How to Install a Car Alarm

Co-authored by wikiHow Staff

Last Updated: August 4, 2021

Car alarms are an effective way to insure your investment and give you peace of mind. While many cars come standard with some kind of alarm system, some do not. It is no surprise that many people want to install an aftermarket alarm system either because they don’t have one or because they feel that they need to upgrade. Installing a car alarm might seem like a difficult task, but with a little bit of preparation and information, it becomes nothing more than a series of small tasks.

Part 1

Part 1 of 3:

Planning the Installation

1
Devise an installation plan. Planning ahead can shave hours off your installation time. In particular, gather documents and manuals outlining your car's wiring colors, locations, and polarities.^{[1]
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2
Check each of your doors. You want to make sure that they trigger the "door ajar" indicator on the dashboard. If a door fails to trigger, then the alarm will not go off when that door is tampered with. If a door doesn't trigger the alarm, then your alarm will not be effective.
- Most alarms come with optional pin switches that trigger the alarm when the door opens. These can be a good alternative if your doors are not triggering the door ajar/dome light when opened.^{[2]
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3
Consult a service manual to find wiring diagrams. Make note of which panels will need to be removed and which tools you will need to remove them. Learn the wiring layout of your vehicle and sketch a plan for where you will connect your car alarm.

Part 2

Part 2 of 3:

Installing the Car Alarm Unit

1
Remove the necessary panels. This will allow you to access the wires needed to hook up your car alarm. It will also give you access to places under the dash that you can mount the alarm if you choose. Typically this will include panels near the center of the dash on the driver’s side. They are often located near the floor or underneath the steering wheel.These panels will be different for each car, but you can refer to your car’s service manual to find the correct panels.
2
Mount the car alarm. Try to install the car alarm in a hidden place. Some people will mount it inside the dash if there is room, or under a seat. Just keep in mind that if the alarm is visible, a thief could tamper with it. The harder it is to find your alarm, the more effective it will be.^{[3]
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Research source} Install the mount with screws and accessories provided with the car alarm.
- Be sure to look on the other side of any surface before running a screw through. You do not want to accidentally screw into wires or other parts.
3
Drill through the firewall. You can avoid this step if you run a wire from the car alarm to the siren through an already existing hole in the firewall. It often works well to run the wire alongside the heater core hoses, an amp power wire, ignition power source, or any other lead going to the other side of the firewall. If there are none conveniently located, you will have to make one. If you have to make a new hole, be sure to look on both sides of the firewall to ensure that the spot you’d like to drill is safe. As long as the drill will not hit any other parts, it should be fine to drill there.
4
Run a wire through the firewall. This wire will connect the car alarm to the siren.
- It is important that you insulate this wire with a rubber boot or fill the hole with silicon. This keeps the wire from being damaged by friction with the firewall and shorting out. You should also aim to put the wire somewhere that it will not be kicked or tugged by yourself or your passengers for the same reasons.^{[4]
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Mount the siren. You can choose a place on the firewall or somewhere else in the engine bay that has enough room. It is important to mount the siren face down to prevent water from accumulating in the voice coil.^{[5]
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Part 3

Part 3 of 3:

Wiring the Connections

1
Run the valet switch. This will be a wire off of the car alarm module. This switch will allow you to disable the alarm features while it is engaged. This will make it easier when someone else has your car, like when you leave it with mechanic.
2
Run the LED light. The car alarm LED light indicates when the alarm is armed. This light is usually mounted on the dash by drilling a small hole in the dash and running the wire to the car alarm module. The light is then mounted on the dash with superglue to keep it in place.^{[6]
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Research source} Be sure to look on both sides of the dash panel that and be sure that your drill will not hit any other parts. There is no need to take the dash off for this step as long as you can find a safe place to make your hole.
3
Run the external antenna. If you have an external antenna, it extends the range of your wireless remote or keypad by receiving the signal from outside the car and transmitting it to the car alarm module. Most external antennas are glass mount antennas. This means that you will mount a receiver on the outside of your glass and a repeater on the inside of the same glass. The signal will be transmitted through the glass with no need to drill holes and the repeater wires directly to the antenna wire of your alarm module.
4
Connect the siren. The siren should have two wires, one negative and one positive. Most car alarms output a positive siren, so connect the alarm unit to the positive wire of the siren and connect the other siren wire to a ground.^{[7]
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5
Connect the alarm sensors. The sensors are responsible for signalling that something is amiss and the siren should sound. You will connect the sensor wires from the car alarm to the wires that signal your door ajar or dome lights.^{[8]
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Research source} You can also connect them to the trunk and hood position sensors and the ignition switch depending on the features available with your alarm system.
- It is recommended that you crimp these connections and do not simply twist them or use a wire nut.^{[9]
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6
Hook up the power wire. You should hook the power wire of your alarm module to the batter or other constant power source. This will ensure the alarm is powered even when the car is turned off. Your alarm is now powered up and active.
7
Replace the panels on your vehicle. Be sure to replace them in the correct order so that all of the panels fit together properly. It is a good idea to consult your service manual for this step as well.

Community Q&A

Question

How can I disable my alarm?

Community Answer

Unhook wires going to the battery. If that's not an option go to the control box, usually under the dash, and cut the red and black wires going in.
Question

Will it be possible for a new key to be programmed for a car alarm without necessarily going through the installation process from the beginning?

Community Answer

It depends on the car. You can take your car to the dealer and they can reprogram the key for you. It will cost a bit of money, but it would be the best thing to do if you want it done correctly.
Question

What do I do if my car alarm fob isn't working and my car alarm is going off?

Community Answer

Likely the wire is shorted. Verify if there are shorted wires: use a multimeter. Reconnect the wires if you're confident or have the mechanic fix it.

Warnings

It is strongly recommended that you test the wire’s polarities before making connections.^{[10]
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