Mercury 75 Operation Maintenance Warranty Installation Manual page 148

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
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d
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