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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?

banncee
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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.

Pᴀᴜʟsᴛᴇʀ2
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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.

F Dryer
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