I am interested in getting to know how brake-by-wire system works in today's cars. This video shows that the Electronic stability control/program (ESC/ESP) intervenes by braking individual wheels. So I assume a car with ESC is able to brake-by-wire since the electronics brakes by itself and there is no driver braking action present. So how exactly is brake-by-wire done technically? Is there an electric motor attached to every single brake or does it work by hydraulics somehow?

I am involved in a research project that need to use brake-by-wire actually so I am interested in getting to know whether there are ways of how to initiate braking electronically. Do I need to hack the ESC unit to see how the system works or are there general principles used for all ESC systems?


So I may be able to answer myself finally. After some more research I found that there may be some confusion in how the systems are named. What is meant by a brake-by-wire system is usually a braking pedal that is mechanically decoupled from the rest of the braking system. This is different from what I actually need to do, because I do not need any pedal action at all, I just need to control the brakes electronically even on say a "non-brake-by-wire" car.

Real brake-by-wire

This is the system with a decoupled braking pedal. This site mentions two types of systems - electro-hydraulic and electro-mechanical. You can research them there. These systems seem not to be standard on today's car at all, there were few models by Mercedes and Toyota in the past 10 years or so some of which had to go through recall because the system was unreliable.

Braking by electronics (ESC/ESP etc.)

Obviously ESC/ESP can brake individual wheels by itself which is a kind of "brake-by-wire" to me but the industry does not call it so. This "autonomous braking" seems to be done by Hydraulic Control Unit (HCU, sometimes HECU) which is usually common for both ABS and ESC. This unit sits behind the master braking cylinder and has 2 fluid inputs but 4 fluid outputs (one for each of 4 wheels). For normal braking the unit just lets the braking fluid pass through without any intervention. The unit has an electric pump attached to it and if ESC wants to brake a wheel it seems to choose one of the 4 outputs and pressures it by itself (without the pressure input from master cylinder).

The bad thing here for me seems to be that the HCU is a black box in terms of software - it gets inputs from various sensors (wheel speed, yaw rate etc.) and decides by itself which wheel to brake. So in order to be able to initiate a braking action externally one would need to either send input signals that would result in all 4 wheels braking (need to reverse engineer the unit first and I also doubt such signal combination even exists) or hack the electronics to in lower level to pressure all 4 brakes in the same time.

Another solution for me may be to leave the HCU alone and make the system pressurized somehow else. I found there is a part called "electric vacuum (booster) pump", although I have not done deeper research on it yet. Any hits are welcome! :)


It indeed looks like there are ABS/ESC HCU units today that can accept external signals to initiate the braking. These units are used in cars with Autonomous Emergency Braking systems. The TRW EBC460 is said to have AEB feature and seems to have a CAN bus so I suspect it may be able to accept certain can bus messages that initiate the braking.


AFAIK most hybrid vehicles provide means of doing brake-by-wire in order to allow for "regenerative braking". They do not 100% mechanically decouple the brake pedal from the brake system itself (there remains a certain mechanical connection for emergency case) but allow for programmatic braking control for the most braking scenarios (probably all except panical emergency braking). This is why hybrid vehicles are very popular for autonomous car research (Google's Toyota Prius etc.) because they in general are very easy to convert to 100% programmatic control (steering, accelerating, braking). The brake-by-wire really is a "killer feature" of hybrids and 100% electric vehicles for autonomous car research purposes.

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I'm not very familiar with the pure brake by wire setups where the brake pedal is completely decoupled from the hydraulics, or the systems where they're using regenerative "load" style brakes. In a electronically controlled system where your pedal is still coupled to the hydraulics, there is an ABS unit that all the brake lines run through. This has motors in it to allow it to apply hydraulic pressure to the lines running to each wheel to apply the brakes. This can also block your pedal from activating the brakes while it's in control.

As for understanding this system, cars produced in the last few years have a control bus network tying together the various electronic modules.


Some of the modules can act independently but also can be controlled by the ECU. Typically on advanced cars with things like avoidance control, active handling, automatic braking, etc... all of the various sensors are tied in through the CAN and with the ECU or a dedicated module will interpret the data and send out the commands to the ECU/PCM to control throttle, the ABS to apply brakes, etc.... as needed.

I know there are diagnostic tools that will activate these systems for testing. There's also been news recently about research in to "hacking" cars to control them.


Anyone who's worked on car electronics, this isn't really news that you can send commands across the control bus. I very much doubt manufactures would release documentation on how their systems work, but there are groups doing this kind of research. If you have physical access to the car you should be able debug the system to isolate the commands and send new ones.

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  • I see I limited myself to exploring the ESC/ESP HCU only, while you are right that some cars today with front assists have to be able to brake autonomously with all wheels (compared to ESC) so there actually may be a way of how to initiate braking electronically on a stock car. The problem is since front assists are quite new there is not much technical info on how the whole systems work actually in theory on mechanical level. I also am not able to find car parts for newest cars in any catalog, dont know if such catalogs even exist. I will try to do some more research. – Kozuch Jul 10 '15 at 16:50
  • The implementation of automatic avoidance and automatic braking are fairly new for cars, but the underlying technology has been around for some time. en.wikipedia.org/wiki/Chevrolet_Corvette_C5_Z06#Active_Handling This active handling system was introduced in 1998, and while it doesn't "drive for you" as in avoiding things, the computer uses those same sensors, throttle, and braking control systems to stabilize the car when you're driving beyond your ability/skill. The ESC/ESP system in the video seems to be just additional control logic over the same components. – elmerfud Jul 10 '15 at 17:01
  • I fear the "Active Handling" is black box as ESP and can not break whole car (=all 4 wheels simultaneously) for you. What I am looking for is a device that will brake all wheels and can possibly also modulate the braking force. Such device must be present with front assists. – Kozuch Jul 10 '15 at 17:30
  • This is actually exactly how they work, it's just the control logic is different depending on how new/advanced your car is. A 4 channel ABS system can apply braking at each wheel independently or at all wheels at the same time, and can modulate the power applied to the brakes. There are scan tools available that cycle all the ABS solenoids to allow for proper bleeding of the braking system as air will get trapped in the ABS unit. I suspect what you're trying to achieve will involve reverse engineering the control bus commands to issue them. – elmerfud Jul 10 '15 at 17:48
  • Well if a diagnostics can operate the solenoids then full braking could probably be done. Could you recommend a scan/diagnostic tool that can do it? Are the commands sent via standard OBD/CAN? If its standard CAN then the HCU could probably be taken out of the car and connected directly to diagnostics to sniff the commands on the bus. I also thought ABS can only release brakes and not apply them itself. – Kozuch Jul 10 '15 at 18:26

On the systems I am familiar with the ABS module (which is a part under the bonnet through which all of the brake pipes are routed) is able to electronically apply or remove hydraulic pressure to a specific brake line (thus braking or releasing the brake on a specific wheel). These modules are pretty expensive to replace too!

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It uses the hydraulics via the ABS module. It modulates each channel of the ABS block-valves.

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