like pressure (like 1000s of PSI) to the brakes when you stand on the pedal.  The clamping force is enough I would think ... but then car makers have obviously made other idiotic enginieering design changes, so perhaps they boosted the brakes to the point that they are no longer fnctional without the vac assist.
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bblackwood

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Re: Toyota: -Something smells fishy to me...
« Reply #17 on: January 28, 2010, 11:01:52 pm »

ScotcH wrote on Thu, 28 January 2010 20:33

Brakes will ALWAYS overpower an engine ... even at full throttle, a functional brake system will stop the car.

In the recording of the Lexus crash, iirc the brakes had faded and eventually failed. I suspect they tried riding them to control the vehicle before deciding to really step on them and by that point they were too hot...
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KB_S1

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Re: Toyota: -Something smells fishy to me...
« Reply #18 on: January 29, 2010, 05:43:48 am »

ScotcH wrote on Fri, 29 January 2010 02:33

KB_S1 wrote on Thu, 28 January 2010 14:40

Keith.


Try manually activating the throttle yourself whilst downshifting or applying a bit of left foot braking whilst accelerating.

Pretty sure the car will not comply and probably give you a row.
Unless you have sneaked a Fezza or Lambo purchase that you have kept quiet?


The point is that the brakes are mechanical.  Even if what you say is true, it will be the accel pedal that will not function, unless of course the computer controlled ABS is somehow preventing you from appying the brakes ... which would be ridiculous, and indeed very scary.

Brakes will ALWAYS overpower an engine ... even at full throttle, a functional brake system will stop the car.  Simply standing on the brakes will do it.  I'm not sure how these poeple are dyning (very tragic), but there are always options.  brakes is one, neutral is another, even if the engine cannot be shut off.




My point was that DBW throttles will not open if the brake pedal is depressed.
A computer controlled decision. ANything more than about 0.5s will be over-ridden by the ECU.

Brakes will NOT overcome a lot of modern powerful  engines if there is already momentum.

Fortunately I am unlikely to ever own an automatic so I will always have the clutch and neutral option.
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MDM,

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Re: Toyota: -Something smells fishy to me...
« Reply #19 on: January 29, 2010, 07:03:28 am »

I think my BMW has this potenziometer-controlled gas pedal from the feel of it. Shocked

it feels like it's doing an average as I step on the pedal.. doesn't respond quickly.

if the motor is under full power, would I be able to push it out of drive and into neutral then?

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KB_S1

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Re: Toyota: -Something smells fishy to me...
« Reply #20 on: January 29, 2010, 07:43:49 am »

MDM, wrote on Fri, 29 January 2010 12:03

I think my BMW has this potenziometer-controlled gas pedal from the feel of it. Shocked

it feels like it's doing an average as I step on the pedal.. doesn't respond quickly.

if the motor is under full power, would I be able to push it out of drive and into neutral then?




What year? Automatic?

Newish BMWs tend to have a couple of weird things going on.
There is the DBW throttle which in non M cars has a noticeable delay.
In manuals there is also a clutch delay valve which I find incredibly frustrating.

My GF's Fiat 500 has these things too and I can not drive the bloody thing smoothly at all.
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ssltech

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Re: Toyota: -Something smells fishy to me...
« Reply #21 on: January 29, 2010, 07:47:54 am »

MDM, wrote

it feels like it's doing an average as I step on the pedal.. doesn't respond quickly.



There is no perceptible lag due to throttle motor reaction time in either my wife's or my DBW cars. I can take a picture of the vacuum/boost gauge in any one or more of my cars, in an effort to show that the difference between the mechanical-linkage variations which I own behave just as quickly as the DBW ones...

Which BMW is it? IIRC BMW were using these about 20 years ago on their dual-intake-plenum V12 motors, (balancing the vacuums struck me as a challenge)

MDM, wrote

if the motor is under full power, would I be able to push it out of drive and into neutral then?



Yes. There is NO connection between linkage operation and motor load in most automatics. Dual-clutch transmissions especially... although in those both clutches are computer-controlled, and operated by pump-driven hydraulic actuators... even more potential points for a catastrophic failure.

In a traditional manual transmission engine drive and shifter position is directly linked, so motive force will come into direct 'contact' (and you can 'feel' this in your wrist when performing a clutchless-shift).

ScotcH wrote

Basically you're dealing with unboosted brakes at that point like many old cars, and plenty of broken cars ...



I've driven, and played with many un-assisted cars. -Take the old VW beetle. The brake arrangement in them is NOT SUPPOSED to be boosted, and the 'gearing' of the brake pedal (. in specific terms the mechanical advantage, the 'leverage' of the pedal/actuator relationship) is MUCH greater than in boosted cars.

As a result, if you disable the servo in a modern boosted vehicle, or if it fails (such as a MkIV platform VW GTI... an example which occurred to a colleague only last week, which is why it is fresh in my mind) then you'll have discovered the world's fastest-acting laxative.

ScotcH wrote

my wife's Fiat spider has no vac assist on the brakes (I diconnected it cause it was leaking), and it stops just fine ... I can lock up the whels no sweat.



I've never worked on a spider, so I honestly can't say with any certainty, but I suspect that it may not have the same assistance ratio that many modern cars do... but that's conjecture. -Certainly, in last week's VW brake servo failure incident, the driver (who is bigger than me, -I'm just over 6' tall- and stronger too) could NOT lock the wheels. In addition, he had to drop a gear -FAST- throw BOTH feet on the pedal, and pull the handbrake, and STILL couldn't stop in the prescribed distance... locking up the wheels was utterly out of the question. -I know, because I tried the car before I fixed it for him.

ScotcH wrote

While the effort is certainly MUCH greater, it is still perfectly effective to use the brakes while unboosted. Yes, the pedal will feel rock hard, but there is leverage at work, and you're still appliying like pressure (like 1000s of PSI) to the brakes when you stand on the pedal. The clamping force is enough I would think ... but then car makers have obviously made other idiotic enginieering design changes, so perhaps they boosted the brakes to the point that they are no longer fnctional without the vac assist.



See above. -Did the Spider also come in un-boosted versions? -Either lower-spec or earlier/older models?

If so, it's quite probable that the amount servo-assist was only very small, just to 'assist' in the typically-understood sense of the word, rather than being a significant and essential part of the required braking force.

Certainly, I assure you that you could NEVER stop a Lexus with no vacuum in the chamber and a stuck throttle, and sadly neither could any of the people who died in California last August.

KB_S1 wrote

Try manually activating the throttle yourself whilst downshifting or applying a bit of left foot braking whilst accelerating.



I'm heading to work now, so I'll give it a go, -at least I'll TRY. Mine is a DSG, so things are a little tricky, but I'll watch the vac/boost gauge. It's a very reliable indicator of throttle-plate position, so you can actually see the throttle response.

Hmmmm, in a manual, how would you do a 'heel-and-toe' rev-match cog-drop on a corner entrance if the throttle is inoperative... Let me see what it does.

Keith
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

ssltech

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Re: Toyota: -Something smells fishy to me...
« Reply #22 on: January 29, 2010, 09:11:04 am »

Okay, the drive in may have puzzled the people behind me (for the second-described test I made certain that there was nobody for ~400yards back; basically I assured myself that there was plenty of space) but here's what I found:

I tried it two ways; brake-before-throttle and throttle-before-brake... I'll describe brake-before-throttle first:

Brake-before-throttle test: From a ~15MPH slow-roll, I lifted off completely, allowing the car to coast-down, then braked firmly. The brakes weren't 'banged-on', but it was pretty firm. Then as the car was decelerating, I mashed the gas pedal into the floor, and watched the gauge. The throttle went from ~24inHG vacuum to ~8psi boost before I had to let go of the brake pedal... I was almost at a stop.

Conclusion: strong brake pedal operation did not prevent subsequent throttle operation.

Throttle-before-brake test: -At 65MPH on interstate 4, having dropped a little way behind the cars in front, in 48
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

compasspnt

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Re: Toyota: -Something smells fishy to me...
« Reply #23 on: January 29, 2010, 09:49:46 am »

Keith (A), please try to be a bit more specific in your next post...
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Roadster

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Re: Toyota: -Something smells fishy to me...
« Reply #24 on: January 29, 2010, 10:56:52 am »

OK. Here's what we know so far. Brakes will absolutely be useless after you've gone over a cliff.
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Rich
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Jay Kadis

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Re: Toyota: -Something smells fishy to me...
« Reply #25 on: January 29, 2010, 12:01:17 pm »

Roadster wrote on Fri, 29 January 2010 07:56

OK. Here's what we know so far. Brakes will absolutely be useless after you've gone over a cliff.
That's what air brakes are for.

KB_S1

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Re: Toyota: -Something smells fishy to me...
« Reply #26 on: January 29, 2010, 12:57:21 pm »

Hmm, it does get more mysterious.

Heel and Toe in a DBW should be possible as the maximum throttle time should be under 0.5s.

I am surprised by your findings. Of the 3 or 4 DBW cars I have tried they have all got huffy when attempting to left foot brake for experimenting.
I know that a lot of the current batch of DBW cars can manipulate the brakes whilst cornering to act as a stability/traction control device.
I have experienced this and it is very strange feeling.

Perhaps DSG cars in particular allow more overlap as they are programmed to blip throttle on downshifts (as does the manual shift on Nissan's 370Z).
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Roadster

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Re: Toyota: -Something smells fishy to me...
« Reply #27 on: January 29, 2010, 02:02:47 pm »

Jay Kadis wrote on Fri, 29 January 2010 11:01

Roadster wrote on Fri, 29 January 2010 07:56

OK. Here's what we know so far. Brakes will absolutely be useless after you've gone over a cliff.
That's what air brakes are for.



I knew that.  Smile

So good to go, as long as you're in a semi or a bus.
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Rich
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Hallams

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Re: Toyota: -Something smells fishy to me...
« Reply #28 on: January 31, 2010, 07:58:06 pm »

In Melbourne last year there was a well publicized incident of the "unstoppable vehicle syndrome". It involved a 2002 model Ford Explorer.


 http://www.heraldsun.com.au/news/driver-taken-to-hospital-in  -shock-after-miracle-escape-when-cruise-control-jammed-on-ea stern-fwy/story-e6frf7jo-1225810668864
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Chris Hallam.
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Collins

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Re: Toyota: -Something smells fishy to me...
« Reply #29 on: February 01, 2010, 02:10:51 pm »

<    http://www.thetruthaboutcars.com/exclusive-ttac-takes-apart-    both-toyota-gas-pedal-assemblies-denso-unit-looks-cheaper-ru mored-to-be-recalled-too/>

More on the CTS vs. Denso pedals...Toyota now says condensation is the cause?...not floor mats wedged under the accelerator as stated before.  
I'm feeling a software bug here.

I've had my 09 Tacoma since early last year...no issues.
The clutch does have a weird squeak in really hot weather though,
but overall, it's a nice truck. 2.7liter 4cyl., 5 speed, 4WD.

PC
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