How Do Car Door Sensors Work?

"Your door is ajar." Back in the '80s, you could listen to a lady's voice say that to you should you decide to turn on the ignition and put your foot to the pedal without completely closing your door. Although her voice could become a little annoying, she was doing you a big favor in reminding you to close the car door all the way. But how did she know your door was ajar? It all has to do with the car sensor system. Although "your door is ajar" isn't something heard as often today, the principles of car door sensors are still the same.

Your car's electrical system is the main driver behind your car door sensors. Within that system are a number of wires and sensors throughout your vehicle, starting with your car's battery. The battery provides power to a junction box full of fuses and switches. A wire would run from the junction box to the individual sensor system. Each sensor system consists of a way to signal whether the electrical circuit is opened or closed, depending on the function of the signal. Car door sensors are part of this same system. There is a wire that runs from the box to different parts of your car, depending on what your vehicle is designed to signal. For example, your main driver door may have different sensors and signals for locked doors, unlocked doors, child locked doors, doors that are ajar or windows rolled down.

The way the sensors work can vary. One type of sensor can be an electrical proximity one for the "your door is ajar" signal. On the vehicle itself would be a wafer-thin magnetic strip; the door would have an opposing strip that has a wire running inside the door and into the junction box. The junction box would then have another wire running into the car's computer that has yet another wire attached to a small speaker in your dash board. As long as the door was closed, the magnetic strip on the door could read the opposite one on the car, keeping the electrical circuit closed. But if the car door was open and the door magnet was too far away from the car magnet, the circuit would be open. The computer would then read that the door circuit is now open. It would send a signal to the speaker to make the announcement that your car door is ajar.

Other car door sensors operate on the same principle, except the sensor system may be different. Trunk locks usually have a spring-loaded switch that opens or closes the electrical signal, depending on the status of the door. If your car had an alarm system, it may employ a "shock" sensor that can shake based on whether your car was touched. When the shock sensor shakes, it signals the computer to turn on the small loudspeaker that emits a horrible alarm sound. The sensor could also signal for your lights to flash off and on.

By understanding the principles of your car door sensor, you could be saving yourself some money from the dealership by performing some simple do-it-yourself maintenance like checking to see whether any sensors are stuck. Just pray that it isn't a faulty electrical one that says "your door is ajar."