Charge needed to disable a car ?

Nope. If you have points and a coil I doubt the EMP shot from a nuke would stop the car. It might blow the radio but that is about it.
I bet the electronics in a new car are a lot tougher than they would have you believe too. I doubt getting hit by lightning would stop one. I have seen them hold up to the ignition cables getting away and sparking around the engine compartment.

I'd agree, but I've seen car disabling devices that will stop a car when it is driven over. A wire sticks up and you can see a spark as the car passes over and is disabled. How this is accomplished I don't know.
My best guess is a high voltage capacitor is discharged. The very high current causes a high voltage gradient around the points of contact that knocks out the computer.
John

Possible .............yes Practical........... not yet.
The amount of voltage would be staggering. Lightning is what your talking about, or an huge welding arc.

Probably not, if you want to stop a car check out the stuff the Iraq folks are turning in for $$, remember this stuff was in their house.

Since the tires are farily good insulators, the car would become a charged cap. It wouldn't stop the car, but you might get a charge out of it when you touched the door handle.

It is not voltage that shuts off a car. It is current AND where that current passes. For example, it might be possible to run hundreds or thousands of amps across the chassis and therefore cause a few milliamps to pass through a critical sensor or controller. But then that would be unique to each model and unique to where the current enters and leaves the vehicle.
This is the point. To cause a shutdown, critical function must have both an incoming and outgoing current path. Current - not voltage - is the critical variable. High voltage may or may not be necessary to create that current. But how current passes through a critical component determines whether that vehicle can be halted.
Many think of electricity as if it was a wave crashing on the beach. Apply a big voltage (a big wave) and that voltage wave overwhelms the circuitry. Electricity does not work that way as was even taught in elementary school science. First electricity must have a complete path - both an incoming and outgoing path though the circuitry. As the current increases equally and everywhere in that path, only then does a control circuit become overwhelmed and fail.
Furthermore, auto manufacturers spend significant time planning, testing and modifying so that this will not occur. Vehicles are even placed in anachoeic chambers to be tested with high concentrations of radio waves at every frequency. So yes, it is possible. But that means a unique weakness must be discovered for that unique model.
Such problems were more common with mid and late 1970s America cars. But then 'car guys' were no longer designing the product. Back then, bean counters would often eliminate such testing and design reviews to cut costs. 1979 American vehicles were some of the highest failure products in automotive history. The 1980s is a story of car guys again taking over vehicle design - which is why, for example, Ford went back to profits in the early 1990s.
S> A friend said that a high voltage charge applied to the body would

Shoot a 12 gauge thru the radiator.
?????

Coincidentally, I was at the top of Mount Wellington the other day, which overlooks Hobart, and rises to 1200m. There's a road to the top (I'm no mountain climber). In the shelter (from the unbelievably strong wind) there were warnings that the high radiation intensities from transmitters located at the peak were enough to prevent the proper operation of some cars. In particular, they could stop car security systems from recognising the presence of the ignition key. They suggested moving the car a bit if this happened (presumably to exploit the intereference patterns).
But as for the original question, since the electronics are grounded to the body, I wouldn't have thought that applying a charge to the body would have any effect, and certainly nothing more than a transient effect.
Sylvia.
Sylvia.

In Needham, MA, there are three 1300ft transmitter towers in like a triganular area. There are many businesses and homes beneath them. If you live or work in the area, you are advised, when you buy a security system, to tell the provider of your location. They provide a "special" system which will operate in the vicinity of the towers. I learned about that too late. When it's humid or the ground is wet, I have to put my remote up agains my driver's side window in order to activate/deactivate the security system. On some day, it doesn't work at all. In non-tower areas, my remote has about a 50 ft range.
When the early ABS were installed, in some instances they would be activated by radio transmitters at the mouths of tunnels. Nice! Why were the transmitters there? Dunno!
I heard that some police depts were working on car disabling systems which consist of laying a wire in the path of a car under chase. The high voltage is supposed to zap the engine module. Some day, you will see, the engine modules will have the capability of being disabled by remote control. Boy will the hackers have fun with that!
And don't tell me the hackers can't do it. They have broken virtually all systems to date.
Al
Al

That's a somewhat different scenario. Because the underneath of the car is not usually covered in metal, it is conceivable that an electromagnetic field can be created that would be enough to damage the electronics.
I think 'hackers' have acquired an unreasonable reputation of being able to break into any system. In practice most attacks that do not require the victim's unwitting cooperation have been variations on a theme - exploitation of the buffer overflow programming error (I call it 'error' here - what I call it in private doesn't bear repeating in polite society).
There's no good reason a car's security system shouldn't be immune to hacking attacks. Of course, that doesn't tell us it will be. Depends whether it's developed on the cheap.
Sylvia.

If I was an urban legend buff I would say it was to detonate RF activated bombs before you get to the tunnel but the pragmatist in me suspects it is a repeater for emergency vehicles in the tunnel.

If the government is the intruder, is this any less a "hack"?

Read an earlier post. A principle taught in elementary school science about how electricity works: Apply current to the metal body or to metal that covers the entire underside of a car. Now we have the incoming path. But where is the outgoing path? No outgoing path - no current. No current, then no problem.
To be completely immune to hacker attacks, then a security system must be so secure that even an auto mechanic cannot repair it. So secure that a locksmith cannot enter the car when the security system fails. What is the purpose of a security system? To keep the honest people honest.
Sylvia Else wrote:

I think you'll have trouble explaining how antennas work with that theory, or maybe even capacitors. The above principle is used very often, but in electrostatics or e/m field situations it may not hold.
j

Yes, this aspect bothered me too.
If you suddenly dump a collection of electrons onto one part of a car's metal surface, there is a net electric field between that point and every other point on the car. This field accelerates the electrons which start spreading out along all the conducting paths. Eventually, they have distributed themselves so that there is no (additional) electric field along any conducting path. There will be a period of damped oscillation until this state is reached.
Prior to the equilibrium state there are currents travelling along conducting paths. Where the end of a path is not conducting a charge builds up, which ultimate stops the flow (and reverses it, hence the osciallations). But in the mean time, the charge at that point (say somewhere in the electronic control system) may be high enough to break the insulation in a component such as an IGFET. Bye bye control system. The vulnerability of some such electronic components to static charges is well documented, and is why we get them in protective conducting bags, or on conductive foam.
So the only real question here is how much charge would you need?
Sylvia.

That is a tough one. When I read the original question, along the lines of 'is it possible', I can only think to myself, yeah, sure I guess it's possible. I don't know how it would disable a vehicle, I don't even know which component(s) it might effect. I don't know if it would be a reliable or very unreliable method of disrupting a car's operation. In fact I kind of have to take some liberties with the question; 'apply a high voltage charge' is kind of funny wording.
Maybe charge rushing onto the car's body causes B fields that link some circuit(s) and cause havoc? Doesn't seem too too likely or too dependable.
I assume that the excess charge would want to spread out as much as possible, even to the conductive surfaces of the 'positive' side of the electrical system, if it had a way. Maybe charge buildup on the 'negative' side of the car's electrical system flashes across spark plug gaps. That could mess up timing pretty bad. Maybe that could cause a stall. But it would suck if the guy could just fire his car up again and go. I wonder what happens at the battery. I can't see it preventing the flow of charge, since its job is to make charge flow. Perhaps beaucoup charge flows and a fuse thereabouts pops. If that isolated the ignition system from the battery and the charging system, then the car would die pretty quick, and stay dead, and not have any serious damage. After applying a charge and disabling the car, a probe could then ground the car's frame, so that some poor dumb bastard doesn't get zapped getting out of his dead car.
In short, I don't know, I've got nothing but some not-quite-uneducated WAGs. I guess that's why I didn't respond earlier.
j

Two ends of a wire - an 'antenna' - may have different charges at either end of that wire if that wire is located in an EM field of proper frequency and orientation. What does this mean? We now require specific numbers. If electromagnetic fields are applied in just the right way to the right length wire, then we have small currents (not major transistor destructive currents) that leave one end of the wire and return to the other end (again the complete circuit requirement). IOW an electromagnetic field applied to an 'antenna' that can discharge current through a circuit. But the necessary outgoing and incoming path still required. That 'antenna' must still connect to a complete circuit that is incoming and outgoing via electronics. A complete circuit is still required to have current flow.
All of which is irrelevant to post that does not apply an electromagnetic field; contains both electrical and magnetic fields. The original question is simply about applying a high voltage charge to car metal. There is no significant electromagnetic field to be received by an interior 'antenna'. Furthermore a complete sheet of metal exists between a charge located under the car and all contents inside the vehicle - a shield against electromagnetic fields.
1) High voltage is not both electrical and magnetic fields. 2) A complete sheet of metal exists between the charge and interior electronics - in essence faraday cage protection. 3) Even a charge induced on an antenna by fields requires a complete circuit out of and back into that antenna. No complete circuit path means no current from and back into that antenna. Connecting just one end of an antenna to electronics does not result in a current through that electronics. A complete circuit must first exist. Connections to both ends of the antenna are required to create a current. Again: Again return to the earlier post: But again we still have that essential requirement. Both an incoming and outgoing circuit path must exist - else no current flow.

Electrostatics:
Take two metal spheres, floating in space, isolated. Say one has no net charge, it is neutral. Say the other had a huge amount of charge placed on it.
Bring them together. A lot of charge flows from one sphere to the other. Moving charge aka current. No return path. No complete circuit.
Take a heavily charged rod, touch it to an uncharged car. A lot of charge flows to the car. Current. No return path. No complete circuit. Heavy relevance to a question about a charge being applied to a car.
Capacitor:
A capacitor has no conductive path through it. There is no complete circuit. Charge still flows.
Antenna:
Take a straight length of a single wire. Apply a voltage source to the middle. Charges race up and down the antenna. Moving charges ... no return path. In fact if there were charge returning on some return path, the fields of the return charge and original charge would largely cancel, and an antenna wouldn't radiate much.
Top-posting:
Avoided by some, only as a matter of etiquette.
Please stop repeating elementary school science to me. This is an electrical engineering newsgroup.
j

That's rude. Let me withdraw that and say, you don't have to dumb it down for me, if you want to discuss something I said, just give it to me straight and concise.
j