The major disadvantage to adding a sensor to the OEM alarm is the lack of a "warn away" stage. Most quality sensors have two stages - a warn away that simply beeps the siren a few times if someone lightly bumps your car and then a 2nd stage that sets the alarm off if the car is hit hard.

If you're just looking for a little extra piece of mind adding a shock sensor and possibly a blinky LED will deter most people from messing with the car. This can be done very cheaply.

If you want to go all out stick with a name brand alarm (DEI, Compustar, etc). Going with cheap electronics off of Ebay is bound to lead to problems.

I have a two way Clifford 50.5x that cost about $230 with all parts and I installed it myself. Look to spend as much on the alarm for installation at a typical shop. Also, make sure you know where you are going and trust the shop, there are many shops out there that will hack your car up!

 

I think some of the warn away sensors could still be used with a stock alarm with a simple relay setup. With the car off the relay would be armed and the set of contacts associated with the light impact would set the horn/siren off. The harder impact set of contacts would go directly to the factory alarm.

However for me the big benefit I would like is the ability for me to be warn that there is something going on with my car. While I may not move right away, I may want to pay attention if it continues. On the other hand I may want to go look and see if I can see who or what is causing it.

I have seen some installations of a $400 alarm end up at $1000 installed so that really is a good point since the install can vary greatly especially when you toss in some of the other options such as remote starters.

 

I probably aught to just get regular alarm...I may be overlooking the power of prevention. A simple impact sensor will not keep the @#$%^ from breaking my window...which in reality is most likely more than an alarm system. With my stereo cost waaaayyyy over budget..I would be a fool to not have a good alarm.

 

No alarm is going to keep someone from breaking your window. A proximity sensor may be what you want to consider. Some will detect someone near your window, but that can of course bring in some false alarms as well.

 

The only thing that has a chance or stopping someone from breaking your window is a visual deterrent (like a flashing LED). Criminals, more often then not, are opportunists - if they think your car has an aftermarket alarm system on it they'll move on to an easier target.

 

Right...a visual cue or an audible one...something to get the theif to evalute the cost vs. benefit ratio... To pfarmers point...certainly any car can get broke into, but perhaps the deterents will shift the likelyhood to a more favorable outcome. Who knows...maybe I'm giving punk theives a bit too much. Either way...for 300 bucks I could be really stacking the odds in my favor.

 

Great thread. Thanks for the info.

 

^ agreed