It’s in the code
VW has an engine cut out feature if both the brake and accelerator are pushed at the same time. How hard is that to engineer in software?
OK Gang,as the resident auto mechanic here ( it no secret,just ignored by me as a rule)let me explain the analog ( electro mechanical) side of this mess.The sensors need to use a physical state to obtain a reading in terms of a voltage to allow the computer to decide what to do.This is accomplished in the case of the gas pedal in the very same way your gas guage( petrol for you Brit guys)takes a level reading.A scale grid has a movable wire that runs across this scale and increases or decreases the voltage sent to the computer(In millivolts).Your pedal is connected to the movable wire side. A delay constant is incorperated into the firmware( non variable software) so you cant get instantanious maximum acceleration.Prior to the drive by wire change this function was accomplished with a similiar device called the Throttle position sensor and was affixed to the throttle blade( air valve opening) where the air entered the engine.This design had a fail safe in the sense that even if the sensor went high or low voltage wise ,if you removed your foot from the pedal it would return to a zero point( reference voltage).All sensors have this zero point at startup ( cold engine).With the drive by wire system this fail safe is removed if dirt or moisture contamination or mechanical failure occurs inside the scale grid/movable wire component( along with IC component failure between the scale and output wire to the computer).The computer will continue to SEE whatever voltage was last read and continue to maintain this reading till all Hell breaks loose.
Hope this helps finish the job Drew started out explaning. Or maybe I just added fuel to the fire.
Most likely the latter.Hey OCM: is your Toyota truck a Tundra, or a Taco/Coma?
My 2000 Isuzu Rodeo did this to me except that it just refused to increase the engine speed above 1200RPM(32mph). Kind of embarassing when you are tootling south on 495 outside of DC. Drove it back to my mechanic and he replaced a sensor in the throttle assembly.
Posted by emdfl
02/11/2010 at 08:30 PM
Rich K in europe most ecu run cars have a fail safe that if something is too far out of normal it goes into “limp home” mode that gives a guess at the timing and ignition, i have not had this to fix on a fly by wire throttle setting vehicle, some cars (BMW for one ) kill the motor when the airbags fire, I would guess it would set a safe but drivable revs setting. On some high end VW group products it will do this if you disconnect one of the sensorrs even when the key is out of the ignition! I did a cam belt change on a VR6 and it was flat afterwards, no go, eventually after a $200 bill at the dealers they reset the ecu and off it went. The thing no programmer can predict is the greatest analog device ever, man, we do things that cannot be predicted and that defy logic, perhaps if an impatient driver taps the pedal 168 times it does something that the ecu thinks is a request for some test function (or something equally bizarre) as oldcatman said put it in neutral, maybe it is a Darwin test, I’ve had cars stick on full throttle, mostly I was worried about damaging the motor, not crashing was never an issue, neither was braking except to keep the revs down while finding the ign key.
ChrisPS most cars get recalls, GM and Ford got hammered by the idiot Nader, I would suspect that both branches of Obamamotors could use some leverage against the worlds biggest car manufacturer
Posted by Chris Edwards
02/11/2010 at 09:32 PM
Chris - most newer cars have a rev limiter in them, so you can’t prang the engine by over-revving it. So in an emergency, don’t worry about the engine. Actually, in an emergency, don’t worry about the engine even if it doesn’t have a rev limiter. Your safety is more important.
My old 97 Satrun has Limp Mode too. It was triggering all the time until I replaced the oxygen sensors last year.
Rich - thanks! You’re saying the same thing I did, from a mechanical perspective. Bad or dirty sensors mess things up. I didn’t know that the pedal sensor readings were buffered but that makes sense, it would allow a sigma delta function by reading the whole buffer (take the average of the last X readings and use that value ... this could be your throttle delay function from a software perspective.) Things are far more complex under the hood than they were back in the days of the mighty Thermoquad and a soft spring in the accelerator pump.
OCM - most people aren’t having a problem. Just a few. But that’s what recalls are all about hopefully. Fix a problem on your vehicle before it happens.
With what Rich wrote, I’m wondering if there isn’t some common factor in all these situations. Like the car was a filthy pit, full of grit and ciggy smoke or whatever. Or the drivers were all tap dancers blipping the pedal to the beat until it wore out 14 years ahead of schedule.
Drew, I know that, since the late 1960s anything with Bosch electrics had a limiter but they are designed for under load, free running at limiter revs will throw a rod if the motor is not 100%, neutral is kind of like that! the point was that a sticking gas pedal is no real big problem for safety, more of an inconvenience, if you have reasonable ability to operate the machine then you will sort it. I dont see how a dirty position sensor will do that, more likely to cause a drop in revs, the reports I have read mostly say the car surged, an increase and the computer has no fault codes stored, I would expect any unusual sensor info to have produced a code.
On the throttle position sensor, it gives a theoretical max opening, the software limits the rate of opening based on all sorts of parameters, one of which is government emission and economy measurement points, that is why chipping can give significant accelleration improvements in non turbo cars. Toyota have been working on this for a while, most of their cars have a cable and the transducer is under the hood so it must be an enigma!Posted by Chris Edwards
02/11/2010 at 10:58 PM
This is just one of so many things that go wrong with ECM controlled engines. Yes, most cars use Limp Mode for total sensor fail ( one or multiple).There aint time or space to even start this conversation.Let just hope the new batch of pedals rectify the issue.
NOW, back to our regular scheduled broadcast of moonbattery and silly pet tricks,Cue Peiper!
