The Therac-25 Incident (2021)

There was a news story from Norway last year where a car allegedly accelerated by itself, causing the car to fall off the second floor of a parking garage and kill the driver.

There are plenty of "car allegedly accelerated by itself" incidents, and usually the root cause is the driver mistakenly pressing the accelerator pedal when they think they're pressing the brake pedal. And then swearing blind afterwards that they were braking as hard as they possibly could but the car kept surging forwards.

Time and time again the introduction of electronic throttle control has spiked the number of reported unintended acceleration incidents.

There's a chart here that shows it clearly for Toyota's rollout:

https://www.embedded.com/unintended-acceleration-and-other-e...

That chart is not clear.

The Camry, the Solara, and the RAV4 are all the exact same engine hardware and software, at nearly all times. Especially the Solara, which is just a Camry with a shinier body. It uses an identical engine, throttle body, and ECU flash and is even considered a "Camry Solara".

That Camry "Unintended accelerations" jumped while Solara didn't means that it isn't the hardware. Instead, they all started at the same time, about 2002. Similarly, the Scion TC is also literally the exact same hardware, software, and throttle body as a Camry. The entire Scion line is just Toyota software and hardware in a different body shell.

Indeed, if you look at the Corolla, the jump in unintended acceleration cases start with mechanical throttle bodies still the norm, and do not change with the switch to electronic throttle control.

IMO this graphic handily shows how the media affects average people. The media went on a large blitz about how Toyota was unsafe now that they had electronic throttle bodies, and so owners of those cars complained, but the average consumer does not realize that the Solara, Camry, and Scion TC are all identical vehicles under the body shell and don't realize that they "should" also be complaining about those cars if the problem was actually caused by the electronic throttle body code or design.

Important note: People who report unintended acceleration events like this almost always say that the brakes didn't stop it. That seems.... hard to believe. The brakes on all Toyotas are fully hydraulic and cannot fail electronically. All toyota vehicles in that chart (maybe excluding some Tacomas and the top line Lexus model) have brake capability far exceeding their engine power. A V6 Camry can be at wide open throttle but hard application of the brakes would still overpower that engine with no problem.

Interestingly the NHTSA's opinion is that the Toyota models in the graphic ARE defective: They allow faulty or improperly installed weather mats to interfere with the pedals. Toyota also believed this take, as they kept their handling of fixing this defect off the books, and changed the pedal positioning in their newer models. They eventually fined Toyota over a billion dollars for their handling of this situation, and claimed there was another "sticky pedals" problem that they were covering up.

> The brakes on all Toyotas are fully hydraulic

No, most are hydraulic with vacuum boost.

If you aren't expecting it, the increased force required to drive the brakes hydraulically from the pedal without boost assist is significant and can be surprising. I assume most folks haven't had an engine fail going downhill, but for a large pickup I was standing on the brake pedal and had to push my leg down by pulling on the steering wheel to stop it.

> That seems.... hard to believe

Anyhow, the vacuum booster is driven from the engine airflow. At wide open throttle the vacuum available to the booster is minimal because the restriction is as open as it can be.

You can test how it feels by rolling at a medium speed in a parking lot, shifting into neutral, and killing the engine. The vacuum reservoir may provide you one or two brake pumps and then you're on your own.

Another test: after shutting down your car after a regular drive, try depressing the brake to the floor a few times. You'll soon exhaust the boost reservoir and the brake pedal will become very stiff now that it's fallen back to full hydraulic operation. In this condition if you hold the pedal halfway down when you start the car you'll feel the brake boost kick in soon as the engine starts.

Note that in many engines, vacuum is provided by a separate vacuum pump, which is driven by the rotation of the engine or rarely by an electric motor. This is always true for a diesel engine (because there is no intake manifold vacuum) and many petrol engines too (if they are direct injection, they also may not have sufficient intake manifold vacuum).

Even if you have a total engine failure, if you're going down a hill, if you keep it in gear then the motion will be turning the engine still, which will be generating vacuum regardless of whether that is by a separate vacuum pump or the intake manifold (unless the throttle valve also fails or you get a gaping hole in the side of your intake, but that seems unlikely).

So braking should still keep working as normal, as long as you keep it in gear and don't disengage the clutch.

TIL! Haven't worked with anything that new.

Note that Toyota only introduced vacuum pumps in 2017, well after the changeover to electronic throttles. All plotted unintended acceleration complaints would've been without a separate vacuum pump.