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Kill switches...

Joined
11 July 2014
Messages
1,432
Location
Chicago, IL
It has come to my attention that I may need to consider a kill switch. I suppose I ought to look into this idea. My recollection of a kill switch was something located under hood and easily accessed.

Any of you have any creative setups?
 
Theft deterrent. Thanks for asking.

All you need is a relay and use it to disconnect the 12v feed to the fuel pump. No fuel no run.

You can also use a replay to bypas the 12v feed to the ECU or main relay. The ECU has some negative side effects so that would be my last choice.

Dave
 
I'm thinking of installing a secret kill switch so I'm reviving this old thread. The main relay seems like a good thing to interrupt, and not to hard to find?

The fuel pump seems easy, but is there wiring inside the car to access?

(I have a recollection that it's bad for the fuel injectors/fuel injector pump to run it without fuel. I'd prefer the fuel cutout since it would seem to be on and cranking, whereas interrupting the main relay would result in no dash lights, making the thief look for the kill switch.)

Any other suggestions? I'm still waffling since a thief can still tow it away.
 
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The main relay should be behind the center speaker. The connector is pretty common, so you can create a T-harness splice without cutting the factory wiring, and makes testing/reversal super easy.
 
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The vehicle already has a starter motor interlock (starter cut relay). Adding a second hidden mechanical switch into that circuit is relatively easy which would give you the same functionality as having the clutch interlock switch button drop out. It is easy for a knowledgeable thief to by-pass this by jumping the starter motor directly.

If you want a second level of interlock and you want to have the dash warning lights operational during power up when the security interlock is enabled the ECU has to power up normally. That leaves you with two options, separately disabling the ignition or the fuel supply. Personally, I like the idea of an interlock in the fuel supply because cranking the engine until the cows come home with a dead ignition system just fills the engine with unburnt gasoline which is much worse than any risk associated with operating the injectors dry (that is not a problem that I have ever heard of).

The fuel pump intercept is also relatively easy to do. The main FI relay is located inside the cabin on the back firewall and is moderately easy to access. The main FI relay is two relays in one. One relay powers up the ECU and the other relay powers up the fuel pump. This is the diagram for the main relay circuit.

Main FI relay.JPG

The fuel pump portion of the main relay powers up after it gets a ground enable signal from the ECU on the green/black wire. If you put a switch in this circuit you can permanently disable the ground enable circuit preventing the fuel pump from running even though the ECU has powered up. I like the intercept in this circuit because it is low current ( probably less than 0.050 amps). You could also put the switch in the main supply to the fuel pump (black/yellow wire); but, this is a high current circuit (6 -10 amps) and requires a much more robust switch plus you are adding additional wiring (and resistance) into the fuel pump supply which is never a good thing.

Note that the relay in the diagram confusingly called the fuel pump relay is not the fuel pump control relay. The fuel pump relay is really the fuel pump resistor by-pass relay. Intercepting the ground on this relay with a switch will not disable the fuel pump and could cause problems with high load engine operation if it failed open.

If you want to add additional sophistication you could add Drew's TK-103a into control the circuit so that you can shut off the fuel pump.
 
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The main relay should be behind the center speaker. The connector is pretty common, so you can create a T-harness splice without cutting the factory wiring, and makes testing/reversal super easy.
Can you give me more details about this? Is there some place I could look up a part number or how to find these parts to make a T-harness splice?
 
I found the connector between the main cabin harness and the fuel pump.

Just matched it up with a M+F connector set at the junkyard with at least 2"/50mm of wire on each side of each connector. Should be a version of the Sumitomo HD090 series.

Use butt crimp connectors to connect the wires leaving the fuel pump hot wire open and routed through a relay. I recommend a relay as the power going through it will be higher than a small switch can handle. Get the relay from junkyard as well, so it matches well with your existing equipment.

It won't be obvious to a potential thief if everything looks like it "belongs"

You can buy all the parts from CycleTerminal.com, but you will need special crimpers for the terminals.

PM/DM me and I be glad to send photos and help out. If you don't mind paying for shipping from NZ, I probably have what you need in the garage.
 
The vehicle already has a starter motor interlock (starter cut relay). Adding a second hidden mechanical switch into that circuit is relatively easy which would give you the same functionality as having the clutch interlock switch button drop out. It is easy for a knowledgeable thief to by-pass this by jumping the starter motor directly.

If you want a second level of interlock and you want to have the dash warning lights operational during power up when the security interlock is enabled the ECU has to power up normally. That leaves you with two options, separately disabling the ignition or the fuel supply. Personally, I like the idea of an interlock in the fuel supply because cranking the engine until the cows come home with a dead ignition system just fills the engine with unburnt gasoline which is much worse than any risk associated with operating the injectors dry (that is not a problem that I have ever heard of).

The fuel pump intercept is also relatively easy to do. The main FI relay is located inside the cabin on the back firewall and is moderately easy to access. The main FI relay is two relays in one. One relay powers up the ECU and the other relay powers up the fuel pump. This is the diagram for the main relay circuit.

View attachment 173062

The fuel pump portion of the main relay powers up after it gets a ground enable signal from the ECU on the green/black wire. If you put a switch in this circuit you can permanently disable the ground enable circuit preventing the fuel pump from running even though the ECU has powered up. I like the intercept in this circuit because it is low current ( probably less than 0.050 amps). You could also put the switch in the main supply to the fuel pump (black/yellow wire); but, this is a high current circuit (6 -10 amps) and requires a much more robust switch plus you are adding additional wiring (and resistance) into the fuel pump supply which is never a good thing.

Note that the relay in the diagram confusingly called the fuel pump relay is not the fuel pump control relay. The fuel pump relay is really the fuel pump resistor by-pass relay. Intercepting the ground on this relay with a switch will not disable the fuel pump and could cause problems with high load engine operation if it failed open.

If you want to add additional sophistication you could add Drew's TK-103a into control the circuit so that you can shut off the fuel pump.
Thanks for the info @Old Guy & @drew, and especially the wiring diagram. I'm starting to understand how the "main relay" (or should it be main relays?) works in order to add a kill switch to stop fuel delivery, but the fuel pump relay with two supplies to the coil is confusing.

So, how does the "main relay" work? I can see it contains an ECU relay and a fuel pump relay. (As you noted, the relay labelled fuel pump relay should actually be labeled fuel pump resistor bypass relay.) Since the main relay has multiple same-coloured wires, I'll call them, starting clockwise from the top left: 1-YEL/BLU, 2-BLK/YEL, 3-BLK/YEL, 4-BLK/WHT, 5-YEL/BLK, 6-GRN/BLK, 7-BLK/YEL, 8-BLK.

This is the issue: I don't want just simple on/off switch that I need to remember to turn off when I leave the car. I want to use a relay that is activated with a momentary contact but that shuts off each time the ignition is turned off. (Can do this with a diode from the switched lead of the relay back to the coil, which then keeps the relay activated after the momentary switch is released until power to the relay is terminated by shutting off the car. I'll probably use a small PLC controlled relay so I can disable it when visiting the dealer, but this can only supply ground or hot based on supply, not connect the black/green wire so I'd need another relay. See timers.shop) I'm thinking of installing it inside the main relay.

As a result, if I interrupt the fuel pump with my PLC relay, the fuel pump 12V will power my relay. I have to be sure that that supply won't get interupted by something else (like the ignition/fuel getting cut by the rev-limiter) which I suspect could be the case for the main cabin harness running to the fuel pump. Thus, the rev-limiter cutout won't reactivate the fuel pump until I press the secret switch again. So interrupt prior to "main relay"?

Guessing: 2-BLK/YEL (switched on by the ignition key?), activates the PGM/FI relay coil, providing voltage from (always hot?) 1-YEL/BLUE to 5-YEL/BLK which supplies the PGM FI ECU. I assume that the other side of that relay coil 8-BLK goes to a fixed ground which also seems to ground the ECU (and the AT controller)? Internally, the 5-YEL/BLK also powers the coil of the fuel pump relay although as you noted the ECU controls that relay by controlling whether 6-GRN/BLK is grounded. Interestingly, the Fuel pump relay coil can also receive voltage from the 4-BLK/WHT which I can't figure out, or BLK/WHT from the ECU, which I also can't figure out. (The ECU grounds the red wire of the fuel pump resistor bypass relay to raise the voltage to the fuel pump - I agree I definitely don't want to affect that and potentially overpower or underpower the fuel pump.)

Does the ECU rev limiter "unground" the 6-GRN/BLK to cut power to the fuel pump as well as stopping the fuel injectors via their individual ground controls and presumably the ignition too? That seems to be the only reason to not just have the 6-GRN/BLK not just go straight to ground via 8-BLK. Or is it monitoring the fuel pump relay voltage?

I ask because, in that case, I need to interrupt 3-BLK/YEL, not 7-BLK/YEL, because if the ECU cuts power to the fuel pump this way and I've interrupted 7-BLK/YEL, my kill switch relay will also shut off and have to be reactivated each time the rev limiter is activated.

I was considering interrupting 1-YEL/BLU, but I don't think that would work if either a) it's always hot so won't reset the relay when the ignition is shut off, or b) the fuel pump relay (and the ECU) still be powered by the WHT/BLK.

Thanks for any insight,
 
a kill switch is a nice peace of mind mod. I have one on the SUV and it presents a hurdle to would be thieves . A smash and steal is thwarted and would require a tow vehicle ect...

For newer cars there are very exquisite switches such as ravelco and Igla..
 
First off, the easy bit. The main EFI relay is powered up by the ignition switch. The internal relay that powers up the ECU and ignition coils stays powered up as long as the key is on. That relay also applies +12v to the coil of the relay that controls the fuel pump. However, that relay is actually ground switched on - off by the ECU. The ECU switches the relay on for about 2 seconds at initial power up and any time the engine is turning (cranking or running).

Second easy question. Based on all (which definitely is not all the ECUs that exist) the ECUs I am familiar with, the ECU does not implement a fuel cut or ignition cut. by shutting off the power to the pump or the injectors. It will implement a fuel cut by suppressing the signal to the fuel injector or ignition coil switching transistor (hard cut) or dropping the fuel PW drastically or retarding the ignition timing drastically (soft cut). The cuts are all software functions.

Can do this with a diode from the switched lead of the relay back to the coil, which then keeps the relay activated after the momentary switch is released until power to the relay is terminated by shutting off the car.
I am having trouble following exactly what you are proposing to do. You would need to attach a diagram showing your idea.

As noted by Drew, what you want is a latching relay or a relay with a seal-in circuit. The circuit linked by Drew is an electronic latch with an electromagnetic relay output. Depending on the application you could make it all electronic by replacing the relay with a MOSFET or power transistor. The 'all mechanical' equivalent of this is a relay with a seal-in contact like shown below - should be self explanatory. It just requires a DPDT relay.

seal in relay20230223.jpg

In the circuit linked by Drew, the trigger circuit can be separate from the relay supply circuit. It can be just about any voltage level and is low current so it could be logic level and it can be momentary. In the seal in relay circuit, the switch can be momentary (push button); but, it is the same supply circuit for the relay coil so the supply circuit must be 12 volts and capable of supplying the relay coil current. Both circuits go dead when power is shut off to the circuit driving the relay coil.

Commercial 12 v latching relays are available. Searching Mouser or Digikey should get you some hits. Amazon lists some for a low price; but, because the descriptions are intentionally opaque or because they used Google to do a Chinese - English translation when writing the descriptions, you are never really sure what you are getting - one listed the color as 5 volts.

There are a number of different ways you can wire this up. As a general rule, it is preferable that it be wired up as fail safe so that if the 'kill relay' fails to operate / coil burns out / whatever - it does not stop the car. This means the relay and kill circuit is powered up while the car is off which means additional parasitic current load on the battery. It would be desirable to select a quality relay with the lowest possible coil current to minimize parasitic current draw and maximize battery life.
 
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the " low rent " kill switch is pulling fuses... 🥸 ;)
 
the " low rent " kill switch is pulling fuses... 🥸 ;)
A valid observation. A workable solution if you drive the car infrequently; however, it will likely fall into dis use because of the inconvenience factor. Its like those steering wheel locks. They get used for the first month of so and then they eventually end up on the floor in the back seat and eventually make it to the trunk where they collect dust.
 
haha human nature....simplicity trumps complexity..
 
I'll test the YEL/BLK wires to the relay for what's hot when/fuel pump & post.

I'm nowhere near smart enough to use @drew 's proposed circuitry. Zoiks! @Old Guy 's double pole relay shown will engage with with the momentary switch, but only disengage IF the 12V supply is ignition-switched. The 2nd pole would allow it to disconnect the BLK/GREEN ECU grounding the fuel pump relay coil. This was actually the circuit I was going to wire up last year until I ran out of time. For other reasons, I want to disconnect the fuel pump wire directly and I'm not that concerned about losses from my relay when Honda put a relay and a huge fuel pump resistor in the circuit (most of the time) already.

Per @docjohn 's request for simplicity, this is the super simple hidden kill switch made from a momentary micro switch & a standard single pole 30A automotive relay (free from auto wreckers now) to control something up to 30A. It engages with a button press after the ignition is turned on and shuts off with the ignition so you can't forget to turn it off. Technically, it's not really a kill switch but an secret activation switch. See picture:
  1. Cut a high current wire to whatever you want to disable, like the fuel pump, that is usually switched off with the ignition.
  2. Mount a female spade lug on the supply side of the high current wire and connect this spade to relay lead 30. Before crimping, merge in a low current wire and run that to the secret momentary switch.
  3. Mount a female spade lug on the device side of the high current wire, crimp in the diode as well. That spade goes to relay lead 87.
  4. Run another low current wire back from the momentary switch and connect a female spade lug for lead 85, also crimping in the diode coming from lead 87.
  5. Lead 86 to ground.
(Be sure the diode is the right way or it won't continue to power the relay.) Momentary contact activates the relay, which powers both the fuel pump and also the relay itself after the switch is released, until the ignition is switched off which stops relay voltage and shuts everything off. It also works with a wire instead of the diode if your switch wire and momentary switch can handle the current of the supplied item (fuel pump in this case), as current can safely flow thru the momentary switch and its wiring while the momentary switch is pressed.
PXL_20230224_031701979-min.jpg
 
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Lol I'm old enough to remember the SNL sketch when a regular reporter posted stick figures for a news story of a big court case..LOL your diagram reminded me of that...
 
Just noticed this.

Your design may have some wrinkles depending on where you intercept the power to the pump. If you intercept the power anywhere on the output side of the main relay supply to fuel pump (blk/yel Main efi relay to fuel pump relay or blk /red fuel pump relay to pump), the is only powered up when the ECU commands the main relay to power up the fuel pump. You will have to press and hold the pushbutton before you turn the key to start. Once the engine has started you can release the pushbutton. If the engine does not start or quits, you need to press the button again before attempting a restart. If you connect the power intercept circuit into the fuel pump power supply to the main relay (blk/yel wire from ignition switch B1), that is switched power. Once you press the pushbutton the relay will stay energized until until you turn the ignition key off so that should eliminate the hassle of having to press and hold the button during the start sequence.

I am not a fan of diodes in control circuits; but, in this case I am not spotting any obvious problems. Make sure that you purchase a good quality high current relay. The 30 amp general purpose automotive relays that Canadian Tire sells don't seem to deal well with carrying significant currents on a long term basis.
 
I generally use old OEM relays. I have a dozen or so I got from an auto wrecker at least a decade ago and I haven't had one fail yet. But for this, I'm going to use a 10A programmable timer switch from timers.shop
 
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easiest thing to do is bypass the clutch pedal microswitch with a switch and use that as a kill switch - that microswitch fails eventually anyways.
 
easiest thing to do is bypass the clutch pedal microswitch with a switch and use that as a kill switch - that microswitch fails eventually anyways.
VERY easy to bypass the entire starter circuit in the engine bay, as @Old Guy has noted in post #8. You have to go after the fuel supply or the ECU.
 
the " low rent " kill switch is pulling fuses... 🥸 ;)
This right here! I used the pull the fuel pump fuse for my CRX Si. One day I had to go to a conference for work and parked it right next to Central Park. A million people walking by every hour. Some thief jimmied the door lock and pulled the ignition. He couldn't start the car and literally tore up the dashboard with his screwdriver looking for the killswitch. Never found it because it was just the fuse, LOL. I needed a new dash and it was a pain to change but pulling that fuse saved my car.
 
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