Mercruiser 5.0 is not charging battery. alternator is good

by Jamie
(Tamaqua, PA USA)

I recently purchased a 1995 Chapparral boat with a Mercruiser 5.0 in it. It doesn't appear to be charging. Pulled alternator and had it tested. It is good. Any ideas?

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Jun 19, 2014
more info...
by: Anonymous

I did learn some since posting this.

I checked the voltage when running and it was 12.6 volts. I have cleaned the connections. They are ring terminal screw on type. This didn't affect the voltage. The alternator has an internal regulator which is good. There are two small wires coming off the back of the alternator in addition to the large Red Hot wire and the large black ground wire. The electric shop that checked the alternator for me explained that the EXC wire should have 12 V all the time. The S terminal should have 12 V when the key is on. So, it would seem to me that I need to trace those two wires and see if they are broken, rubbed through, etc. I should find an issue in one of those two circuits. I can always run a new wire to either one of these posts if necessary. The battery is a brand new Interstate that appears in good condition. Checked 12.8 V after I just charged it. (Running the boat last time killed it because I was running on just the battery.)
Thanks for the replies!!!

Jun 18, 2014
Good alternator not charging battery
by: Bruce

What is the voltage of the battery before you try to start charging it ? With the engine running take a voltmeter and put the negative and positive lead from the meter on your battery posts and see what voltage is being supplied to the battery. It should be at least 14 1/2 V. If you're voltmeter is not reading at least 14 1/2 V, clean your battery posts and all associated connections. The battery posts may look clean, but underneath where the clamp attaches to the battery post is where the corrosion can be hiding.
Check the condition of the electrolyte in the battery. If the battery is dead it may not accept the charge. It may have an internal short in the battery which will not allow it to charge.
Is there enough electrolyte in the battery to cover the cells inside the battery? If the plates inside the battery are exposed to air, that will greatly shorten the life of a battery. Sitting to long without charging the battery will kill a battery also.
Check your wires from the alternator to the battery and see if there is any damage to them.

Jun 18, 2014
by: Don

Did they say if it had a build in regulator or not .
If it doesn't have a build in regulator then their is a external one .the wires coming out of the alternator,see where they go , if to a little box
about 3" square or so then that is your regulator,
that is bad .

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