Foo - How does the alarm on the '99 Jetta go off?

My girlfriend leaves the door to her '99 Jetta unlocked. Perplexed, I asked her why. She said that the alarm goes off when she unlocks the doors and turns the car on. This makes no sense to me; VW wouldn't devise a mechanism that performed this way.

And of course, it never happens when I lock the doors and turn the car on. Is there a rationale behind this? My '93 accord doesn't have car alarms and I'm not sure how they work. Is there a sequence of events she might be following that would trigger the alarm?

This car. It has character.

The door usually has to be locked in order for the alarm to be set. When it is unlocked with the key or alarm fob, it should disarm it. If the car is unlocked, the alarm is not set.

MkIII, right?

http://forums.vwvortex.com/showthread.php?585949-The-Official-DIY-FAQ-MkIII-Post&p=12124795&viewfull=1#post12124795

Finally, a question I can answer with some certainty. I'm a VW mechanic with 20 years experience. The Jetta uses a complicated series of micro-switches inside the doors on the handle mechanism to tell the alarm control unit if all of the doors are locked or unlocked. If you unlock the door and the micro-switch doesn't send a "door unlocked" signal to the control unit the alarm will sound when you try to start the car. Basically, the alarm thinks someone broke into the car and climbed in through the window, or didn't unlock the door with the fob or key. So when you go to start the car, since it didn't see the door unlock "properly", it sounds.

Also, sometimes the wires break in the door jamb. Look for a thick rubber boot in the jamb near the hinges pull it back and see if any wires in there are broken. If that's not the problem you may have a malfunctioning lock micro switch assy. You might have to take it to a pro for this repair.

Finally, a question I can answer with some certainty. I'm a VW mechanic with 20 years experience. The Jetta uses a complicated series of micro-switches inside the doors on the handle mechanism to tell the alarm control unit if all of the doors are locked or unlocked. If you unlock the door and the micro-switch doesn't send a "door unlocked" signal to the control unit the alarm will sound when you try to start the car. Basically, the alarm thinks someone broke into the car and climbed in through the window, or didn't unlock the door with the fob or key. So when you go to start the car, since it didn't see the door unlock "properly", it sounds.

Also, sometimes the wires break in the door jamb. Look for a thick rubber boot in the jamb near the hinges pull it back and see if any wires in there are broken. If that's not the problem you may have a malfunctioning lock micro switch assy. You might have to take it to a pro for this repair.

Wow, great response. That makes perfect sense, and the inconsistency of symptoms definitely points to a mechanical issue. I know the boot you're talking about, and I'll take a look underneath it. If it's not an easy fix I'll try to simply disable the car alarm as per the instructions in that forum thread.

A VW with electronics issues?! *gasp*

Seriously, just pull the alarm. :)

Or shoot the car.

My daughters car had the same issue. It would lock the doors while driving, set off the alarm, arm the alarm at other times and it would go off as soon as you opened the door to get out. When it did that you had to roll the window down, insert the key outside and unlock the door to get out. We had to relace the alarm module and have the wiring checked as above.

She affectionately named the car HAL.

Rule #1 of owning a German car: never let the warranty expire.

Dump her for a new girlfriend with a later model car and better taste in boyfriends.

Dump her for a new girlfriend with a later model car and better taste in boyfriends.

A Paradox?