My 2011 Audi A4 B8 has cruise control and although not adaptive, it does engage the brakes if I set a cruising speed well below my current speed or if the car picks up speed during descents.

The car has ABS, ESP and I think also EBD. As far as I know, the brake pressure is only created mechanically by the driver pushing the pedal, and amplified by the brake booster. The pressure is then handled by the aforementioned systems. How is it then, that the car can brake on its own without me touching the pedal?

2 Answers 2


The original ABS systems only modulated pedal pressure. Some systems only worked on two of four wheels.

"Better" or later systems could modulate all four wheels. Even more developed was the ability to modulate each wheel independently.

Modern systems have an electric pump, which pressurizes a reservoir. Sometimes you can hear them whine for a few seconds on startup (modern Honda Civic comes to mind).

These systems can compensate for "too much" as well as "not enough" pedal pressure. Through sophisticated electronics, they can possibly even do a variety of other tasks:

  • Retard cruise control or steep slope descents
  • Emergency braking triggered by proximity radar
  • Traction control to redirect torque across a differential away from the slipping wheel
  • Dynamic handling assist that can figure out which wheel to drag in a corner and help steer/pivot or prevent spinout.
  • Automated parallel parking assist
  • Launch control

These are just a few off the top of my head. I'm certain there are several more, and more to come.

While you won't find Lauch Control in all but hyper sports cars, some Volvos have Dynamic Stability Control (they may call it something slightly different).

I'm not current with BMW and Mercedes offerings, but I do know that they offer a menu of features beyond "anti-lock braking" on the higher-end [more expensive] vehicles.

Even my lowly 2018 Toyota Sienna van (don't get me wrong; I love it) has emergency braking, traction control, and some other high end features which are effected with the ABS "pump".


The abs system can provide the function to engage the brakes.

  • The diagram I looked at showed the ABS pump cycling a valve to open or close a chamber of pressure before the slave sylinder. Is the pump also providing brake pressure then? I'd be happy to read any articles if you know of one that explains it - I haven't found any
    – Erik
    Commented Jan 12, 2019 at 15:55
  • 1
    Hmm, wonder what "pump" stands for? If the other systems like ESP and EBD call for braking at one or more wheels then that needs to come from somewhere...
    – Solar Mike
    Commented Jan 12, 2019 at 16:05
  • well I suppose it the pump provides pressure for braking, but I'm not sure so I'm asking you. As far as I've understood, in traditional abs systems the pressure is used to cycle a valve, not provide brake pressure.
    – Erik
    Commented Jan 12, 2019 at 16:13
  • The valve cycles to allow or prevent the pressure getting to that particular brake...
    – Solar Mike
    Commented Jan 12, 2019 at 16:22
  • Yes, that I understood. But the pressure created by the pump is, as far as I understand, not used for anything else than cycling the valve in traditional abs systems. So my question is then, does the pressure from the pump also provide braking pressure in my car and other fairly new models?
    – Erik
    Commented Jan 12, 2019 at 16:24

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