erratic voltage, bogs motor

True North

Member
Sep 11, 2012
165
Live: DeWitt, MI,West Michigan (Ludington)
Boat Info
2005 SR 260DA
350 MAG MPI
Bravo 3
Engines
(1) 350 MAG MPI w/Bravo III
1988 SeaRay 270 Sundancer, twin 4.3L motors, original, approx 1400+ hours on each. Many hours trolling. Boat is mechanically and cosmetically in great condition for a 30yr old classic.

Last season I noticed my starboard motor had developed an irratic rpm drop while trolling. I had also noticed while underway, my motors would go in and out of synch. I thought it was a carb issue, but yesterday I ran both motors on the muffs and noticed that my starboard motor voltage would read 13-14V on the dash voltmeter, but then would jump to 17-18V and that's when I would notice the drop in rmps, almost like the motor was under load.

One thing to note, it is my port motor that has an alternator on, starboard has the power steering.

So what is the significance of my voltage spiking and causing load on the motor? There doesn't seem to be any rhyme or reason of how often or how long it stays in that "bogged down" state. Sometimes from a few seconds to probably a minute or two. I have no clue where to start outside of the internet search, and I thought I would start here. I thank you in advance for your assistance.
 
Sounds like you have a loose ground. check and clean all the cables from batteries to the engine. also check the plug on the engine where the harness plugs in. I had a Bayliner that the harness plug was loose and that caused all kinds of different problems including voltage spikes.
 
will check those things. I already checked the batteries (3) to make sure all was tight and no corrosion. All good on that front. Not sure if the issue is a loose ground only because there is very little vibration when the boat is idling while sitting on the trailer that would cause the issue to come and go.

Any other thoughts on what would cause voltage spikes on only 1 of the motors???

Thanks for replying!
 
Check your wiring, sound like you have a 12volt + grounding occasionally. Check the heavy wire going from the battery to the alternator
 
I thought it could be an alternator/voltage regulator issue, but it appears I only have one alternator and it is on the port motor. The voltage issue I am having is on the starboard motor, without the port motor even running, so I'm not sure if it could be that? I plan to go back later this week and will check all connections at the batter, motor, and main electrical plug to see if I can find the culprit. If not, and I still have the same occur, I will be back on here looking for more guidance. I just called my "local" marine dealer and he just said they were doing away with service this year because they can't keep good mechanics. The larger marine dealers where I keep the boat in the summer are 3 hours away and I'd like to resolve before I splash. Thanks for all the suggestions and I will report back!
 
That isn't making sense to me...How could you have a voltage spike without an alternator involved?
 
that's what I'm trying to determine from those more experienced than me. My port motor has an alternator, my starboard does not. It's the starboard motor that is experiencing the voltage spikes. Keep in mind when it occurs, it makes the motor sound like there is a load on it, drops the rpms when it spikes, but then will return back to normal. I can actually tell when it is spiking by the sound of the motor without looking at the voltmeter gauge.......
 
Got a pic of that voltage spiking motor that has no voltage generating equipment on it ?
 
I can get one once I get back to the storage unit later this week. Maybe I'm wrong, but I don't recall that motor having an alternator, only a power steering pump. But Ill double check! :)
 
Almost positive there are alt`s on both motor, whether there connected properly is the question
 
Has this problem been steady for a while or just an intermittent low rpm thing? If there is an uncontrolled voltage spike from an engine with no alternator then maybe a starter hanging up on the ring gear could do this? Have never heard of these symptoms for a dragging starter but without an alternator, something is loading the engine and producing that spike. Not a lot of options.
Doesn't make sense as mentioned above. Without an alternator on that engine it will be just below 13V at best while running alone.
 
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Yea, doesn't make sense with only one alternator. because one charges the house batts. and the other the start batts. if there is only one alt. then the only other thing that is producing voltage is the coil. a bad coil can do all sorts of things. but ive never heard of one sending voltage back through the system. just a thought. unless there is a battery charger that is plugged in.
 
ok, so this wasn't making sense to me either, so I made a trip out to the storage unit and took a closer look. Low and behold the starboard motor DOES have an alternator, it's just hidden below and behind the power steering pump. I only have power steering on the starboard motor (for sure :)) I have a 3 battery system, 2 starter batteries and a house battery. I checked all battery and electrical connections including the main harness to the motor. Everything was tight except on of the small wire contacts on the front side of the mercathode was a little loose, so I tightened that. I did not get a chance to run the motor after that, but doubt that was the problem.

So, now that I know I DO have an alternator on that motor, is the alternator/voltage regulator suspect for the voltage spikes? And if so, do I just get an automobile alternator for that year and size motor or do I need a Mercruiser marine alternator? I have exchanged an alternator on an old car, but never a marine motor so don't know if there is a difference? Was going to pull it and head to NAPA.

The voltage spikes occur regardless if the motor is at idle or higher rpm. I even noticed it while on plane towards the end of the season last year, but again thought it was a carb issue because I hadn't noticed the voltage issue before.

Any way to know for sure if a new alternator will cure this before spending the coin? I don't think you can return an alternator once purchased. Thanks for your continued assistance on this one!
 
Find a shop that specializes in alternator and starter repair. Have them test it or rebuild it. Don't go to Autozone or other similar chains. Could be a cheap fix. Don't do automotive.
 
for sake of time, I don't mind swapping out the whole alternator vs. waiting for someone to repair the one I have, even if it's going to cost more, but does the issue I am having sound related to needing to replace my alternator or is there a way to test/verify? I am trying to splash the first weekend in April and I am out of town the last week in March, so I am trying to rectify this before middle of next week (including running it on the muffs to confirm if fixed). I plan to pull the existing alternator tomorrow and head to napa. Looking online it looks like there are 3 variations of alternators for my year and motor, I want to make sure I get a direct replacement.

One last question, why no automotive? Are they truly different?
 
There are several reasonably priced marine alternators available on Ebay. I replaced mine on my old 260DA with one purchased online and it was still working well 4 years later when I traded it in.

For the price I would go for a swap out with one designed for marine use.

-Kevin
 
A Marine alternator is either completely sealed or has screens over any open areas so a spark can not ignite any gas fumes. Flames can't burn or pass thru a screen. Plus there are other minor changes for the damp/hotter environment that marine engines operate in.
 

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