I got to thinking about this one, because the alarm I just bought and am preparing to install also uses a normally closed relay to kill the starter. If the alarm brain is found and unplugged, the starter kill relay will open and the starter will engage.
My car has been stolen once successfully, and almost stolen again a few weeks ago. The only reason I still have it right now is because I had the ultimate starter kill - the battery was dead.
I bought an alarm to put in to try to save my car from a third attempted theft. Don't get me wrong, I don't expect the alarm to prevent my car from getting stolen, I just wanted the alarm for the lightshow to keep a thief from wasting his and my time and money.
It's the starter kill that is important to me, and I don't want the starter kill to be defeated by simply unplugging the alarm brain. I only want the starter to work under the alarms supervision while unarmed.
So, I came up with this idea, and I'd like comments. If this is crazy, please let me know. I drew a diagram, but I don't have anywhere to host it, so bear with my description. I am not a professional installer, and I'm still just learning about relays, so forgive my terminology.
First, I'll have the alarms usual starter kill relay on the ignition starter line. This will keep the starter dead while the alarm is connected and armed.
Next, I'll hide a second relay under the hood. Power from the ignition (+) will come in to both the relay coil(86) and the common(30) of the relay. power(+) will continue to the starter from the closed contact (87). The other side of the coil will go to to a line that will travel through the engine firewall to a third relay to the closed contact (87).
The coil of this third relay will sit inline (series) with the constant hot line that usually feeds the alarm brain. (86 & 85). The common(30) of the relay will go to ground.
The idea is that if a thief finds the alarm brain and pulls the plug, the coil of the third relay will collapse(open) because the circuit to the alarm brain will be broken(open). This will cut the connection to ground that is needed by the second relay under the hood to allow the starter to turn over.
The coil in the second relay (under the hood) is only energized while cranking, so the plan would only put one extra relay coil to use while the car is sitting armed. I'm not sure how much of a current draw that would put on the battery. Now that I think of it, the third relay could probably be fed from the ignition line going to the alarm unit, still in series with the alarm so that if the brain gets unplugged the circuit would be open while cranking...
Anyway, I'm sorry for being so long winded, but that's my idea. Comment away.