Denny Graham,
If you were to install a GM 10SI alternator on your 'big old truck' and happened to tap off the coil for a handy source of switched power to excite the alternator, then there is a chance that when you turned your 'big old truck' off, it would refuse to shut down.
After the alternator gets up and running, DA#1 creates voltage that can power the coil even when the key is turned OFF resulting in engine run-on. The diode prevents this from happening.
Some folks don't want to crawl through the firewall to the ignition switch for power...the coil location is handy. Remember, don't shoot the messenger!
Stuart
