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.

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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.

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.

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.

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

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.

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.

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.

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...

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.

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.

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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

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


...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

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.

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.

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.

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

I simply bought a chalon. Problem solved.

not like the CD box is worth risking due to replacement cost,I have a nice little switch that breaks the pump power supply,its not easily seen and works good.

I know the OP doesn't want to pop open the lid, but if I'm worried about my car I just reach in there and remove the fuel pump relay. Even have it marked with a silver Sharpie (including rotation direction) so I can grab and reinstall it easily.

I mean, it's not like this is an Ocean's Eleven crew looking to steal a Porsche 914, equipped with a well-versed Porsche Master Technician to troubleshoot a no-start...one or two good turns on the screwdriver-key and they're likely to just walk away.

The OP is from Poole, England. Yes, here (North America), the manual trans helps weed out a lot of would be joy riders. Probably less so in the UK...

I wonder if the 914 is a left hand drive car. That could be a slight deterrent in itself in a right hand drive vehicle country. A cabin switch like any of the posts would be the best.

I wonder if there is something like this for older cars?

https://www.mobilistics.com/nostart-remote-...mp-immobilizer/

Best anti-theft I ever saw was my brother hooked up a button under the carpet that killed the ground to the fuel pump. It worked, of course then the thieves came back and hooked a chain to the car, dragged it three blocks and stripped it.
But it worked the first time.

works here now as well.
no one under the age of 30 can drive a stick or work a third pedal.

--

i have 74.
have a switch wired off the seat belt interlock relay stuff under passenger seat.
which is my driver seat (rhd!).
convenient.
but if you got an earlier car not available to do.

Move to a safer neighborhood & park your car in your garage.

I am more worried about totally forgetting I did any custom wiring….

Get one of these?

Don't be so Cruel on yourself, 80's style was Rad !

Not really stealth, but it's what I have. Most idiots would not know what it is.

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If they really want the car, they are gonna show up with a trailer, or a tow rope at least .

hell yeah. emery is on to something.

but

just when he thinks he is safe and can go charming grandmothers down at the mall parking lot, he will turn his back and have it nicked because its the thing some 20 year old has to have. i reckon he is in the high risk bracket.

mullets are in. watch out.

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Truthfully, the chalon only protects me from uppity Porsche collectors but that might just be enough.

I would bet they use trailers too.

If I wanted to stop the more casual thief I would run one of those cheap new remote relays you can get and put it on the solenoid.

Examples
You can probably find one that has a "on" failsafe.

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Go get one of those 123Dizzy's with bluetooth, it has a immobilizer in the phone app...
Another very very old idea is to put and keep criminals in jail. I am going out on a limb here but, I bet car thief's have practice from all the times DA's let them off because of.....


Matt

The bluetooth looks good but I have had way too many sketchy moments with bluetooth pairing.

THis one looks interesting. I like that it has a manual button on the relay in case the fob or button doesn't work. Tempted to hook this to the ford solenoid hack.
https://www.amazon.com/dstfuy-Wireless-Ceil...sr=8-5&th=1

I added an under the dash cut-off switch for the fuel pump on my 76 (front mounted pump) & carbed. I find it useful if I'm chasing light bulb issues and don't have to have the fuel pump running.

Some really interesting suggestions here, thank you all.

Being in the UK, the manual gearbox is not a problem and even the selector pattern is easily overcome. The car will pull away in 2nd gear and reverse can only be in one of four places! LHD is problematic when overtaking but not an anti-theft deterrent.

I'm liking the ideas of either cutting power to the fuel pump or using the interlock device. I also like the idea of easily unplugging any changes I make and returning it back to stock should I need to for troubleshooting etc.

I apologise for being somewhat economical with the truth earlier. Whilst the engine is from '73 the car is actually a Californian spec '76. Therefore there is a kinda interlock system under the passenger seat. This is much simplified for '76 as there are no pressure plates in the seats. The car runs fine without the relay in place. All the relay seems to do is illuminate the fasten seat belt sign for a few seconds. I believe the yellow wires are bypassed at the factory. I don't want to put a switch in line with the yellow wires as they look to be taking a fair bit of current. I'm not sure how I can take advantage of this with a switch under the dashboard though.

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The yellow wires are not for the fuel pump. They connect the ignition switch to the starter to make the engine crank. If you interrupt that, the engine won't crank over, but the car could still be push started.

I once put an anti-theft switch in a 280Z for a customer. I put a button under the carpet between the clutch and brake pedals. You had to put it in neutral, and the step on the right spot on the carpet to get the engine to crank. The customer quickly got used to it.

For a 914-4, you can ground the tach wire on the back of the tachometer. Or kill the fuel pump, or something similar.

There are also GPS Trackers. https://www.amazon.com/GPS-Trackers/b?node=617650011 With one of these, just let them steal the car. then go get it back when it stops moving. Or notify the cops and let them get it back for you.

I use a battery cutoff switch with a removable handle. I seem to always be working on some electrical upgrade/problem and it is so easy just to turn and remove the handle. At this point even those pesky electrons cooperate. I have used both types of battery cutoff switches, the type that simply disconnects the battery from ground and the type that also connects to the alternator, to try to save the alternator diodes in the event of an emergency battery disconnect. Pegasus Racing has always been my source for these.