Best kill switch technique?
 

I want to put a kill switch on my 2009 2.3 liter auto. Not to get into a huge story buy there are occasions I need to leave it unlocked with the keys in the glove box or whatever so one of my co-workers can use it when I'm not around. I want to add a hidden kill switch so when I do this a theif can't just take off with my truck but my co workers would know where the switch is. Don't need anything james bond style, just a good idea what wire under the dash would be an appropriate one to cut and splice in a hidden toggle switch?

Try Fuse #21 in cab fuse box(SJB), 5amp
Pull it out and try to start truck, starter motor should NOT work

When key is turned to START ignition switch sends 12v to fuse #21, 12v goes thru the fuse and out to Neutral switch and then to Engine fuse box to activate the starter relay.

ignition switch-------(blue/white stripe wire)-----------fuse 21-------(blue/grey stripe wire)--(firewall)-(neutral switch)-----------starter relay

If you can find the Blue/white stripe wire under the steering column then you could add toggle there, I would add a 5amp fuse between toggle and ignition switch because a short there would burn out ignition switch and you would be stuck where you sit, lol.
And toggle needs to be rated at least 5 amps at 12v.

But test when fuse 21 is removed you have a no crank first, diagram for 2009 shows it that way..............but it ain't a Ford diagram

You could also extend the #21 fuse with in line fuse holder and have it out above passenger kick panel so under dash but fuse can be inserted and removed without pulling kick panel off


If you want to get fancy you could use a dual relay circuit and momentary contact switch.
Each time key is turned off the relay opens, so no power can go to fuse 21
When key is turn on, relay stays open UNTIL someone presses the momentary contact button, then relay closes and stays closed until key is shut off again.
So no "remembering" to disable fuse 21 circuit

Scroll down to #3 diagram here: https://www.12voltplanet.co.uk/relay-guide.html

Is your truck and automatic or manual transmission?

While disabling the starter is an idea you may want to disable the fuel pump via hidden switch - Thief may get it started for about 3 seconds from primed fuel pressure then try to restart it. People pay attention to a truck that just cranks and cranks or stumbles for a few seconds but never fires. Makes a buttload of noise trying to start a truck and thief will panic and leave it while alerting anyone nearby. Will definitely wake anyone up. If the starter is dead they can sneak in try to start and it does nothing then sneakily leave unnoticed.

or use a battery disconnect under the hood with a small 15A fuse going around the disconnect (keeps computer powered, radio presets, things stored in memory etc but the moment someone tries to start it without the disconnect made the starter pops the 15A fuse)

Yes, the Inertia switch is in the cab on passenger side under the dash, you could use a Relay on the OUT TO PUMP wire, purple/white stripe in 2009, to enable and disable fuel pump power.

Although a problem with this added circuit would cause engine to die while driving, and you would be stranded until you could find out why it failed.
Not that it would fail, just that it could.

Starter motor disable, at worst means you can't start/move the vehicle, until repaired, not ideal, but a far cry from stranded on the side of the road, or in traffic, if it dies at rush hour

Just playing devils advocate and applying Murphy's Law and the addendum to it
Murphy's Law - "whatever can go wrong, will go wrong"
addendum - ", and at the worst possible moment" (Finagle’s Law)

Yea the starter cut off does seem simpler and safer. Im not looking to fool professional thieves here, just stop some teenage jerk from trying to hop in and ride off. If the starter doesnt engage they would probably just assume the keys were in it because the battery is dead. Im gonna go dig around under the dash to see if i can find the wires ron mentioned.

If fuse #21 is the correct fuse you can also use a "fuse tap," or "piggy back" in the fuse panel so no wires need to be cut.
Run that to a switch on drivers side

Then it can be removed at a future time, no harm done

https://cimg1.ibsrv.net/gimg/www.ran...6072404b4b.jpg
Just wanted to give an update since I finally got around to installing my kill switch today. I did as ron suggested and used the #21 starter relay fuse. I made a fuse extension harness with a toggle switch inline. I just ran the harness up and mounted the toggle behind the glovebox in a conveniently placed pre-existing hole. You cant see the switch with the glovebox open, you literally need to pull the glove box off and lay upside down to see it so I think its in a great spot, but still easy to reach behind the glovebox through the little hole at the top and turn it on and off from the drivers seat. Check out the pics and feel free to criticize if you see any potential issues. Like I said previously this wont stop professional thieves but I'm just trying to deter drunks and crack heads lol. And I really like the fact that I can just yank it out at any point and put the fuse back where it was. No wires were cut or any modifications to the truck itself were done. Hardest part of the whole thing was cutting apart a fuse to steal the terminals and then soldering the terminals onto the wire that go into the fuse block. (Second pic below)

https://cimg0.ibsrv.net/gimg/www.ran...be8d69d215.jpg
https://cimg8.ibsrv.net/gimg/www.ran...4b32818079.jpg
https://cimg7.ibsrv.net/gimg/www.ran...cc8ea96844.jpg
https://cimg1.ibsrv.net/gimg/www.ran...fff8881de7.jpg
https://cimg6.ibsrv.net/gimg/www.ran...616867259e.jpg
https://cimg4.ibsrv.net/gimg/www.ran...55f7366ebc.jpg
https://cimg3.ibsrv.net/gimg/www.ran...37ba3aa05a.jpg

Nice install


Does it work?


Oh and what happens if someone takes the keys and walks off?
You got a spare set around?

Works like a charm, even played a joke on a couple people at work before i told them about the switch lol. And I have a extremely hidden spare key on my truck just for the reason you mentioned.