Help with charging problem on my boat?

Question

I have a ski boat with a 3 liter mercury inboard. Last year I started having dead battery issues. I had the battery tested and it was supposedly okay, so I took the alternator off for testing. Auto zone and 0'Reilly's couldn't test it. I was referred to a guy working out of a shop at home. He tested my alternator and said it was bad and sold me a generic delco boat alternator. I put that one on. Went to the lake and the starter went out. Took the starter to the rebuild shop and they tested my old alternator and said it was good. Okay. I got ripped off. Rebuilder told me I probably had a bad battery and or keyswitch. I replaced the keyswitch and battery and bought a digital voltmeter for the lighter socket. I've cleaned every contact I can find and used dielectric grease on them. Cleaned the ground to the engine block, the main plug and everything else. At the boat shop they suggested I put the old alternator back on. I still get only 12.33 volts running and 12.6 with the engine off.

Answer 1

The alternator you should be using is an enclosed, marine rated Mando from merccruiser.  A very common fault is a corroded charge wire from the alternator.  physically check this wire( largest red from the alt.), a continuity check will lie!  Replace if suspect.  Use a voltmeter to check output directly from the alternator, you should have 14-15 V at about 1500 rpm.  good luck.

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The charge wire runs from the alternator connection to either the positive terminal on the battery, or to the main positive cable on the starter, depending on year.  Do a volt test directly from the alternator.

Answer 2

I don't know for sure what's wrong with your boat - the marine mechanic knows a lot more he could be right on .

I had a similar problem years ago on an outboard - the big ground wire from the motor to the battery was cut inside the insulation - it was hanging on a couple of thin wires only for a long time - causing dead battery symptoms. good luck.

Or buy a new boat