I joined the forum just to answer this question. I just installed a similar kill switch in car on the parking light circuit and then decided (belatedly) to check if anyone else had approached it in the same way.
It is actually a very simple and clean way to install a kill switch. It requires no extra wires into the cabin, no drilling of new holes (a problem I faced) through the firewall and it can easily be returned to stock standard with a few deft snips of wire. It requires basic car electrics knowledge and an understanding of relays. I suggest you buy a simple circuit tester and read this:
http://www.autoshop101.com/forms/hweb2.pdfSkill with a soldering iron is ideal, but you can be as messy as you like because you'll cover it up with insulating tape afterward.
Materials & Equipment
- 15Amp Wire
- Automotive Relay
- Solder
- Electrical Tape
- Soldering Iron
- Crimp Connectors (if you don't want to solder)
- Cable Ties
Purchase a 60Amp relay. This way it will cope with ANYTHING required of it. The input amps on the coil are normally only 20A but you might as well allow for spikes in current draw. Most relays of such a high amperage will be 5 pin relays, which have a normally-on position and a normally off position so that with the switch off power still runs through on one pole but with the switch on the power runs through the other pole, deactivating the normally on pole. This way, you can do fancy things like wire a switch to flick between two devices.
Step 1: Find your ignition coil and then identify the live wire from the parking lamp nearest to the coil. Cut it and add a new length of wire at the cut. You'll have the power coming along, hitting the join and then going to the parking light and to your loose-end of wire.
Step 3: Wire the loose end of that wire to the comms-pin of your relay. The comms-pins are normally a smaller size than the actual power pins as they only have to deal with the small current running the switch and the bigger pins can run the actual device (in this case, the coil).
Step 4: Wire the other comms-pin to the earth of the car (any bit of bodywork). There are always bolts/screws tapping into bodywork which can be lifted and have a wire attached to them (either use a proper connector shaped like this: --O with a hole on the end to stick the screw through or just wrap the wire around the shaft of the bolt or screw and tighten around the wire. As long as you tighten fully it'll still do the trick.
Step 5: Detach the positive IN from the coil. Cut it halfway along its length, and add extra length of wire on both cut ends. You then attach each of the loose ends of your new wire to the power-pins on the relay. Make sure you attach it to the 'Normally Off' pin so that without the parking lights on it won't start. Reattach the connector the the coil.
Step 6: Insulate all your joins with plenty of electrical tape.
Step 7: Use your cable ties to hold the relay up and out of the way of rainfall or areas where water may accumulate. Cable ties are ideal because they are temporary and will allow you to easily remove the relay should you decide to change your security solution or remove it entirely.
Step 8: Use cable ties to tidy up the wiring.
Here is a schematic:
------- ----
|Lamp| ______________|Join|_______________<<Power in for lamp<<
------- -----
|
|
|
----------
>>Coil Positive IN>>___|RELAY|______>>Power going to Coil
---------
|
|
|
_____
___
-
Earth
The beauty of this solution is that it is easily removed either to return the car to standard (remove relay and rejoin wiring at the cuts on the positive IN wire and the parking lamp wire) or if some part of it fails. Most importantly, it is imperceptible from the cabin. With the parking lamps off, the starter motor will turn over, but there will be no spark. It will sound as though the car is out of fuel. With the headlights or parking lights on, the car will spring into life. They also say parking lights in daytime increases the visibility of your cars, so it's road-safety by necessity.
Now, any car thief worth his salt knows that to start a car you only need to:
1. Break the steering lock.
2. Wire the coil straight to the battery
3. Get the starter motor going by briefly jumping it with a cable to the battery.
All the under-dash stuffing about is pure Hollywood. In short, if the thief is anything more than a joy-rider or a heroin-junkie, you need to find a way to keep the engine bay secure and inaccessible. Joy riders will smash up your dash to rewire the ignition key barrell. Professional thieves will rewire your under-hood electrics.
For the cheap junkheap I wired my killswitch into, all I'm trying to deter is joy-riders (as my make/model is notoriously easy to break into and hotwire) and due to a front-shunt to the car you need two people to open the hood; one to pull the release cable, the other to lift up the hood the first part of the way (before you stick your fingers in to un-latch it). For classic Mercs, you might want to look at securing the engine bay somehow so they can't hotwire it through the coil easily.
