You're invested in your car and you don't want anyone messing with it, or worse, taking it. That means it's time to fit an alarm. Maybe it didn't come with one from the factory, or perhaps you're looking to upgrade. Many modern systems include a very useful cell phone app. You could go to an installer and pay an arm and a leg, or you could do it yourself.

Car alarms are surprisingly easy to fit. In most cases the hardest part of the project is reaching the wiring harnesses beneath the steering column. That means sliding the seat all the way back, opening the door wide and twisting yourself into position! But so long as you're pretty flexible there's no good reason for not doing this job yourself.

Here we'll explain how to install a car alarm. Some alarms come with a remote starter. Alternatively, you may want to fit a separate starter kit. As the two projects are very similar, we'll cover how to install a remote car starter at the same time. The sections we'll go through are:

  • Whats in the box
  • Tools, materials and information
  • The mounting process

Whats in the box

The parts that come with an alarm depend on its specification. At a minimum you'll have:

  • Instructions for mounting
  • Control box
  • Siren
  • Starter kill relay
  • LED indicator
  • Shock sensor
  • Wiring harnesses
  • Keyring remotes etc.

Some of these might be combined into a single unit. The siren might be built into the shock sensor. You might also add other components. For example, an ultrasonic or RF proximity sensor will protect a convertible left with the top down.

Tools, materials and information

The single most useful thing to have is a wiring diagram for your vehicle. These are usually found in workshop manuals. This will tell you what color wires you're looking for and where they are. In addition, you will need:

  • Multimeter with test probes. This lets you see when a wire has power. A wire probe or test light will do the job too.
  • Screwdrivers
  • Pick tools
  • Electric drill
  • Wire strippers
  • Pliers
  • A strong light (a headlight device might be useful.)
  • Electrical tape
  • Cable ties

Depending on how you like to join wires, you may also use:

  • T-taps
  • Soldering iron
  • Heat shrink sleeving

Soldering is the best method because the last thing you want is a bad join. That will probably leave you unable to start your vehicle. However, T-taps can work if fitted with care.

The alarm mounting process

This splits into seven steps:

  1. Overview of the process
  2. Siren mounting
  3. Control box mounting
  4. Door and hood switch connections
  5. Other components
  6. Remote start
  7. Test

1. Overview of the process

Depending on how your alarm is designed you'll be mounting two or three main pieces. These are the siren, the controller and possibly a shock sensor. The siren goes under the hood with a wire fed through the firewall to the control box. The shock sensor needs mounting to a rigid surface. Under the kick panel, (the panel in front of the door,) is a good place. There may also be an LED to show when the alarm is armed.

The control box mounts close to the ignition wiring harness. That's best reached under the steering column. The controller goes here because you'll be connecting to three wires in the harness. These are: a 12V constant (always on,) the Ignition wire and the starter wire. The Ignition wire is live when the engine is running but the starter connection only comes on when you're cranking the engine. In addition, you'll need to provide a good ground.

Most alarms connect to the hood and trunk opening sensors. Consider adding these if your vehicle doesn't have them.

The steps for fitting a remote start are very similar to those for an alarm.

As with any electrical project on your vehicle, disconnect the battery before starting work. This prevents any short circuits.

2. Siren mounting

Find a solid mounting location under the hood. (The firewall is a good place.) Orient the siren so it points down to prevent water getting in. Drill mounting holes and fit the siren, then thread the wires through the firewall into the cabin. If the alarm instructions suggest a ground under the hood, make this happen.

3. Control box mounting

This usually goes under the steering column, near the fusebox and ignition wiring harness. If your alarm has a separate ignition kill relay mount this in the same place.

Remove the plastic trim panels, and also any metal brackets that may be in the way. (Check there are no airbags down here. They need careful handling!)

Join the wiring harness supplied with the alarm to the controller. Also connect the siren.

Now you need to decide which of the ignition harness wires to connect to. Even if you have a manual telling you what colors they are, check this with your multimeter. (You will need to reconnect the battery first!)

If making soldered joins, strip each wire and solder it to the appropriate wire from the alarm controller. (They'll be identified in the instructions.) Alternatively, make good connections with the T-taps. Cover these with electrical tape. Ensure any ground wire in the alarm harness is connected to the vehicle body.

Install the ignition kill relay, making the appropriate electrical connections.

Mount the control box and relay securely. These are usually small so can be cable-tied to the wiring harness.

4. Door and hood switch connections

You'll want the alarm to sound if the doors are opened. You do that by tapping in to the wires from the door switches. These are usually found behind the kick panels. The manual will indicate the wire color. Again, confirm you have the right wires with your meter.

Follow the instructions to join the alarm controller wires.

If you are linking to the hood and trunk switches, make those connections at the same time.

5. Other components

If your alarm system has a proximity sensor, LED indicator or other components, fit them now. Follow the instructions in the box. It's a good idea to hook in your lights to the alarm as flashing lights make it obvious which car is being broken in to. To do this, connect those wires to the controller as explained in the instructions supplied.

6. Remote start

The process for fitting a separate remote starter is similar to that for the alarm controller. It goes under the steering column and connects into the ignition harness. Use your multimeter to find the wires you need. These are the constant 12V, the cranking circuit, the running circuit, (sometimes termed “Ignition 1” and “Ignition 2”,) and the Accessories. As with the alarm controller, connect to these wires following the instructions supplied with the remote start kit.

7. Test

Before refitting the various trim pieces it's a good idea to test that the system works correctly. Reconnect the battery if necessary, then go through setting and activating the alarm. If you installed a remote start, check that operates as expected.

If you find any problems check that wires are all connected as shown in the instructions. Then make sure each connection is good. (Your multimeter will help.) Also check that you have a good ground. If the alarm still isn't working after doing this contact the manufacturer for support.

Go steady, don't rush

The thought of fitting an alarm system or a remote start can be pretty daunting but it shouldn't. It's a job someone with just a little experience of vehicle electrical systems can undertake. Start by reading the instructions thoroughly. Then get familiar with the system components and go slowly. In a couple of hours you'll be enjoying greater security and convenience.