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Voltage dropping

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I have a 1999 Malibu Sunsetter with an Indmar 325 engine.  When the engine is under load, the voltage drops to around 10V.  It occurs at various rpms, but it only happens on the water under load.

I had the alternator rebuilt, replaced the alternator with a new one, replaced both batteries, checked the V belt, and replaced the V belt, but it still happens.  Nothing else is on while the engine is running (no bilge, no stereo, no lights, etc.).  I even replaced the ignition coil because it was in the electrical schematic for the charging circuit.  I also ran heavy gauge wire from all three alternator connections directly to their respective locations (ground, battery, and battery).

Does anyone know what the issue could be or what to try next?  I'm out of ideas.


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I read the voltage on the analog gauge cluster, the digital gauge, and a voltmeter.  They all read the same (about 10V).  When it's not having the issue, they all read about 14V and everything is normal.


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Something has a bad connection.  Clean the engine ground lugs.  Dive under the dash and clean the bus bar connections as well as the other terminals such as key switch, etc.  If that doesn't do it, use your meter and start looking for voltage drop along specific wires and connectors while it is acting up.  Since you are losing about four volts it will probably be a larger connector.

It might also pay to check things like your auto bilge pump to make sure it isn't actuating and drawing a crazy amount of current.

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On 8/4/2022 at 9:27 PM, BusterTex said:

I also ran heavy gauge wire from all three alternator connections directly to their respective locations (ground, battery, and battery).

ground, starter, and battery?

Are you starting the day with fully charged batteries?  What does your DMM read at the alternator, starter, and batteries when the boat is off and the batteries are charged?

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I have two batteries and I measured them without charging them up.

One was 12.68V and the other was 12.71V.

I used the first battery and measured the voltages at the locations you suggested:

Alternator (12.67 V)

Starter (12.67 V)

Battery (12.68 V)


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I also removed, cleaned, and tightened the engine ground lugs and all the positive connections I could find in the engine area.  Next I will do the same under the dash.

And I have a manual bilge (not automatic), but I've disconnected it completely in the past and didn't see any difference.


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It appears to drop from 12.6 V to 11.7 V temporarily, but goes back up to 12.6 V fairly quickly.  I rotated the throttle lever through the whole range and it didn't appear to change.

I pulled out the "in gear" button on the throttle and rotated the throttle lever again and didn't notice any voltage change at all.

Should the voltage change at all?

The area I usually see the issue is about 1/2 to 2/3 throttle, but only under load in gear on the water.  When I use a "fake a lake" and put it in gear, the voltage never seems to drop at all.


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I think there might be a partial short circuit that really only appears when under way (vibrations from transmission triggering it or something similar). I don't think the voltage should be dropping should you put it in gear; in fact, it should be doing the opposite based on the thread below.

Found this thread on the neutral safety switch, which in fact just grounds the starter relay signal and disconnects it from the ground when in gear. Can you try testing your boat to see if it will start while in gear? There might be some funky wiring issues going on. Maybe the starter relay is going bad and partially short circuiting internally.

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  • 9 months later...

JustGary - You were correct!  I finally figured out the issue after fighting with this for over a year.  There was a bad connection.
When I was removing some other items over the winter, I found a positive battery cable connection tucked up under the seat backs that I never new was there.  I've owned the boat for 15 years and done all kinds of maintenance on the boat in that area.  I removed the connection, cleaned all the terminals, and put it back together.  When I took the boat out for this first time this season, everything worked great.  Woohoo!

The connection looks like a jump start terminal, but I'm not sure why it's in it's current location.  It would be easier to access the batteries than this terminal if the boat needed jump starting.

I did get a new neutral safety switch, but I never had to install it.

Thanks to everyone for all the help and comments on things to try.  :-)

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