starter disable

it all depends on how it fails.  the starter wire is cut, and wired through a relay (with a diode) to the normally closed pins when the coil is not energized.  The starter kill output grounds the powered coil and opens the relay, disconnecting the starter wires.

if the relay fails open, she no worky, no matter what.  One thing we do here to prevent the relay from actuating EVERY time you arm/disarm (to save literally wearing it out), we use starter signal to power the coil.  So when the alarm is armed, it grounds the coil, but ONLY if someone tries to start the car while armed will the relay click over and open (+12V on the starter wire energizes the coil).  In most cases, the relay NEVER moves, and if it fails, it fails closed (driveable).

If someone wants starter kill, I tell them about all the horrible realities if it fails, and that I assume no liability if the relay decides to die and hang them up.  If they choose to go ahead, we do it.  Have yet to have problems (knock on wood).

b