Simple Anti-Theft Switch, Wiring suggestions Options

[FlatSix]

So I want to install a switch hidden under the dash to stop the engine starting.
Having done a search, the black/purple wire going to the tachometer seemed a likely candidate to break the circuit. However, when I disconnected it from the tach, the car would still start!
Which wire should I use?
The engine is a 1973 2.0 four cylinder with fuel injection.

[jrmdir]

Hi:

There are some good ideas here and perhaps the most straightforward, as has been mentioned, is the grounding of the tach wire approach. But apparently having too much time on my hands (and wanting to avoid working on taxes!) this off-the-wall approach came to mind and I'm sharing just for fun. (Still needs to be validated with a prototype if I can get around to it.)

The idea is to interrupt the +IGN wire at the coil by inserting a simple latching relay as shown. When energized, the engine can be started normally. Because of the latching connection, the relay will stay energized until the key is turned off.

So all that's needed is a momentary +12 application to the relay coil and it seemed to me that the heater fan switch would be a cool way to to do this from the driver's seat. Pull up on the lever to latch the relay, then push it back down. Start the car and you're off.

Since this scheme ties the +IGN together with the fan +12, a suitable diode is needed to keep the fan from running all the time.

However, any momentary shot of +12 would work if you want to run the wires (e.g reverse lights, fog lights, maybe even a turn signal.)

I'm probably missing something so feel free to poke holes and or suggest enhancements. (One thought is an optional bypass switch in the engine compartment in case the relay and/or heater switch or fuse fails at some point.)

Cheers,

Ron

[GregAmy]

I'm a seriously KISS guy. And one issue with your design...

When energized, the engine can be started normally. Because of the latching connection, the relay will stay energized until the key is turned off.

...is that a failure of that system leaves you dead in the water and unable to drive it away. Or worse, fails while you're in stop/go traffic on the Interstate -- in the middle lane, of course.

My recommendation is to design something that fails "safe", as in you can still drive it. Which will be 99.9999% of the time the car is attempted to start.

GA

[Superhawk996]

My recommendation is to design something that fails "safe", as in you can still drive it. Which will be 99.9999% of the time the car is attempted to start.

GA

(IMG:style_emoticons/default/agree.gif)

Per previous post, in today's Merica' a manual transmission is an anti theft device. If you still want to double down on that with a kill switch a simple on/off switch on the coil will do it.

Beyond that, not sure exactly what the goal is here.