The Hungry Hacker's Explanation of Everything

Home » Automotive

1985 Suburban: Killswitch (conventional and alarm operated)

5 May 2006 No Comment

A well thoughtout killswitch is extremely effective in preventing the theft of your vehicle. I hate starter-kills, because they are silent in operation, and by the time the thief gets to the point of making noise, the switch has been defeated. If you disable either ignition, or fuel injection, the car will make quite a bit of noise in the time that the thief is unsuccessful. Many thieves will abandon a car that’s making noise, if they can’t start it in 60 seconds – so it’s critical the starter makes noise, but the thief is unable to start the car without several minutes of fooling.

However, never assume you are smarter than the thief.

You will need:

  • Spade terminal matching that which connects the ignition wire to the low side of your ignition coil.
  • 12vdc normally closed relay. Ensure it will be able to remain energised during cranking!
  • Between 3 and 10 feet of wire to match the ignition wire on your coil. Ideally you want it to appear stock.
  • SPST switch, or alarm with starter kill wire.

Common Steps to both methods:

Locate the ignition coil of your vehicle. On my 454 V8, the HEI ignition coil is built onto the distributor. The wires we’re interested in are on the driver’s side (USA) at the top of the distributor. One is marked “BAT” and the other is “TACH”, disregard the tachometer connection for now, BAT is the one we’re interested in. You will also need to locate a 12vdc wire which is hot when the ignition is on. You can use this same wire if you like, or find another. Once you have discerned which wires you need, I heartily recommend disconnecting the battery from the vehicle. The wire connects to the location circled in figure #1, and you can see me holding it loose in figure #2 (Note that my distributor cap is tilted forward to ease access).

Disconnect the wire from the coil, cut the wire close to the terminal, discard the terminal and purchase a new one. Drill a hole through the firewall into the cabin – I actually drilled from the cabin out, being less likely to hit a heating/ventilation component that way, and because I already had such components removed. The lower to the ground (and thus, futher behind the motor) this hole is, the harder it is to spot.

Run the coil wire through a rubber grommet of suitable size, and then send another wire through as well (for best results, choose a wire of matching color, mine was a faded red). Press the grommet into the firewall and seal if necessary. Attach the new wire to the new terminal, solder (optional, but I did) and re-attach the terminal to the coil.

Inside the vehicle, solder or otherwise attach the two wires to the switched poles of the relay. Take the hot wire (not the new one that goes to the coil, the other one) and jumper it to the positive side of the relay. You want these connections to be secure, lest your engine die on the highway. Tape ’em up pretty good, because a blown fuse or a fire would also be bad (and kill your engine as well).

Here’s where we divert depending on how you’re wiring it.

Conventional Kill switch:

The last wire on the negative side of the relay coil needs to go to a regular switch. Hide this switch pretty well, but not so well that you fail to use it due to inconvenience – a switch you don’t use is useless! The other side of the switch goes to a good ground, and you’re done. Verify it works (you should be able to flip the switch and kill a running engine), and ensure that everything’s clean, secure, and sealed/insulated.

Alarm-operated Ignition Kill:

The last wire on the negative side of the relay coil runs to the “starter kill” wire of your alarm module. This is assuming your alarm module’s starter kill wire is the “switch to ground when armed” type. If it’s hot-when-armed, or another variety, you’ll need to rethink this layout. Consult your alarm manual, and if you can’t figure it out, scan your alarm manual and email me, and I’ll try my best to help you out.

Ensure all connections are secure, and with any luck trying to start your car while the alarm is going off will be ineffective. I like to use these with a pager and the alarm on silent mode – it stops the thief from being able to take off with the car, lets me know, and generally doesn’t give the thief that the car is anything other than a difficult-to-start piece of shit until you arrive on the scene with baseball bat in hand.

The Good:

Starter kills are ineffective, as I said above, whereas this switch is quite effective in stopping all but the most determined and experienced thieves. If you connect it to an alarm, it’s virtually painless to use – I have my alarm default to silent, and it is enabled when I hit the lock button.

The Bad:

It’s still possible to hotwire the car under the hood still, simply with a wire jumping from the battery to the ignition coil (I’ve started cars like this before, and it’s in my opinion easier than tampering with the ignition switch in the dash/steering column). If you want to put a little more thought into it, perhaps switching the negative wire of the ignition coil would work better, as it would require the thief to know something’s up, and use two pieces of wire to get spark.

The Bonus:

If your alarm is like mine, the “starter kill” is activated when the ignition is on and the panic button is pressed. Having the “starter kill” do this is absolutely pointless, but when you have wired your car the way I’ve described here, you get a neat bonus: anti-carjacking.

On my alarm, when the ignition is on and the panic button is pressed twice, the alarm will begin to sound, and 30 seconds later the “starter kill” circuit is activated. This effectively lets someone get about a mile down the road before the engine dies – far enough away from you that they won’t take their anger out on you, but not so far that you can’t recover your vehicle.


  • Search for alarms on eBay.
  • had a rather nice article about using a reed switch with a magnet as a “key” to hide the killswitch, but it appears to be down. I’ve linked the google cached version.

Leave your response!

Add your comment below, or trackback from your own site. You can also subscribe to these comments via RSS.

Be nice. Keep it clean. Stay on topic. No spam.

You can use these tags:
<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

This is a Gravatar-enabled weblog. To get your own globally-recognized-avatar, please register at Gravatar. Note: By filling out this comment form or emailing us you are signifying that you have read and agree to the terms laid out on the Contact Us page.