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Self Setting Car Alarm

I have a 1997 Plymouth Voyager SE that has had the engine replaced. The current problem is that the alarm sets itself and the anti-theft device will not allow the car to remain started. I have lost the key fob and have been told that the car is not recognizing that the alarm should be turned off. My mechanic was stumped and suggested that we just unhook the battery to reset and then the car would run. This worked for quite awhile but no longer does. We have tried other methods of resetting the computer. Any ideas?

Howdy

 I know you have a chrysler but for quite some time GM used a little device on the ignition key that on the shaft close to the head of the key sits a little black plastic "square" that had a very small metal "chip" in it. That was the early anti-theft systems. I'm unfimiliar with Chrysler however, but some keys now have a magnet inside of the actual head of the key. 

 Funny you mention having the motor replaced because that's making me wonder about something electrical that was not given special attention to during the pull/install. I don't know if these new systems make it difficult to "hot wire" or if that is what the systems' job is to prevent. Perhaps there's a short in the ignition system  and/or alarm system. It could very well be something as simple as a relay for the alarm system. Start small. 

 I'm not one for taking it to the dealer, but if you can find one nice enough to help you over the phone or lets you use their books or look up Mitchell On Demand or go to local library. I SWEAR by Mitchell books!!!!

ONE LAST THING TO TRY! Does it constantly go off even with key out? Try shutting the door and unlocking the car manually with key. On some models that will deactivate the alarm. I learned that the hard way on a friends car. It's actually a funny story!! Want to hear it? Okay.

 Short version: My friend had a 1994 Nissan Maxima. I had some valueable items in his car so I locked it when we got to another friends house. He looked at me and screamed, "No!!" He left his keys in the console. Taxi came out and jimmied the lock and got it open and sure enough alarm started blaring. Being 16 and watching alot of car movies I looked under the dash for a "kill switch". I thought I found it. For some reason my friend decided to push in the clutch. I started screaming like I was on the set of a Halloween movie. I was actually pushing a clutch switch. To no avail. Calling the Nissan dealer, they advised that we pull a certain harness and he warned us that the ONLY thing that will work is the engine, EVERYTHING else will be dead. Speedo, all guages, windows, locks, AC!!!, nothing worked!  

 We were actually on our way to a wedding. So in the summer heat at every red light, we ALL opened our doors and my friend was cursing me. I actually was starting to find it humorous. We happened to talk to the right guy at the wedding. He came out opened the hood plugged the harness back in... (HONK!  HONK! HONK!) He asked everyone to shut their doors and he used the key in drivers door and unlocked it and... silence!

This friend was kinda inept to cars. I have many more funny stories about him and his automotive blunders, including new engines, transmission...

Hope this helps.
Please let us know what you find.
JP#3