As been said, modern car electrical systems (and databusses) are
INCREDIBLY robust. You really cant "crash" them, no matter what you do. Not to mention, modern automotive electrical systems are incredibly clean as well. Im not sure why everyone thinks that modern car electrical systems are still the "dirty nightmare" that they might have been back in the 1970's when you had points opening and closing etc... People need to remember that pretty much everything on cars today is driven by microcontrollers and computers. The automotive engineers know this and have made HUGE strides in making modern car electrical systems perfectly safe for electronics of all types. Alternators are all solid-state regulated and smoothed, etc.
For example, all of the GM trucks and SUV's have had something called "regulated voltage control" since 2005. Basically there is a module dedicated JUST to control efficiency of the electrical system, the alternator, and how the battery is charged. There is a module that has an inductive (?) clamp/ring around both the battery cables and the cable coming out of the alternator. That module continuously monitors how much current the truck electrical devices are drawing at that second, it records battery temperature, battery health, battery sulfation info, what voltage the battery dropped to at last startup and how long the vehicle was sitting, and if it senses that any of the battery cells are getting weak/wearing out, it changes the whole charging rates and manages the electrical system loads differently, and exercises the battery carefully to preserve the life left in it. Alternator charging is all solid state controlled by the ECM (the main engine control module) with a PWM signal, and a databus signal.
I dont know much about other vehicles, but GM has used several different databusses over the years. In pre-2005ish vehicles, Class 2 for low-speed body communications (instrument cluster, radio, HVAC, amplifier, windows, doors, lights etc), which is a single-wire databus, common chassis ground. I think it floats around 3 volts and is pulled up to 7volts for a "1" and pulled to chassis-ground for a "0". 10.4kbps, pretty slow. Tons of error checking, priority headers, etc. You can even do cool things like send ASCII text to the message-center display in the instrument cluster. For ECM-TCM (trans control module) communications, because those were higher priority real-time messages they used an isolated J1939 CAN bus, twisted pair, 250k. In later vehicles, they went to their proprietory GMLAN databus in two forms. Low speed GMLAN, 33.3kbps, technically a "single-wire CAN", which otherwise works the same as the old Class 2 bus, common chassis ground and then 1 "databus" wire. Then high speed GMLAN for powertrain comms, 2 wire CAN, twisted pair, 500kbps, 120ohm terminating resistors at either end. If you were to send an unrecognized/misformatted/totally whacked-out command, the modules would simply flag the message as "jibberish", ignore it, and go about their business. They will only follow a command unless it was directly addressed to them. And even then, anything "safety critical" physically CANT be commanded by a databus command...the modules just wont do it because they only understand and will only process specific messages that have been programmed into their software from the factory, no matter what you tell them. You cant just "hack the bus" and send a data message to the airbag control module and make it deploy the airbags....it just doesnt work that way.
Yes, everything is databus controlled, from the radio to the amplifier to the ABS module etc. But you have to remember, all of those modules are still "standalone" in their own form. Automotive databusses are not designed with a central node and while all modules share info over the databus,
they are NOT dependent on eachother to function.
Example A. If you cut the databus wire going to the airbag control module and you are in an accident, your airbags will still deploy just fine, because all of the sensors are hardwired directly to the airbag control module. Now what WONT happen (because you cut the databus wire, or shorted it) is that after you crash and the airbags deploy, the airbag module wont be able to command the doors to power-unlock (obviously you can just flip the manual door lock lever, just like you did in the days before power locks), it wont be able to command the interior lights to turn on, the 4-way lights to flash, and it wont be able to send a data message to the OnStar module to automatically summon help "SOS"....but so what...
Same thing with ABS. Again, its a standalone controller. Yes, it communicates with the ECM (to request torque reduction during a stability-control event), instrument cluster (to turn on the ABS warning lights if theres a problem), etc. But if you were to severe its ties with the rest of the computers in the truck....you'll still have power brakes (again, its a mechanical system/brake-booster), you'll even still have ABS....only thing you will lose is traction control and stability control. But again...so what...most cars older than 10 years or so dont even have stability/traction control to begin with.
Even if you cut ALL ELECTRICAL POWER going to the ABS module, you'll still have power-assisted brakes. You just wont have ABS....
If you cut the databus wire going to the engine computer once the engine is running, it will keep running just fine....again....all critical sensors (such as the drive-by-wire throttle pedal) ARE HARDWIRED DIRECTLY TO THE ECM. The things that will happen is you'll lose your instruments (tach, speedo, coolant temp, fuel level, oil pressure), and if you shut the engine off, it wont restart until the databus wire is back online because it uses the databus to exchange anti-theft passwords with the transponder in the key.
As said above...I cant reiterate enough how incredibly robust/failsafe automotive computer systems are. Modern car electronics are designed to operate in harsh environments, and they are designed to cope with and isolate direct short-circuits, loose connections, etc...
And second of all, saying you could lose things like power steering and brakes by having a PICAXE do something funky on the databus is flat out ridiculous, no offense. Power steering is a mechanical system driven by a hydraulic belt-driven pump on the engine, and power-assisted brakes are either hydraulically boosted by the mechanical-belt-driven power steering pump, or manifold vacuum off the engine. So unless a PICAXE command messing with the databus can make your power steering hoses spring a leak, you dont have anything to worry about as far as a total databus meltdown causing a "dangerous loss of control" under ANY circumstances.
Ill gladly take a video of me directly shorting any Class 2 or CANbus wire to ground OR to battery positive voltage, just to prove that nothing happens, and the databus is plenty capable of isolating and ignoring any severe problems...ill do this with the vehicle safely parked in the driveway of course though.
I was helping a friend of mine fix the harness going to his instrument cluster...on the 24 pin connector, the battery +12v wire is directly above the Class 2 databus wire. We accidentally put the pins back in the wrong place when we were repairing the connector. Started the truck and the instrument cluster wouldnt turn on/boot up (because it wasnt getting a data message to "wake up!"). Basically it was backfeeding 12volts directly onto the Class 2 bus. I plugged in my scan tool to figure out why the cluster wasnt "waking up" and found that a bunch of modules were setting "loss of communications with instrument cluster" trouble codes, and had then isolated themselves from the bus. I rechecked the wiring, found our mistake, swapped the pins, cycled the ignition once, and the cluster booted right up....I cleared all of the "loss of class 2 comms" codes, and havent had any problems with anything else since...that was 30,000 miles ago. No magic smoke, no fires, no expensive burned-up computers.... This is the 21st century. Cars have gotten a lot smarter than what they used to be.......
for what its worth....
Ben