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.

[Shivers]

[GregAmy]

However, when I disconnected it from the tach, the car would still start!

As expected.

But if you GROUND the tach wire, the car won't start.

But I don't really recommend it. Install a battery cutoff switch instead, with a removable handle.

[jvmarino]

An easy anti-theft method is to pull the coil wire to distributor. Even if you leave the wire in the engine bay, I would expect most thieves would figure after lots of cranking and no starting, the car is just an old car and is not working, and then walk-away.

[FlatSix]

Thanks.
I'm really looking for a solution that doesn't involve opening the engine lid and disconnecting wires or pulling the rotor.
What are the implications of grounding the black/purple wire? I don't want to cause myself any unnecessary problems further down the line.

[mepstein]

Years ago I had my shop wire a cut off switch into the stock fog light switch. It was pretty stealth.

[GregAmy]

What are the implications of grounding the black/purple wire?

The tach wire? The coil never collapses, no ignition fire.

It's the same condition as when the points are closed. As long as you don't leave the key on for hours it probably won't hurt anything.

[914e]

In the US just being a manual transmission is anti theft. May not work in other countries where people know how to operate a vehicle. However you end up I would suggest you break the connection and extend wires at a connector that way you can return it the the way it was without cutting into the harness.

[ClayPerrine]

If the car is injected, then just put in a switch to cutoff the fuel pump. That way the potential thief thinks there is something wrong with the car because it starts and then dies. If you do this to a carbed car, it will run on what is in the float bowls then die.

Be careful with grounding the tach wire on a six. The CD boxes don't like you doing that.

[vitamin914]

I have a switch for the fuel pump and auxiliary oil cooler fan. Both are in plain sight and unlabelled on the dash (plus, I have a battery disconnect for storage and electrical work). More than once I have forgotten to turn on the fuel pump only to have the engine die shortly (carbs). Fooled myself several times thinking something failed because I forgot about the switch - D'oh!

Maybe reconnect the the seatbelt interlock under the passenger seat that most have defeated (by connecting the yellow wires together). Then mount the switch under the passenger seat. Nothing under the dash to worry about in that case. Not sure if the '73 had the interlock...

[Chris914n6]

Problem is all the important wires are in the engine bay.

I don't remember if the 73 has the seat belt relay under the pass seat, but that would be easy to tap into. Both engine power and start go thru it. If you install a start relay, like you should anyway, it will be a low load and any switch will suffice.

[Superhawk996]

What are the implications of grounding the black/purple wire?

The tach wire? The coil never collapses, no ignition fire.

It's the same condition as when the points are closed. As long as you don't leave the key on for hours it probably won't hurt anything.

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[jhynesrockmtn]

When I bought my 1970 the yellow switch was a bit of a mystery. The car has factory fog lights. This was thought to be a rear fog light switch possibly but no sign of that in the rear.

Turns out after talking to the original owner this was his anti theft switch hidden in plain sight. He did it in the 70's but was disconnected much later when the engine was rebuilt.

He figured it looked stock. You had to pull the switch to start the car.

[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]

You may want to rethink that relay plan.

The ignition coil is subject to flyback EMF (high voltage spikes) that many relays don't respond to very well without a flyback diode across the relay solenoid coil.

[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

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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.

[mmichalik]

I know you wanted a simple solution but, the Bluetooth 123 distributor has a function where you can set it up so the car will not start without your phone. They call it the "immobilizer" button.
Works very well as an anti-theft device.

[roblav1]

I did the fuel pump switch thing on mine... about the 3rd Porsche I did that.

[emerygt350]

I simply bought a chalon. Problem solved.