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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.
CA873
Diagnosing EFI Problems (If Equipped)
NOTE: All references to EFI models apply to EFI and MPI engines.
Your Authorized Mercury MerCruiser Dealer has the proper service tools for diagnosing problems on Electronic
Fuel Injection (EFI) Systems. The Electronic Control Module (ECM) on these engines has the ability to detect
some problems with the system when they occur, and store a Trouble Code in the ECM’s memory. This code
can then be read later by a service technician using a special diagnostic tool.
CA624
Water Separating Fuel Filter
CAUTION
The electric fuel pump and factory installed water separating fuel filter have been carefully designed
to function properly together. Do not install additional fuel filters and/or water separating fuel filters
between fuel tank and engine.
The installation of additional filters may cause:
• Fuel Vapor Locking
• Difficult Warm-Starting
• Piston Detonation Due to Lean Fuel Mixture
• Poor Driveability
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