Car Alarm Wire Harness: How To Wire a Vehicle’s Alarm Security System Using 8 Steps

As the name suggests, a car alarm wire harness is the wire bundle set that connects an alarm security system’s components to your vehicle to make it work. 

In most cases, 18-gauge is the ideal wire diameter for these systems, but power wires should be 16-gauge or thicker. 

The wiring process can be a bit complex. So, let’s list the installation steps for this security system in your vehicle to help you do it as a DIY project.

Table of Contents

Required Tools

You will need the following items.

  • Panel remover
  • Wrench
  • Flat blade screwdriver
  • Electric drill
  • Heat gun
  • Multimeter
  • Soldering iron
  • Wire stripper
  • Crimping tool
  • Butt connectors (or heat shrink tubing for other connectors)
  • Your vehicle’s wiring diagram

Precaution

It is vital to note that modern vehicles have keyless entry immobilizers that must sense the key or fob’s chip to unlock the doors or start the car.

We don’t recommend installing an alarm system in such vehicles because they can mess up the signals going into the computer.

A person placing the key fob near the push start button for the immobilizer to detect the chip

A person placing the key fob near the push start button for the immobilizer to detect the chip

But you can use a preprogrammed vehicle interface module for your specific car to tell the onboard computer that it is okay for an aftermarket system to unlock the doors without detecting the key or fob chip.

This module communicates with the computer via the data bus.

On the bright side, wiring an alarm system in these vehicles is easier than in other cars because you can reach most sensors and actuators via the data bus. So you don’t need separate wiring.

You can use the T harness between the car and the interface module to attach the alarm system, then use the existing data bus.

How to Wire a Car’s Alarm Security System

Follow these steps to set up the system.

Step 1: Mount the Alarm System’s Controller

Install this controller near the vehicle’s center but out of sight. Some locations that can work include under either of the front seats or the dash.

If you have installed an amplifier under the seat, don’t mount the alarm controller there because the electromagnetic radiation can cause interference.

An electrician installing a car alarm controller in the dashboard

An electrician installing a car alarm controller in the dashboard

Generally, the most suitable location is under the dash because it is near other car wire harnesses, easily accessible, and high enough to give a good range for your alarm’s remote control.

Connect this system to the car’s battery via a fuse in the fuse box.

Step 2: Mount the Siren

Install the siren in the engine bay somewhere high up to ensure it is safely away from intense heat sources, such as radiators and exhaust manifolds.

Then, place the siren facing downwards to keep water from collecting and staying inside the horn. Water can cause the siren to malfunction or emit low noise.

A car alarm siren is mounted in the hood when facing down

A car alarm siren is mounted in the hood when facing down

Ensure you position the car alarm wire harness away from reach so thieves cannot cut the bundle. The siren should be in a safe space, as well. Consider the inner fender panel.

Wire the siren to the controller by channeling the wires into the firewall through a factory-cut hole.

Step 3: Install the Flashing LED

Cars with alarm systems usually have a blue LED mounted on the dashboard. This unit is a deterrence measure because it warns thieves the vehicle has an alarm system.

Install this LED on a visible part of the dashboard. You can drill a hole for this unit or channel the wires close to the windshield.

Step 4: Mount the Sensors

Alarm systems rely on shock sensors to detect sudden movements on all vehicle parts. Mount these sensors in the cabin and trunk at any place where they can’t get wet. Avoid installing them in the engine bay because water can get in.

Try to fix them near the vehicle’s center and adjust their sensitivity to the right setting. Too sensitive sensors yield false alarms, but those with minimal sensitivity can keep the siren from going off. So, balance between the two.

Wire these sensors to the controller either under the dash or seat.

Step 5: Tie the Alarm System into the Existing Pin Switches

Pin switches detect when you open the doors or trunk. You can tie into the existing wire harnesses from these pin switches on the floor level or behind the kick panel.

These switches can have a positive or negative trigger. Your vehicle’s wiring diagram will show you the polarity of the system. But you can use a multimeter to check it.

A car door switch

A car door switch

The pin switch wire harness usually has brown/blue wires on one side and white/brown color wires on the other.

If they have a positive trigger, these switch wires will have a potential difference of 12V when the door is open and 0V when closed. But negative trigger wires will measure 12V when closed and 0V when open.

If your car does not have these pin switches, you’ll have to buy and install a new set. Connect these wires to the controller on the respective terminals.

Step 6: Wire the Controller to the Parking Light Circuit

When a car alarm system goes off, the controller should activate the siren and parking lights. So you have to connect the controller to these lights.

Some vehicles have a parallel circuit in their parking light switch system, while others have a single circuit.

You’ll have to connect the alarm controller to both parking light output wires when wiring a vehicle with a parallel system.

And you’ll have to use a dual-make relay for this connection.

Take care not to link the controller to the dimmer circuit. Use your multimeter to measure the voltage on these wires when you dim the dashboard lights.

Step 7: Wire the Starter Interrupt

The car should not start when the alarm has gone off. So, you have to connect a starter interrupt relay to the wire running from the ignition switch to the starter solenoid.

This wire does not draw as much power as the one from the solenoid to the starter motor, so it is safe to tie into it.

A car starter motor with the solenoid mounted on it

The starter disabler or interrupt should connect to the controller via a 5-pin connector.

Step 8: Mount the Valet Switch

The vehicle’s remote control for the alarm system should be a private key for only you and your family to know about.

A valet switch allows you to switch off this system when handing the car to other people, such as valets or mechanics, to ensure they don’t have access to the remote disabler.

Mount this physical switch or button at a hidden spot in the cabin and connect it to the controller.

Valet switches also help reprogram or reconfigure the alarm system, and you can only activate them after disarming the alarm and with the key on in the ignition.

Wrap Up

While the alarm system’s controller functions as the brains behind the operations, the car alarm wire harness is the nerve system that sends and receives the power and data signals. 

So you must install these wires carefully and crimp or solder the electrical joints firmly to form reliable connections. And remember to ground the system to keep the power flowing. 

Check out these additional car anti-theft tips and comment below if you’ve successfully installed an alarm system in your vehicle. Keep us in the loop.