Mercury Optimax User Manual page 112

Dual Outboards
Connect a common ground cable (wire size same as engine
battery cables) between negative (–) terminals on starting
batteries.
b
a - Red sleeve ‑ positive (+)
b - Black sleeve ‑ negative
(–)
OUTBOARD INSTALLATION
a
c
(-)
a
b
(-)
c - Ground cable
d - Cranking battery
104
d
d
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