Multiple Efi Engine Battery Precautions - Mercury 496 MAG Bravo Owner's Manual

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Also, if you partially disassemble an engine or drive assembly and are unable to repair it, the dealer's
mechanic must reassemble the components and test to determine the problem. This will cost you more than
taking it to the dealer immediately upon having a problem. It may be a very simple adjustment to correct the
problem.
•
Do not telephone the dealer, service office or the factory to attempt for them to diagnose a problem or request
the repair procedure. It is difficult for them to diagnose a problem over the telephone.
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Your Authorized Dealer is there to service your power package. They have qualified factory-trained
mechanics.
It is recommended you have the dealer do periodic maintenance checks on your power package. Have them
winterize it in the fall and service it before the boating season. This will reduce the possibility of any problems
occurring during your boating season when you want trouble-free boating pleasure.
CB840
NOTE: All references to EFI models apply to MPI engines.
CB841
NOTE: All references to ECM apply to all engine control modules including the propulsion control module
(PCM).
CA782

Multiple EFI Engine Battery Precautions

Situation
Alternators: Alternators are designed to charge the battery that supplies electrical power to the engine that the
alternator is mounted on. When batteries for two different engines are connected, one alternator will supply all
of the charging current for both batteries. Normally, the other engine's alternator will not be required to supply
any charging current.
EFI Electronic Control Module (ECM): The ECM requires a stable voltage source. During multiple engine
operation, an onboard electrical device may cause a sudden drain of voltage at the engine's battery. The voltage
may go below the ECM's minimum required voltage. Also, the alternator on the other engine may now start
charging. This could cause a voltage spike in the engine's electrical system.
In either case, the ECM could shut off. When the voltage returns to the range that the ECM requires, the ECM
will reset itself. The engine will now run normally. This ECM shut down usually happens so fast that the engine
just appears to have an ignition miss.
Recommendations
Batteries: Boats with multi-engine EFI power packages require each engine be connected to its own battery.
This ensures that the engine's Electronic Control Module (ECM) has a stable voltage source.
Battery Switches: Battery switches should always be positioned so each engine is running off its own battery.
DO NOT operate engines with switches in BOTH or ALL position. In an emergency, another engine's battery
can be used to start an engine with a dead battery.
Battery Isolators: Isolators can be used to charge an auxiliary battery used for powering accessories in the
boat. They should not be used to charge the battery of another engine in the boat unless the type of isolator is
specifically designed for this purpose.
Generators: The generator's battery should be considered another engine's battery.
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