Mercedes puts a rechargeable battery in the alarm siren itself. Any attempt to disconnect the siren when the alarm is armed, will set it off. Unfortunately, a bad or intermittent connection will also set it off, making the owner think the car went "haywire." Are car alarms worthwhile? I don't think so.
Most cars today come with car alarms - particularly upscale cars. Usually, the alarm is packaged as part of the keyless entry system, such as on some older BMWs. Disabling the car alarm may also disable the keyless entry.
While an alarm may seem like a logical thing to have on a $50,000 car, when it gets to be an old, used car, worth maybe $10,000 to $15,000, the alarm seems more of a nuisance, particularly if it goes off at random times.
For example, the alarm went off one night on the X5 while we were camping. We had left the trailer plugged into the car, and it drained the car battery overnight (while trying to run the refrigerator on 12 volts - bad idea). When the system voltage reaches about 10.5, the alarm goes berserk and sets itself off.
To avoid the problem again, I unplugged the siren (under the hood, easy to access) and learned to live without a car alarm.
A friend came by last night with a 2006 Mercedes 500 CLK, a 300 HP monster with the AMG trim package. A real German hot-rod. But for the last several weeks, the alarm had been going off, while at work, much to his dismay. And when I researched the issue online, I discovered that Mercedes actually put a battery pack in the alarm siren itself. So if a thief tried to disable the alarm by disconnecting the car battery, the siren would be set off - and keep going off.
The problem is, the siren connector was intermittent (or the siren itself was going bad) so it kept thinking someone was disconnecting the battery, and then going off at odd times.
The local mechanic refused to touch it. "You need to take it to the Mercedes Dealer!" he said, "Otherwise, the computer will go haywire!" I suggested he find a new mechanic.
The siren was located in the right front fender, behind the fender liner. It was a simple matter to unplug it and remove it. You have to make sure the alarm system is OFF before removing it, or the siren will go off and never stop, until the batteries run out.
But the power locking system seems to work OK without the siren, and moreover, the alarm system still works, but it will just flash the headlights if it is set off. Since the car is worth maybe $15,000 at this point, what is the point of an alarm system?
These sort of esoteric problems with alarm systems illustrate why they are annoying as snot and useless to boot. Why?
1. Never in the history of automobiles has an alarm system thwarted a car thief. OK, maybe that is an exaggeration, but in most cities, and indeed suburbs, when a car alarm goes off, no one runs to the rescue of the car, or calls the police, but instead says, "Damn, another annoying car alarm!"Even "old school" car alarms are prone to false alarms - and aftermarket ones in general. My old BMWs had a very simple alarm, but when a trunk lid switch or hood switch would go out of adjustment, the alarm would either not set (and not lock) or it would go off as the car got warm in the sun, metal expanded, and a microswitch, out of adjustment, made contact.
2. False alarms outnumber real alarms 1000 to 1.
3. As car makers embed this technology into the car more and more, it becomes harder to remove it, and when it fails, it can make it hard to start the car. For example, BMW and Mercedes used "Immobilizer" technology to make the car impossible to start, if the key does not electronically handshake with the immobilizer unit. Great technology when it works, it sucks when the car is a decade old.
4. As my friend's experience shows, many mechanics are afraid to work on such complex electrical systems, making "take it to the dealer" the only option. And you never want to take an out-of-warranty car to the dealer for repair.
5. Professional thieves are not deterred by alarms. If they want to steal the car - usually for parts - they will merely flatbed it away, alarm screaming the whole time, and then strip it for parts in their garage. Teenage joy riders are not deterred by alarms, but will likely be foiled by modern key systems.
6. Motor Vehicle Theft Rates are down - way down. Joyriding is hard to do, unless you steal an older car, like one from the 1990's, where all you had to do was smash the column lock (which unbolts in many cars) and jumper two wires. Again, the "handshake" technology puts joyriding out of the reach of the average teen.
Note the peak here in the early 1990's. Many Japanese cars of this era had bolt-on ignition locks that were pathetically easy to disable. Unscrew the plastic housing around the steering column, unbolt the column lock, and then jumper the ignition and starter wires and you are done. Today, this is not so easy to do, with modern cars. VIN plates on body parts have made the chop shop man's job more difficult as well.
7. If you are worried about your car getting stolen, you own too much car for your income bracket, period. The beautiful thing about owning a $10,000 car, is that it is still a nice car, but you can afford to walk away if it is smashed, stolen, or whatever. If you make $100,000 a year and own a $50,000 car, you own too much car - and if you worry about your car being stolen or damaged, then you own too much car for your income bracket. A simpler approach than car alarms is to simply own less car.
Newer cars, like the X5, have motion sensors, which are a pain if you leave the dog in the car (press the alarm button twice to disable the motion sensor). On one occasion, this did work for me - I was walking the dog and left the car windows open, and the alarm went off. I looked over to see a young man loitering around the car, with his arm halfway in the window - reaching for the door handle to do a little quick rummaging through the car.
Of course, the joke was on him, I don't leave valuables in my car (an approach much cheaper than car alarms) so he would not have found anything. If I had rolled the windows up, that would have also deterred him.
When I was in my 20's, my van was broken into (it was the 1970's, gas was expensive, so we all drove vans - go figure). Someone stole the spare tire - net cost, $40. So I spent $75 on a "shaker box" type of alarm system. It would measure vibrations and then go off. Every time a car drove by in the parking garage, it went off. And no one really gave a rat's ass when it did, other than it was annoying when it went off. Eventually, I disconnected it.
And kids do this - put a $500 or $1000 stereo in a car, get ripped off, and then put in an alarm, which of course does no good. Can't afford to lose it? Then you can't afford to own it.
And also it points out that if you live in a crummy neighborhood or are in school (being redundant here) or park in bad neighborhoods a lot, you are better off just not owning fancy crap until you can afford a fancy place to live.
Why? Because alarm or no alarm, you will get ripped off. You dangle candy in front of children, they will go for it, even if you lock the cabinet door.
Car alarms are basically worthless, period.
If you are thinking about installing a car alarm, because you've been "ripped off," I would suggest you re-think it. Because you are just throwing good money after bad. Changes in your behavior (where you park, what you own) are more likely to alter your outcomes than trying to "bulk up" in terms of security.
Poor people buy lots of expensive crap, live in bad neighborhoods, and then try to rely on steel bars, deadbolt locks, viscous dogs, and alarm systems to protect them. Owning less crap and living in a better neighborhood is a more profitable and better lifestyle.
If you don't want to be poor, don't do what the poor do!