I wonder if you're hitting the right nail here. The guy said he wanted it off
when the car is running, not when the key is on.
If that's the case the easiest way is to hook the LED anode to "BAT" and
through a resistor, to the "FIELD" terminal of the alternator. The internally
regulated alternators still require a "trickle" to this terminal to "start" the
alternator. The "BATT" light on the dash usually fulfills this requirement.
Even on a self starting alternator it should work. Of course in addition to the
engine not running, it will indicate an alternator or belt failure.
Another method could be devised using the MAP or TACH signals or the "bypass"
line from the computer to the ignition module.
If all you need is to know if the key is on, of course it is very simple, if
you need absolute reliability, the "bypass" line to the ignition is the surest
bet. It changes states when the engine hits about 200 RPM or something and
tells the module to obey the computer for ignition timing. When it is cranking
the timing is preset inside the ignition module.
If this is required I recommend an enhacement mode FET to drive the light
because I'm leery about loading any of those computer outputs, and it is an
output. The module does not tell the computer when to take over, the computer
tells the module that it is taking over.
Note that since this signal is only 5V you won't be able to saturate the FET,
so stick with an led. If you want to drive something else, buff it with a
bipolar.
JURB
