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A link posted in this answer describes a potential engineering flaw in GM ignition switches that can result in a car losing power while driving, and accidents where airbags do not deploy.

I drive a 97 Chevy Blazer, and the steering wheel will lock if the wheel is turned while the key is not in the ignition...it's an anti-theft feature. I'm not sure how it would respond if the key was in the ignition and the ignition switch failed...but obviously I'd want to stop the car!

So if I were in this situation: key in the ignition position, driving 50 mph, and the switch fails...what systems remain to stop the car?

  • Will steering work, albeit stiff without the pump assistance?
  • Will braking work...at all?
  • Will shifting work (automatic, not manual)?

I'm sure the emergency brake will work...but at 50 mph I suspect it'll need some help. If down-shifting works, it might help slow the car.

So the question is, what should someone do in this situation (car is moving at speed, ignition switch fails)?

Edit: I realize this question isn't asking about a repair or maintenance...but I believe this is the best audience for this question.

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    Step on the brakes, just like how you always stop a car. Is there a maintenance or repair issue here? – cory Dec 14 '16 at 18:28
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    I don't see anything very special in the "failed ignition switch" scenario here. You could get exactly the same situation by running out of fuel, timing belt failure (been there, done that one!), etc, etc. – alephzero Dec 14 '16 at 22:28
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The brakes should still work, but without power assist so they take more force. Same for steering. I would shift to N and then brake while heading to the shoulder. I had a K1500 Blazer of about this vintage, and the steering wheel would not lock until the key was rotated all the way back to the "Lock" position, where you can take the key out. If the failure is just electrical, it shouldn't lock the steering wheel.

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    The brakes still have 1-3 assisted pumps after the engine dies. You can try this after shutting off your engine. – rpmerf Dec 14 '16 at 17:19
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In nearly all situations, the way you stop if the ignition switch is the way you stop if the ignition switch is working: you stop pressing the gas, and hit the brakes. It's literally that easy.

Power steering and power breaks will continue to work for a little while because they have built up a charge (as rpmerf points out, you usually get about 1-3 good breaks in before the hydraulics discharge). After they are discharged, you can still steer and break, but you will not be assisted by the hydraulics. This means you'll have to push on the brake harder. This isn't the end of the world, though it can be a bit of a shock at first. My dad's car from the '80s does not have power steering or power brakes, so he operates in this mode all the time!

The one unusual case would be if the switch somehow failed in a way which engaged the anti-theft feature you describe. This is very different from the GM switch issue, because the failure you are interested in is mechanical. In most cases, you're driving straight, and the same idea applies. Stop driving, start breaking... problem solved. However, if it fails when you are on a sharp enough turn, the anti-theft feature may engage, preventing you from driving straight afterwards.

In this case, the story is exactly the same: stop driving, start braking. The only difference is now it is more likely that you are pointed in a direction which is unsafe, so it would be wise to stop driving faster, and start braking faster.

Being a mechanical system, it is unlikely to fail in this situation mid-drive. I would venture a guess that, if you are worried about the probability of this occurring, you should also be checking your tire pressures before every drive, because improperly inflated tires are far more likely to cause you issues, statistically speaking.

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The brake booster will support a few pumps so use 'em sparingly if in such a situation. After that it'll get harder depending on the weight of your car. An MPV will barely slow down without booster, but a standard grocery getter doesn't take much muscle. The steering will get hard only when the car is slowed down to <15km/h. You're probably out of most danger at that point. A rolling car is rather easy to steer.

I can't predict the behaviour of the automatic transmission, but it doesn't matter anyway. Your engine is shut off so you can't use your transmission for driving. But it won't lock or something. Downshifting may help a little in a manual diesel, in an automatic, probably not so much as the engine does not have a fixed connection with the wheels.

If there's no other option, you can use the hand/parking/emergency brake. Keep in mind that the hand brake applies to the rear wheels. Applying it fully at higher speeds will get your car in an uncontrollable spin.

I don't recommend it, but you can try out what happens by driving to an empty parking place and switching off the ignition while driving sub 20km/h. Make sure there's enough space to stop if it doesn't work out.

Your steering lock will apply only after you've taken the keyout of its place. Keycard cars may engage the lock earlier, but not after engine stall. You don't have to be afraid for that while driving.

Sources: I drove various cars that don't have a brakebooster and power steering and weigh about 1000kgs. Even parking is easy to do i think, but i'm a guy in his twenties.

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    I found the key lock engages when turning off the engine before removing key. However, after stalling car while moving I know the rest of this is more or less correct. – Joshua Dec 14 '16 at 19:54
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    Cars designed for unassisted braking and steering have far greater mechanical advantage than those designed with assist. – Mark Dec 14 '16 at 22:02
  • @Joshua keyless entry cars and such may engage the lock already when you shut off the engine. Stalling is another thing though, the engine knows the difference. Engineers take lots of failure modes in account when designing a car, and this sitation is absolutely considered. In their design they try to minimize (lethal) damage as much as possible in all possible situations.(called failure modes) – Bart Dec 15 '16 at 7:20
  • @Mark Thats certainly true. My car is easy to steered even at standstill. A car with inactive power steering is easy to steer only at >15km/h or so. Braking power doesn't differ very much in my experience. Current brakes are far more effective so even unboosted they still give fair stopping power. Again, the weight of the car heavily influences how well you can handle it when those systems fail. – Bart Dec 15 '16 at 7:25

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