Are you running a three wire alternator? If so, you need to add a diode to the harness on the wire that goes to the ignition switch. That will solve the problem. You can do search to find out what diode to buy, then get it at Radio Shack. Very inexpensive. I soldered mine into the wire just before it enters the alternator plug. Or, if you are installing a 10SI three wire alternator, you can go to eBay and buy a pigtail that plugs into your alternator socket that has the diode installed for about $10.
Matt
