The basic electrical ignition wiring diagram for the 914, early 911 and 356 is shown in Figure 1. As you can see from the diagram, placing an in-line switch that grounds the tachometer signal will prevent the coil from firing. A thief may enter your car and be able to hot-wire the starter, but he will not be able to get the car started because the points will be grounded. The starter will turn over and over, but the car will not fire up because there will be no spark for ignition. This basically has the same effect as disconnecting the coil.
I chose to use a simple SPDT switch from Radio Shack that allows me to alter the tachometer signal cable from the gauge to ground. I placed this switch behind the dashboard so that it couldn't easily be seen from the drivers seat, or from underneath the dash. The switch works really well, and will prevent anyone from starting the car who does not know it's there.
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Here's an additional thought on the subject from one of our readers, 914 owner Bill Kohnke:
Just a thought for the safety of the car and a little theft prevention. I am a Ford salesman and through the years have to explain to many customers about fuel shut off switches that are installed in all Fords. They work in an accident situation by popping up mechanically from the force of the impact and interrupting the fuel pump circuit. Just push the switch down to reset.
I have one mounted to the firewall because if I ever oops & tag the ditch, tree, etc. I don't want the electric fuel pump to empty the tank into an already bad situation.
For theft prevention a little direct tap from a screwdriver handle and the carbs/FI will cease to run very soon or until the switch is reset. This is great because who would be looking for a 1988 - 1998 Ford part stashed above the relay box and can't be seen. The 19 year old has no idea why the car can't run long enough to leave the cellar either.