I have a 1987 mariner 8 hp. recently I had to replace the engine on my sailboat, the connections were different so when I went to change over to the old connection, the colors didn't match up. 2 green wires from the mariner , black and white from the other. I tried to isolate the positive by putting a voltmeter on one end, and using a common ground on the other. nothing both ways, but if you connect the meter dirrectly, it shows it is charging. my ? is, how to find the positive and neg. I dont want to reverse it and blow my charging system.any help would be appreciated.. thanks