7:46 AM
@adamaero ACC used to be a switched B+ to power stuff in the car, but with the ignition system disconnected.
Everything in the car (radio, lights, etc.) would work.
The ignition system in old cars used a mechnical switching system to make the high voltage for the spark plugs. If the ignition switch was left on for a long time but with the motor not turning, the switch contacts could burn up, or the ignition coil.
What isn't obvious is that the points are normally closed. The default is to always conduct. It makes a better spark that way, but can burn out the points or the coil if left energized for too long.
ACC was invented to let people listen to the car radio without running the motor and without destroying the ignition system.
These days, there are a couple of options for how ACC really works.
It could be a switched B+ like in olden days.
It could be a signal to the ECU that tells it "turn on the B+ but don't turn on the fuel pump or ignition."
If it is switched B+, then you don't want to be putting signals or power on it.
If it is a signal to the ECU, then you need to see what voltsge level it uses to mean what.
