Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 45Get this manual
Warning Gauge Harness
Battery Cables
Single Outboard
1Connect the harness extension to gauge and engine
aRed Sleeve (Positive)Black Sleeve (Negative)Starting Battery
Dual Outboard
Connect common ground cable (wire size same as engine battery cables) between negative (-) terminals on starting batteries""""'-L-_ -