On August 22, 2012 I posted about our 2007 Nissan Sentra attempting to accelerate out of control in reverse. We had it checked by Nissan mechanics and they found nothing wrong or in their log. Yep your modern car has a log file in it.
I was even shown that slamming the brakes automatically overrides the accelerator even if your foot is on it. That's a good safety and so I figured if the problem ever resurfaced, we would still be safe - just slam the brakes.
Yesterday, 9/11/14, our 2007 Nissan Sentra accelerated all by itself! It went into runaway mode again but this time in drive!
We were in a McDonalds drive through and had just placed our order. The car was in park. The AC was on but not on Max AC. I put my foot on the brake and shifted to Drive. The car lurched forward but the brake held it. My wife Kelly exclaimed, "what are you doing?"
This time I was ready except I had no camera or smartphone. Damn!
I said to Kelly, "Look! There it is! Look at my foot, it's not touching the gas at all - just the brake. Look at the tach. It's racing up to 2500 and coming back down. It's in drive."
Kelly asked, "What do we do?"
I said, "Watch this!" I slammed the brake to the floor as hard as I could. Everything went back to normal. I said, "see that mechanic was right. Just slam it to the floor (the brake)."
"Are we safe," asked Kelly.
I answered, "I guess so. If anything happens we just slam the brakes. Nissan will never find this. It wouldn't do any good to take it in; they couldn't find it the last time."
"You sure we'll be okay", asked Kelly.
"I don't know but I'm putting it online so that it's at least documented somewhere if anything happens. At least we have a brake override to kill it?"
Kelly asked, "do you think Nissan knew about this?"
I answered, I don't know but at least they planned for it. I wonder if they found it in testing and added the override because they couldn't find the bug causing it?"
-- end of story -
I think it likely has to do with the air conditioning. The engine has to compensate when the air conditioner kicks on or stall. It is likely that the software has an endless loop that occurs when the AC kicks on under just the right circumstances. Or it could be an integrator problem. When trying to regulate speed electronically, oftentimes a PID loop is used.
PID stands for Proportional Integral Derivative. To put it simply, an equation would look at the speed command for the engine, see how far off it is which is basically the proportion, and then use integration to consider prior responses and use derivative to offset the integration and proportion. That is an extremely simplified explanation of PID. If you understand mathematics pretty well, look up PID equation on Wolfram Alpha to help understand the relationship between the P the I and the D.
The problem that can occur is called integral windup. What happens is that each new addition to integral becomes progressively larger until derivative is no longer able to offset the integral. To put it simply, it's like dividing one by zero. Of course it is undefined but if you instead divide by a number as close to zero as you can possibly get, such as 10 raised to the minus one million, then the result of the equation is enormous. Just imagine that number being applied to the accelerator of your car!
I used to always include a safety clamp in software for the PID result just in case of integral windup. Some people frown on such practice and are of the opinion that if the PID loop is setup properly then there's no need for a clamp. I strongly suspect that the software used in our 2007 Nissan Sentra and in many other cares is the result of such debates and probably lacks and software clamp for the PID loop.
At least Nissan has an override, although scary. I wonder about the other car companies...
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