advanced circuit

(First off i would like to thank this website for teaching me what i know about the electrical systems of cars.)

I am trying to install an advanced but still convenient kill switch in my 1999 Honda Civic base model. The goal is to make a starter kill using the stock AC button (my car does not have AC). Theres an LED on the button that lights up when the button is pushed so i figured the kill switch would be disarmed only when the LED is on.

So here is the circuit im working on and what i did:

(red wire signifies my modification)

Because of my limited knowledge with circuits, i cannot really understand why it doesnt work. When i pulled out the multimeter to diagnose the problem, i got some interesting results:

• Before i tapped into the wire, it was outputting 12v when LED was off and .05v when LED was on


• After i tapped it, LED remained constantly on and was outputting 1.3V


• Just for kicks i tested for continuity between the wire and ground and the LED turned on but was very dim and the multimeter read approx. 500ohms

I dont know why the LED on the switch turns on randomly and i dont really understand the circuit diagram. So my question is, is it even possible to make my idea work. If so, how can i do it??

That LED circuit is a grounding circuit.

Your relay coil (like the LED) should be hot (+12V) and it is then grounded when the LED comes on. The diode also has to be reverse, though since it isn't necessary, I'd put it across the coil to suppress spikes - ie, line end to +12V.

But if that grounding transistor cannot handle the coil current, it will blow. (The LED is maybe ~20mA; the relay coil may be 250mA - ie. 14V/<coil resistance in Ohms>.)

It's also not a good idea to cut wires. A better scheme is usually to ground ignitor outputs or other grounded sensors that are required for engine starting or running. (IE - the NC relay contact shorts to ground any sensor signal or the ignitor/points output to the ignition coil(s).)

Ahh yes now that you point it out it makes so much sense! For some reason i was stuck on the idea that it was outputting voltage. Hopefully the transistor can handle the coil. Thank you!