Mercury 75 OptiMax Owner's Manual page 122

Dual Outboards
Connect a common ground cable (wire size same as engine
battery cables) between negative (–) terminals on starting
batteries.
b
c
a - Red sleeve ‑ Positive (+)
b - Black sleeve ‑ Negative (–)
c - Ground cable
d - Cranking battery
OUTBOARD INSTALLATION
b
a
(-)
a
(-)
d
112
d
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