I'm installing a basic car alarm on my 2000's (USA) domestic make (gasoline) truck. This alarm has an immobilize feature that is intended to prevent the vehicle from starting.
The mechanism is a simple SPST relay internal to the "brain" which goes open circuit when the alarm is on/fired. The intention is for the installer to splice said relay's connections into the starter wire, so [bumping the ignition/punching the ignition/hot wiring] doesn't ever energize the starter solenoid, thus the thief can't start the vehicle.
My concern with this approach is that the vehicle still has factory power going to the "ignition" and thus the rest of the electronics; fuel pump, coil, injectors, etc.. So, it could at least theoretically still be started. Particularly it could be bump started, or crossbar started.
The thought occurred to me that, because of the nature of the working mechanism, the immobilizer component could instead be inserted into the ignition wire, rather than the starter wire. This would cause the thief to be able to crank the vehicle, but it still wouldn't actually start.
A side advantage of this other approach is that turning over the engine would make more noise, should the siren portion of the alarm have already been disabled. The downside to this would be that the thief could leave me stranded somewhere by draining my battery dead with too many repeat attempts to start my vehicle. Another downside is that this method is surly not endorsed by the alarm maker, and so probably isn't valid for any insurance discount.
So... I'm stuck trying to decide which is the objectively better arrangement.
I'm honestly more interested in preventing my truck from ever being stolen, period. Keeping that in mind, I feel like the ignition interrupt method is clearly the more secure option. However, In practice, just having an alarm is usually enough to prevent 99% of attempts. A thief usually sees signs of alarms and immediately looks for an easier target. So, changing the install so that it captures that 1% seems like a small return. Add to that the possibility of being stranded, and the loss of any insurance discount, and the choice isn't as clear.
Which is what brings me here. I feel like there may be an AUTOMOTIVE RELATED reason that alarm makers specifically interrupt the starter, and not the ignition... I just can't think of it. What am I missing about interrupting the ignition wire? The factory key/ignition does this all the time, so I don't see it causing any damage.