The Background:
I have a 1991 Talon TSi AWD with the factory alarm.
To arm the factory alarm, all you have to do is open the door and flip the toggle to lock it. At this point, what is supposed to happen is that a green light labeled "Secu-rity" lights up on the dashboard. This light is supposed to remain lit until 5 seconds after you close the locked door.
The Problem:
When I opened the door and flipped the toggle, the green light sometimes came on and sometimes didn't.
The only reliable way to arm the alarm was to close the unlocked door and use the key to lock it from the outside. This would always cause the green light to come on indicating that the system was armed.
The Cause:
When you open your door, there are a number of sensors that tell the car that the door is open. One of these sensors is located on the body of the car at the lower, aft corner of the door. This sensor is easily mistaken as the switch that simply controls the dome light. While this is true, it also connects to the security system.
So the obvious questions is, "How can my dome light work fine and the alarm fail at the same time?"
The answer is that that innocuous little black button actually contains two, independent sets of contact points. Contacts that are made of a copper blade which is forced against a steel (or iron) seat.
The dome light remains reliable over time because it draws enough current to burn off dirt of corrosion that may accumulate on the contacts. The alarm circuit, due to it's low current through the sensor, does not have this ability.
Over time, both sets of contacts get dirty and even the dome light will eventually fail. This is, in fact, how I finally diagnosed this problem.
The Solution:
Luckily, fixing this problem is very easy.
All you have to do is pull the rubber cover off the switch, remove one screw, pull the switch out of the car body, and unplug the connector. Be careful that the connector does not fall back inside the hole as it would be very difficult to get back out.
Once the switch is out, spray the contacts with some contact cleaner and let it soak for a few seconds. Then press the switch and let it snap back so the contacts hit hard. Snap the switch like that a few times to break up any deposits on the contacts.
Reinstall the switch and replace the rubber cover.
You alarm should now arm reliably when you lock the door with the toggle.
Good Luck,
Terry Wells
twells@cp10.es.xerox.com