My friend was recently on a road trip in a 2009 Acura TL (3.5L), when the check battery light came on. About ten minutes later, light came on again and car fully 'shut down', going 60MPH on the road. He said he could not press or pump the brakes at all, and steering wouldn't turn. He skidded nearly 40 yards along the road, and came to rest half way on/off the road. It sounded like all 4 wheels locked up completely.

After a tow and repair, it was the alternator that had failed completely (serpentine belt had to be replaced as well).

My question is: why did the brakes not work at all? Would it not have felt like non-power brakes, which require significant effort to depress and operate?

OR, is this a case where the car has 'brake-by-wire', where the there is no mechanical linkage between brake pedal and master cylinder, rather only a sensor that detects angle of pedal and communicates electronically to the braking system? If this is indeed the case, how do manufacturers account for alternator failure while the vehicle is in motion?

  • So did the alternator fail first, followed by the battery? Why would anything work after that with no enrrgy source???
    – Solar Mike
    Aug 6, 2021 at 17:20
  • I'm not aware of any motor vehicles sold in the USA including Tesla that has a "brake by wire" system. I am aware of electric pump driven power steering and one example of that is Mini Cooper. But even with a total pump failure you can still steer since there is still a mechanical linkage. Your question, however, doesn't make sense since you say "skidded nearly 40 yards along the road". How would the car skid if the brakes were not working?
    – jwh20
    Aug 6, 2021 at 17:49
  • @SolarMike I don’t understand what you are talking about in your comment. How is it helping the OP?
    – HandyHowie
    Aug 7, 2021 at 8:04
  • @HandyHowie how are you helping? I wanted to know if there are any other energy sources and elicit the OP's understanding of the system....
    – Solar Mike
    Aug 7, 2021 at 8:44
  • @Solar Mike Yes, that is my guess. He said the battery not charging light came on, he dismissed it. 10-15 minutes later it came on again, and the car shut down completely. My guess is the alternator died when the light came on the first time, and then the battery drained out.
    – banncee
    Aug 7, 2021 at 16:05

2 Answers 2


The brakes have a vacuum assist. When the engine shut down, there's no vacuum (or the vacuum gets used up quickly). Once the vacuum is gone from the system, it becomes completely manual, which requires much more foot pressure on the brake pedal to make them work ... yet, they will still work. Again, it just takes more pressure to make it happen.

There are few cars today which have "brake by wire" setups, and I believe for the most part, those which do have fail safes if the braking system were to fail. There would still be braking, it just wouldn't be what it is when the brakes are fully functional.

  • He said the brakes were not working as in he could not control them. The skid tells me they were locked up. Just trying to figure out why the brakes couldn't be pumped / used minimally.
    – banncee
    Aug 7, 2021 at 16:10
  • 1
    @banncee - When people get into "panic mode" because their vehicle is failing, it often seems things don't work at all when they don't work as they normally do, such as brakes. If when the car is failing you now have to apply 9x the force to get the brakes to work, then it can seem like they have completely failed. This is what it seems to me has happened with your friend. I'm not discounting their situation, it's just that if brakes fail, they fail (usually) with the pedal in the down position or where there's no resistance at all on the pedal. It sounds from your description, they were hard. Aug 7, 2021 at 16:55
  • Reminds me of a time when I adjusted the steering wheel tilt while driving on the highway; the fellow with me ( an engineer) almost had a heart attack thinking the car could no longer be steered. Aug 10, 2021 at 1:07

Whether electric or hydraulic power steering, once the engine dies, electric power steering relies on the battery until it dies then steering becomes difficult but not impossible. Hydraulic power steering pumps runs off the drive belt so if the engine dies, power assist goes away but steering is still possible. At speed, steering will feel difficult. All low speed, steering will be very difficult but as long as the front wheels turn, steering is possible but requires strong arms.

When the engine dies, virtually every vehicle using power assisted brakes have a reserve for several power assisted braking attempts to stop a vehicle. The power brake boost unit stores engine vacuum all the time the engine's running. This vacuum drives a large diaphragm thru a valve connected to the brake pedal to provide power assisted braking. Once the engine stops while driving, engine vacuum stops but the reserve vacuum stored in the power brake unit will provide several power assisted braking attempts. Once this reserve vacuum is exhausted like pumping the brake pedal, the pedal firms up and may feel as if brakes are lost when it's not. The firm pedal is the hydraulic brake system, always there, applying pressure to brakes. Locking up brakes means the basic brake system was working as it always was and stopped the vehicle albeit in a longer distance. Every vehicle uses this combination of hydraulics and power vacuum boost unit to achieve power assisted brakes. Lose vacuum and brakes are still there, just needing stronger leg muscles to stop.

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