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Home > Technical Articles > An Easy and Inexpensive Ignition Cut-Off Switch

Guest Technical Article

An Easy and Inexpensive
Ignition Cut-Off Switch

Difficulty Level 2

Difficulty scale:
Adding air to your tires is level one
Rebuilding a 911 Motor is level ten

Wayne R. Dempsey


[Click on Photo]

Figure 4
Figure 1: Simple
Wiring Diagram

    When I first acquired my 356B it had a 912 engine installed along with a mechanical tachometer. Seeing how the mechanical tachometer will not work with the 912 engine, I quickly set out to convert it to electric. After studying the electrical diagrams, I soon realized that the tachometer line connects to the output from the points that drive the ignition coil. Thinking about this in detail, I soon realized that the points that trigger the coil would not work if the tachometer cable was accidentally grounded. With this in mind, it became apparent that if I placed an electrical switch that swapped the electric tachometer cable between the gauge and ground, I would have a handy ignition cutoff switch with very little extraneous wiring.

   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.

     Comments, questions or feedback?  Feel free to drop us a line.

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.

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