Multiple Efi Engine Battery Precautions; Efi Electrical System Precautions - Mercury MerCruiser MCM 454 Mag MPI Service Manual

CHARGING SYSTEM

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 only for acces-
sories. Isolators should not be used to charge the battery of another engine in the boat.
Generators: The generator's battery should be considered another engine's battery.

EFI Electrical System Precautions

NOTE: All references to EFI models apply to all EFI and MPI engines.
Avoid damage to the EFI electrical system and components. Refer to the following
precautions when working on or around the EFI electrical harness or when adding
other electrical accessories:
DO NOT tap accessories into engine harness.
DO NOT puncture wires for testing (Probing).
DO NOT reverse battery leads.
DO NOT splice wires into harness.
DO NOT attempt diagnostics without proper, approved Service Tools.
Index
Page 4C-6
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CAUTION
SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999