The Bane of Aftermarket Car Alarms


The humble car alarm has been around almost as long as the car itself, first being developed by an unknown prisoner in Denver, circa 1913. To the security-conscious motorist, they make a lot of sense. The noise of a car alarm draws attention which is the last thing a would-be thief wants, and the in-built immobilizers generally stop the car being moved at all without a time-consuming workaround. Both are a great deterrent to theft.

It may then surprise you to know that I, dear readers, consider the aftermarket car alarm to be one of the most heinous devices ever fitted to the modern automobile. Combining the unholy trifecta of being poorly designed, cheaply made, and fitted by only the most untalented or uncaring people to wield a soldering iron, they are a blight that I myself refuse to accept.

It was my very own Mazda that suffered at the hands of a car alarm system. Two days after purchasing the car, the keyfob died, and thus the car would no longer start. My other car was already out of action due to bent valves, and I needed to get to work, so I figured as a competent hacker, I’d be able to quickly disable it.

In the short term, I was able to find some new keyfobs and get the system back up and running, but in that moment, I knew I wanted it gone forever. Thankfully, this is readily achievable for the average hacker. This guide isn’t intended to help facilitate would-be thieves — the method described is one that takes time and patience, not something that’ll have your car Gone In Sixty Seconds. I primarily write this for the thousands of budding car enthusiasts out there who have bought the second-hand car of their dreams, only to find their beautiful stock wiring has been hacked to pieces by well-meaning fools.

A Tale of Relay Interlocks

The vast majority of car alarms and immobilizers prevent the engine from starting in a very simple way. There are various wires that, when cut, will make running the car impossible. For example, if you cut the wire that runs the fuel pump, the engine won’t get fuel and can’t run. Cut the wire running from the ignition switch to the rest of the car’s electrical system, and the whole car loses power.

Diagrams indicating typical alarm operation in both the armed and unlocked states.

When installing a car alarm, these vital wires are cut. Each end is then connected to a relay, controlled by the car alarm. When the car alarm detects the proper keyfob or other signal, it closes the relay and allows the car to start. If the car alarm instead detects someone trying to start the car without first disarming the system, it will open the relay, no longer allowing current to flow. In the case of my car, with the relays connected to both the ignition switch and the fuel pump relay, the whole car just goes completely dead, save for the now-blinking alarm. Oh, and usually a siren will go off, and your neighbours will hate you.

So, how do we go about removing a car alarm? It’s as simple in most cases as identifying the wires that have been cut, removing the relay, and splicing the cut wires back together. This sounds easy in theory, but the shoddy nature of most installs and the absolute wire spaghetti that results can make it very difficult. Expect to face off against bare wires twisted together with tape, solder joints that are entirely uninsulated, or the dreaded Scotchlok (TM) connector.

This relay was used to cut power to the fuel pump. The green wires were spliced inline with the fuel pump’s 12 V line. The red and yellow wires are the relay’s coil wires, controlled by the car alarm.

RTFM (If You Can) — Then Clip the Wires

I started my job with a Google search. Your results will vary since “security” companies selling aftermarket alarms likely don’t want to details of the systems to be easy to find. I got lucky and turned up a manual for a similar alarm made by the same company, which suggested the alarm would cut the fuel pump power supply and the 12V line from the ON position of the ignition switch. After a bit of digging around, I found exactly these two relays. I disconnected the battery for safety and got down to work.

The relay which cut the fuel pump was buried in the driver’s side kick panel, making an unsightly lump under the carpet. The relay had two fat green leads that were spliced into the blue wire supplying the fuel pump. I took the liberty of cutting the relay out, and twisting the two ends of the blue wire back together.

This nasty fella was tucked up under the steering column, spliced into the circuit coming from the ignition switch in the ON position.

Next, I tackled the second relay, buried under the steering column. Spliced into the ignition switch’s ON wire, it completely shut down the car’s electronics when triggered by the alarm. Again, I cut out the relay and twisted the original wire back together. I was careful to make sure the bare wire wasn’t shorting on anything before I tested my work.

A quick turn of the key, and the car sprang to life! I’d successfully managed to remove the immobilizer part of the car alarm. All that was left to do was to solder the wires back together.

I no longer had to worry about my car being disabled by a failing keyfob or an oversensitive alarm. I decided to leave the rest of the alarm in place for now, sans the control box which went straight in the bin. The next few weeks will see me gently peeling out the rest of the alarm hardware, which interfaces with tilt sensors, brake switches and door locks.

The wires were twisted back together for testing, before being soldered in a linesman splice and covered with heatshrink.

Tackling this job got my car back on the road, and it’s great knowing that my car is no longer at the mercy of a temperamental piece of electronic junk. Check out the video below for more of the gritty details on my own alarm removal. Naturally, we’d also love to hear your stories of auto-electrical nightmares in the comments! Happy hacking.