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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.

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  • Have you a specific wire related to the ignition that you were planning on breaking? Are you talking about the wire from the ignition switch?
    – HandyHowie
    Feb 14, 2023 at 19:47
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    If "domestic make" is relevant, it's not very helpful to a world-wide audience. Better to actually tell us which country it's from. Likely the fuel type (petrol, diesel, LPG, ...) affects the answers, too. Feb 15, 2023 at 14:42
  • @Toby. Noted and added.
    – Charlie
    Feb 16, 2023 at 18:01

3 Answers 3

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The average aftermarket car alarm uses cheap components and is not designed for high reliability and long life. This is not to criticize the particular model that you plan to install, but I'm speaking in general terms from the number of times I've seen problems with dead batteries caused by alarms, alarms triggering for no reason or causing all manner of problems.

If you install your alarm according to manufacturer instructions to interrupt the starter and the alarm goes haywire, the worst that can happen is that your car won't start. Bummer, but it's parked safely and your only problem is that it won't start.

Now suppose you hook it up instead to interrupt the ignition or fuel or both. What if the alarm loses its mind while you are driving and stalls your engine?

enter image description here

Can you imagine the nightmare scenario? Factory alarms / immobilizers won't do this to you, or will fail so rarely that it's practically unheard of. Aftermarket alarms? You're taking your chances.

If you're set on doing this, better to follow instructions and get stuck in a safe place than to stall in traffic.

(Photo: KTLA.com)

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    +1 You make a somewhat reasonable observation. I would ask, however, how is this any different from running any car with lurking problems down the same highway? In the same sense that they make cheap components, they make cheep cars. Furthermore, there are dozens of cars on the road that people drive that haven't had basic tuneup and maintenance in YEARS! I know several people driving beaters down the highway with 300k+ miles on them and no windows like it's sane.... bottom line my truck could just as easily die on a superhighway simply because it's fuel pump is 20+ years old.
    – Charlie
    Feb 15, 2023 at 1:27
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    @Charlie All perfectly true, but the question is posed as a simple X/Y (interrupt ignition or starter). If given a choice on this one decision, and knowing how trouble prone aftermarket alarm systems are, the better choice is to give you a failure mode that can leave you stuck in a parking spot rather than in traffic. The bigger picture addressing the risks brought on by poor maintenance are left for another day.
    – MTA
    Feb 15, 2023 at 2:20
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    @Charlie Just because there's other failure modes doesn't mean you need to make it worse. In fact, it's a reason to avoid introducing more risk. Put another way, if your car isn't the safest one around, it doesn't mean you should drive without a seatbelt and drive in excess of the speed limit, it means you should take extra care in driving.
    – user71659
    Feb 15, 2023 at 4:09
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    I had exactly this nightmare scenario happen in Houston traffic, you would have thought I spit on their baby jesus or ate a steak well-done with how angry people got.
    – John Jones
    Feb 15, 2023 at 21:57
  • I am reluctant to change what I have accepted as the answer, but you have all made very compelling arguments. I have, in fact, decided to install the system as per manufacture guidelines. This was mostly because the alarm in question has auxiliary features that allow ignition system interrupt to be added later should the alarm prove itself reliable over time. TY. +1 all round.
    – Charlie
    Feb 16, 2023 at 18:06
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Most modern immobilizer systems cut off Ignition, Crank and fuel. You could easily use it to cut off the start signal and something like the ASD relay control. That should disable both the fuel & fire.

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  • I'm going to accept this as the answer for now. It would be cool if you expanded your "for instance" I don't need it, I have a good idea what you mean, but more verbose answers please the SE gods and may help others looking at this problem get a better picture of what they want to do.
    – Charlie
    Feb 15, 2023 at 1:31
  • ...With regret, I have decided to change my acceptance of this answer, and instead accept the answer relating to cheap security systems. The comments in that answer make compelling arguments as to why. Furthermore, the system in question has auxiliary features that allow ignition system interrupt to be added later should the alarm prove itself reliable over time.
    – Charlie
    Feb 16, 2023 at 18:09
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In the days of mechanical distributors and contact breakers, and before they became standard fitment, I was taught how to make an effective immobiliser.

The usual way was to put a hidden switch in the low-voltage supply to the distributor, but it was easy to circumvent without finding the switch – just make another connection. A voltmeter would tell a thief what had been done.

But this method was to put a short circuit across the contact breaker, and was much harder to detect (there is voltage present) and to defeat – they had to actually find it, and there was apparently nothing wrong: just no spark.

The short-circuit does no damage: the contacts which it shorts spend most of the time connected anyway, but when they separate, it electrically prevents the circuit from breaking, so no spark is generated on the high tension side of the coil.

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  • +1 for Kettering ignition system. My truck does use a distributor, but it is a more advanced solid state ignition control module for the low voltage side of the ignition coil. I wouldn't be surprised if it was just a transistor as a switch to ground instead of points, with the rest just there to replace the vacuum spark advance with a signal from the PCM. One wire of my alarm holds to ground whenever the alarm is active. I could conceivably use this to override the ICM signal to the coil. A simple test light between the ICM and coil would tell me if that's whats going on there.
    – Charlie
    Feb 15, 2023 at 1:19
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    Review your aftermarket anti-theft wiring schematic and descriptions. Disabling the starting circuit may stop or slow down a thief. A secondary deterrent to disable the ignition system power or ground circuit to prevent spark as another theft deterrent. A distributor with solid state electronics should be easy to disable power or ground wiring.
    – F Dryer
    Feb 16, 2023 at 15:18

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