anti theft device using relay

Hi guys, new to the forum here. I am self teaching myself about car wiring and relays and other such things and now I need some help with my fake car "alarm" I'm trying to wire.

When I bought my car, one of the previous owners wired a kill switch in my car. It basically intercepted the fuel supply wire and ran it through a little switch under my dash. When the switch was off the car would start for a second and then die.

On the other hand, I'm trying to wire a little blinking LED I have soldered to some speaker wire to come on when the kill switch is on. I have a SPDT relay that I know the basics of, but I can't figure out how I could wire my LED, the switch, and the fuel supply wire together.

So in short, I'd like the switch to activate the LED and cut-off the fuel supply in one position, and turn off the LED and complete the fuel supply circuit in the other position. Is this possible using a relay?

What kind of switch was used to intercept the fuel line? A standard SPDT relay won't work in this application because leaving it energized will kill your battery.

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Kevin Pierson

30 to power, 87 to fuel pump. 87A to 12 volt LED or a resistor and a standard LED. Ground to 85, Switched power to 86.

A flashing LED connected between constant +12V and the switch output else fuel-pump supply should work. (LED cathode or -ve to the switch output assuming it is switching +12v.)

The LED will be on (flashing) whenever the switch output (or fuel pump or relay) is NOT +12V.

Guess what cornerpocket, all those answers work but Oldspark is the one I'd go with, it's simpler.
A word of extreme caution, make sure your switch is good quality with decent connections, it can't be allowed to fail as you're passing someone.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Thank you for the answers.

KPierson wrote:

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What kind of switch was used to intercept the fuel line? A standard SPDT relay won't work in this application because leaving it energized will kill your battery.

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The switch looks like this, I'm not sure what the technical name is. It's two prongs on one side and the button on the other. The fuel pump wire was removed from a pin connector under my dash and connected to a piece of black wire and ran to one terminal of the switch, and another piece of wire from the other terminal of the switch back to the pin connector.

i am an idiot wrote:

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30 to power, 87 to fuel pump. 87A to 12 volt LED or a resistor and a standard LED. Ground to 85, Switched power to 86.

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One thing confuses me about this, and it's probably due to my very limited understanding of relays; if the fuel pump is connected to 87 and the coil is energized (the switch is on) doesn't that connect 30 and 87? So isn't that like me taking the 12V Constant wire in my car and crossing it with the fuel pump wire?

oldspark wrote:

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A flashing LED connected between constant +12V and the switch output else fuel-pump supply should work. (LED cathode or -ve to the switch output assuming it is switching +12v.)

The LED will be on (flashing) whenever the switch output (or fuel pump or relay) is NOT +12V.

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I'm sorry I don't understand. Are you saying to ground the LED to the fuel pump wire and use the fuel pump wire as the switch on the relay? Bear with me I'm trying to get it all straight in my head, I think I butchered what you were suggesting haha...

howie ll wrote:

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Guess what cornerpocket, all those answers work but Oldspark is the one I'd go with, it's simpler.
A word of extreme caution, make sure your switch is good quality with decent connections, it can't be allowed to fail as you're passing someone.

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Thanks for the warning, I assume you're talking about the switch failing and the fuel supply being cut off while I'm driving? I never really thought about that but the terminals on the switch do look a little corroded maybe I'll buy my own switch just to be safe.

I appreciate the responses guys. I grabbed these diagrams from my service manual in case they're any help. The GREEN/ YELLOW wire is the "fuel pump wire" I've been referring to. I don't know if this has any significance but the "Hot with engine cranking" part looks promising.

Yes - connect the ground of the LED to whatever you are switching - ie, the switch's on-off end (not its input).
Connect the +ve end of the LED to constant +12V if you want it on or blinking all the time the IGN is off (ie, like an alarm LED).

If you only want it on when cranking (with the switch off, ie, no fuel pump), then wire it across the switch instead - i,e +ve end of LED to the switch input.


Are you sure the switch was in the GRN/YEL wire? To me that looks like the fuel pump signal whilst cranking. The BLK/YEL looks like the normal ECU controlled fuel pump signal - ie, whilst it is running (for carby cars that would be the alternator's charge-light output).

Whilst a GRN/YEL break (switch) will prevent an EFI vehicle from starting, it may not be difficult to defeat depending on how it is wired. Then once it starts, the BLK/YEL supplies the fuel pump as per normal (if I interpret the diagram correctly).
I'd rather break the normal fuel pump signal (BLK/YEL). That means the car will crank and start, but as soon as the cranking (key) ceases, the pump stops and engine stalls almost immediately which is what you described.
(For a carby car, you would definitely break (at least) the normal fuel pump signal (not the cranking signal) because cars will start on the fuel-bowl. The cranking signal is only to ensure the carby is primed.)


And make sure you hide the wiring to the LED. If it's easy to trace the wiring from the LED...