What to Do if Your Car Alarm Keeps Going Off

By: Tom Harris & Sascha Bos  | 
Man talking on a cell phone and holding a key fob beside a black car in a parking structure
No one wants to be struggling with their owner's manual or on the phone with tech support while their car alarm is blaring in the background. EXTREME-PHOTOGRAPHER / Getty Images

You probably don't think about your car's alarm system until your car alarm keeps going off for seemingly no reason (especially if it's wailing from your garage in the middle of the night — we know your pain).

In this article, we'll cover how car alarms work and why yours may be malfunctioning, whether a low battery or a faulty sensor is to blame.


The Parts of a Car Alarm

If you want to think about a car alarm in its simplest form, it is nothing but one or more sensors connected to some sort of siren. The very simplest alarm would have a switch on the driver's door, and it would be wired so that if someone opened the door the siren would start wailing.

The car alarm systems on most modern vehicles are much more sophisticated than this. They consist of:


  • An array of sensors that can include switches, pressure sensors and motion detectors (a problem with any of these sensors could cause your alarm to go off when it doesn't need to)
  • A siren, often able to create a variety of sounds so that you can pick a distinct sound for your car
  • A radio receiver to allow wireless control from a car key fob
  • An auxiliary battery so that the alarm can operate even if the main battery gets disconnected
  • A computer control unit that monitors everything and sounds the alarm — the "brain" of the system

The brain in most advanced systems is actually a small computer. The brain's job is to close the switches that activate alarm devices — your horn, headlights or an installed siren — when certain switches that power sensing devices are opened or closed.

Security systems differ mainly in which sensors are used and how the various devices are wired into the brain.

The brain and alarm features may be wired to the car's main battery, but they usually have a backup power source as well. This hidden battery kicks in when somebody cuts off the main power source (by clipping the car's battery cables, for example).

Since cutting the power is a possible indication of an intruder, it triggers the brain to sound the alarm on the vehicle.


Types of Sensors

­The most basic element in a car alarm system is the door alarm. When you open the front hood, trunk or any door on a fully protected car, the brain triggers the alarm system. Today's vehicles are typically equipped with a variety of sensors that work in different ways.

Door Sensors

Many car alarms utilize the switching mechanism that is already built into the car doors. In modern cars, opening a door or trunk turns on the inside lights.


A valet switch is a manual shut-off that temporarily disables the alarm system (so you can let the valet park your car, for example). The valet switch is hidden in an out-of-the-way spot in the car. The switch pictured here is mounted under the car's fuse access panel.
Photo courtesy Directed Electronics

The switch that makes this work is like the mechanism that controls the light in your refrigerator. When the door is closed, it presses in a small, spring-activated button or lever, which opens the circuit. When the door is opened, the spring pushes the button open, closing the circuit and sending electricity to the inside lights.

All you have to do to set up door sensors is add a new element to this pre-wired circuit. With the new wires in place, opening the door (closing the switch) sends an electrical current to the brain in addition to the inside lights. When this current flows, it causes the brain to sound the alarm.

As an overall protective measure, modern alarms typically monitor the voltage in the car's entire electrical circuit. If there is a drop in voltage in this circuit, the brain knows that someone has interfered with the electrical system.

Turning on a light (by opening the door), messing with electrical wires under the hood or removing an attached trailer with an electrical connection would all cause such a drop in voltage. But a low car battery (or worse, a dead battery) will also cause a decrease in voltage, so if your car alarm is acting up, you may want to check your battery.

Shock Sensors

The idea of a shock sensor is fairly simple: If somebody hits, jostles or otherwise moves your car, the sensor sends a signal to the brain indicating the intensity of the motion. Depending on the severity of the shock, the brain signals a warning horn beep or sounds the full-scale alarm.

There are many different ways to construct a shock sensor. One simple sensor is a long, flexible metal contact positioned just above another metal contact.

You can easily configure these contacts as a simple switch: When you touch them together, current flows between them. A substantial jolt will cause the flexible contact to sway so that it touches the contact below, completing the circuit briefly.

The problem with this design is that all shocks or vibrations close the circuit in the same way. The brain has no way of measuring the intensity of the jolt, which results in a lot of false alarms. More advanced sensors send different information, depending on how severe the shock is.

The sensor has only three major elements:

  1. A central electrical contact in a cylinder housing
  2. Several smaller electrical contacts at the bottom of the housing
  3. A metal ball that can move freely in the housing

­In any possible resting position, the metal ball is touching both the central electrical contact and one of the smaller electrical contacts. This completes a circuit, sending an electrical current to the brain. Each of the small contacts is connected to the brain this way, via separate circuits.

When you move the sensor, by hitting it or shaking it, the ball rolls around in the housing. As it rolls off of one of the smaller electrical contacts, it breaks the connection between that particular contact and the central contact.

This opens the switch, telling the brain that the ball has moved. As it rolls on, it passes over the other contacts, closing each circuit and opening it back up, until it finally comes to a stop.

If the sensor experiences a more severe shock, the ball rolls a greater distance, passing over more of the smaller electrical contacts before it comes to a stop. When this happens, the brain receives short bursts of current from all of the individual circuits.

Based on how many bursts it receives and how long they last, the brain can determine the severity of the shock. For very small shifts, where the ball only rolls from one contact to the next one, the brain might not trigger the alarm at all.

For slightly larger shifts — from somebody bumping into the car, for example — it may give a warning sign: a tap of the horn and a flash of the headlights. When the ball rolls a good distance, the brain turns on the siren full blast.

Window Sensors

­A lot of the time, car thieves who are in a hurry don't mess around with disabling locks to get into a car; they just bust a window. A fully equipped car alarm has a device that senses this intrusion.

A typical crossover unit: Using a specific combination of inductors and capacitors, you can design a crossover unit that only conducts current that has the frequency of breaking glass.

The most common glass-breakage detector is a simple microphone connected to the brain. Microphones measure variations in air pressure fluctuation and convert this pattern into a fluctuating electrical current.

Breaking glass has its own distinctive sound frequency (pattern of air-pressure fluctuations). The microphone converts this to an electrical current of that particular frequency, which it sends to the brain.

On its way to the brain, the current passes through a crossover, an electrical device that only conducts electricity of a certain frequency range. The crossover is configured so that it will only conduct current that has the frequency of breaking glass.

In this way, only this specific sound will trigger the alarm, and all other sounds are ignored.

Pressure Sensors

Another way to detect breaking glass, as well as somebody opening the door, is to measure the air pressure in the car. Even if there is no pressure differential between the inside and outside, the act of opening a door or forcing in a window pushes or pulls on the air in the car, creating a brief change in pressure.

You can detect fluctuations in air pressure with an ordinary loudspeaker driver. A loudspeaker has two major parts:

  1. A wide, movable cone
  2. An electromagnet, surrounded by a natural magnet, attached to the cone

When you play music, an electric current flows back and forth through the electromagnet, which causes it to move in and out. This pushes and pulls the attached cone, forming air pressure fluctuations in the surrounding air. We hear these fluctuations as sound.

This same system can work in reverse, which is what happens in a basic pressure detector. Pressure fluctuations move the cone back and forth, which pushes and pulls the attached electromagnet.

When the brain registers a significant current flowing from this device, it knows that something has caused a rapid pressure increase inside the car. This suggests that somebody has opened a door or window, or made a very loud noise.

Some designs utilize the car's built-in stereo speakers as pressure sensors, but others have separate devices that are specifically designed for detection.

Perimeter Scanners

There are several good ways for a security system to keep tabs on what's going on outside the car. Some systems include perimeter scanners, devices that monitor what happens immediately around the car. The most common perimeter scanner is a basic radar system, consisting of a radio transmitter and receiver.

The transmitter sends out radio signals and the receiver monitors the signal reflections that come back. Based on this information, the radar device can determine the proximity of any surrounding object.

Tilt Detectors

To protect against car thieves with tow trucks, some alarms have "tilt detectors." The basic design of a tilt detector is a series of mercury switches. A mercury switch is made up of two electrical wires and a ball of mercury positioned inside a contained cylinder.

Mercury is a liquid metal; it flows like water, but it conducts electricity like a solid metal.

In a mercury switch, one wire (let's call it wire A) goes all the way across the bottom of the cylinder, while the other wire (wire B) extends only part way from one side. The mercury is always in contact with wire A, but it may disconnect from wire B.

When the cylinder tilts one way, the mercury shifts so that it comes into contact with wire B. This closes the circuit running through the mercury switch. When the cylinder tilts the other way, the mercury rolls away from the second wire, opening the circuit.

In some designs, only the tip of wire B is exposed, and the mercury must be in contact with the tip in order to close a switch. Tilting the mercury switch either way will open the circuit.

Car alarm tilt sensors typically have an array of mercury switches positioned at varying angles. Some of them are in the closed position when you're parked at any particular slant, and some of them are in the open position.

If a thief changes the angle of your car (by lifting it with a tow truck or hiking it up with a jack, for example), some of the closed switches open and some of the open switches close. If any of the switches are thrown, the central brain knows that someone is lifting the car.

When Multiple Sensors Activate

In different situations, all of these alarms might cover the same ground. For example, if someone is towing your car away, the mercury switches, the shock sensor and the radar sensor will all register that there is a problem.

But different combinations of alarm triggers may indicate different events. "Intelligent" alarms have brains that react differently, depending on the combination of information they receive from the sensors.


Car Alarm Alerts

A Neo mini siren, hidden inside a vehicle's front fender
Photo courtesy Directed Electronics

No matter how advanced these systems are, the alarm isn't much good if it doesn't set off an effective alarm. An alarm must trigger some response that will deter thieves from stealing your car.

A lot of the devices that are already built into your car make for effective alarm signals. At the minimum, most car alarms will honk the horn and flash the headlights when a sensor indicates an intruder.


They may also be wired to shut off the ignition starter, cut off the gas supply to the engine or disable the car by other means.

An advanced alarm will also include a separate siren that produces a variety of piercing sounds. Making a lot of noise brings attention to the car thief, and many intruders will flee the scene as soon as the alarm blares.


Car Alarm Transmitters

The keychain transmitter from the Sidewinder security system: The transmitter lets you lock the doors, arm and disarm the alarm and use a "panic" button to set off the siren from outside the car.
Photo courtesy Directed Electronics

M­ost car keys are also portable transmitters. Your key works in basically the same way as radio-controlled toys. It uses radio-wave pulse modulation to send specific messages.

The primary purpose of the transmitter in key fobs is to give you a way to turn your alarm system on and off. After you've stepped out of your car and closed the door, you can arm the system with the touch of a button; when you return to the car, you can disarm it just as easily.


Usually, the "lock" button activates the alarm, and the "unlock" button deactivates it. If your car alarm keeps going off even though you pressed the "unlock" button, check to see if the door locks respond; if they don't, you may need to replace your key's batteries.

In most systems, the brain will flash the lights and tap the horn when you arm and disarm your car. This lets you, and anyone in the area, know the alarm system is working.

Since the transmitter controls your alarm system, the pattern of pulse modulation must act like a key. For a particular line of transmitter devices, there might be millions of different pulse codes.

This makes the communication language for your alarm system unique, so other people can't gain access to your car using another transmitter.


Car Alarm FAQ

What triggers a car alarm?
Car alarms feature sensors that trigger the alarm when motion or impacts are detected. Vibrations, bumps or movement typically trigger the sensors.
Will my car alarm go off if someone breaks the window?
Typically, car windows aren't connected to alarm sensors. A bump or significant motion would be required to set off the alarm; broken windows alone likely won't be enough to do so.
What does it mean when your car alarm goes off while driving?
There's typically an issue with your key fob if your car alarm is sounding without being set off. It could be caused by a dead key fob battery, an incorrect signal or another fob-related issue.
Can I install my own car alarm?
It depends on the type of alarm you choose. Some car alarms have simple, straightforward installation processes that anyone can perform. Others may be more complex.
How much is an alarm for a car?
Basic car alarms can cost anywhere from $30 to $300.