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Below you will find information troubleshooting marine starting systems.   We do not claim to be experts in all fields.  We cannot promise to be correct on all issues, nor can we address the content of these pages.  (Please read our legal disclaimer page)  But, I hope you will find this page useful.  We want to thank Richard Martin, Vice President of ARCO Marine Corp., for his technical assistance with this page.  Remember if you need technical assistance, we are only an email away.  We recommend that you get assistance from a trained individual, if you are not fluent with electrical systems.
ALWAYS START YOUR TESTING AT THE BATTERY:

One of the most important things to remember when troubleshooting marine electrical systems is to start your testing at the battery first.  Regardless of the symptom, the battery is the heart of the system and every electrical device depends on the battery to operate properly.  Boats are stored for months without charging the battery.  All batteries self discharge when stored.  Self-discharge rates are even higher when moisture is present.  Once a battery becomes discharged sulfation occurs.  Sulfation is a chemical action that occurs when batteries are stored for a period of time in a discharged state.  Hard, bulky, crystal-like substances are formed on the lead plates inside the battery.   This causes the pores in the plates to become clogged and useless.  Don't just throw a charge on the battery and expect it to perform properly.  I hear this all the time "the battery is brand new," "I just charged it up, it's good".  You can't assume anything when it comes to troubleshooting electrical systems.
TOOLS REQUIRED TO TEST THE BATTERY.

In order to test a battery properly you must apply a load equivalent to 50% of its cold cranking amp rating for 15 seconds and maintain a terminal voltage of 9.6 volts.  This is the only way you can accurately test a battery.  The recommended C.C.A. rating for a battery that is used to crank most V8 inboards is 650 amps. In order to test this battery you would need to apply a 325-amp load on it for 15 seconds.  You need a tester that is capable of applying at least a 300-amp load to accurately test most batteries used in V8 Gas or Diesel inboards.  These will usually be a variable carbon pile load type.  On smaller engines like outboards, or 4 cylinder Gas engines, you can do this with a smaller handheld 100 amp load testers.  The smaller type of tester is available from most automotive stores.
THE STARTER IS NEXT:

From this point on, most of these tests will require two people.  Many starters are replaced unnecessarily due to low voltage.  Remember the load test for the battery?  We had to maintain a terminal voltage of 9.6 volts for 15 seconds.  Do you know why?  All 12-volt starters are designed to operate and perform efficiently at 9.5-10 volts under load.   As a matter of fact, performance curves for starter output efficiency are performed at 10 volts.  When a starter operates on voltages below 9.5 volts, the output is dramatically reduced and will cause the starter to overheat.  When a starter is first engaged, the current required can be as high as 300 to 400 amps for a V8 engine.  Once the starter is spinning it requires much less current.  If the battery can't supply enough current at 9.5-10 volts to get the starter spinning at its rated rpm, the starter will draw excessive current and heat up severely.
TESTING THE STARTER:

Once the battery is determined to be in good working condition, connect the positive voltmeter lead to the starter where the battery cable is attached.  Connect the negative voltmeter lead to the starter case making sure you have good contact with the metal.  This will insure that the positive and negative circuits will be tested. Disable the ignition spark.  This is important.  On conventional ignition systems, this can be as simple as disconnecting the coil positive wires.  It is not a simple thing on electronic ignitions.  You must consult  the manufacturer's specifications on methods to avoid damage to electronic ignition.  Diesel engines may require that someone holds down the kill button on the engine to stop the flow of fuel so the engine will not start.  Crank the engine for about 10 seconds while observing the voltage reading.  Use of a meter with clip lead ends will aid in this measurement.  A reading of at least 9.5 volts would indicate there is sufficient voltage being supplied to the starter to operate properly.  If the starter does not function, as it should, chances are the starter is defective or the engine has a problem causing it to be difficult to turn.  If you have a flywheel turning tool, you can attempt to turn the engine over by hand.  A voltage reading below 9.5 volts would indicate a voltage loss between the battery and the starter.  For example if we had a reading of 9.6 volts at the battery, and we had a reading of 8.0 volts at the starter terminal.  This would be a 1.6-volt drop in the circuit.  The maximum drop allowed in the starting circuit is 0.5 volts (0.25V per cable).  Corroded battery cables, dirty connections, or insufficient wire size would be the problem here.  This could be any connection in the positive or negative circuit.  Even corrosion between the starter and the engine block can reduce the available voltage.
TESTING FOR VOLTAGE DROP.

Locating the voltage drop is done with a very simple test.  This test should be done where ever a voltage drop is indicated.  To test the positive battery, cable, connect the positive voltmeter lead to the positive post of the battery, not to the battery cable end.  Connect the negative lead to the starter terminal where the battery cable is attached.  Crank the engine while observing the voltmeter.  The reading should not exceed 0.25 volts.  Make sure you remove at least one voltmeter lead before the starter motor is turned off.  The starter motor can sometimes generate a spike voltage and damage the voltmeter.  If the reading is over 0.25 volts, excessive resistance is occurring in that cable.  To test the negative battery cable, connect the negative voltmeter lead to the negative post of the battery, not to the battery cable end.  Connect the positive voltmeter lead to the starter case making sure to make metal contact.  Perform this test in the same manner as with the positive battery cable.


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Boatfix.com specializes in marine engine parts and boating accessories; including risers, manifolds, alternators, starters, water pumps, bilge pumps, pistons, gaskets, lifejackets ( PFD ), hoses for Mercruiser, Crusader, OMC, Volvo, Sea Maxx, Evinrude, etc. (Gas and Diesel)- you name it.  If it goes on a boat, it's in our catalog!