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Old     (ottog1979)      Join Date: Apr 2007       09-24-2012, 11:07 AM Reply   
I'm having alternator issues and electrical is my worst DYI subject. A little over 2 years ago, my alternator stopped working and I had it replaced. Stock alternator for Mercruiser 350 Mag MPI. Just this last week, the same issues are coming up again and the alternator is dying/died. Volt meter goes slowly from 13 down to 10, then 9, then 8 volts and things on the boat stop working simultaneously, then at all. This happens over a day or two (not all at once). Boat will run but when you turn on a ballast or bilge pump, the Perfect Pass will turn off (not enough juice to run everything). Obviously, I need a new one.

But why such a short life? Will a heavier duty alternator fix this issue? There's nothing that unusual on my boat as far as electrical accessories: 2 amplifier stereo, Perfect Pass, basic bilge & ballast pumps. That's it. Could it just be that I need a beefier alternator or should I be checking out other potential issues?
Old     (polarbill)      Join Date: Jun 2003       09-24-2012, 11:18 AM Reply   
What is your battery setup? Do you do a lot of cove partying with the stereo turned up loud? what is the wattage of yoru amps?
Old     (david_e_m)      Join Date: Jul 2008       09-24-2012, 11:36 AM Reply   
One consideration is that pushing and using your batteries beyond their healthy lifespan is very hard on an alternator.

Btw, Brett ^^^ is your alternator expert.
Old     (ottog1979)      Join Date: Apr 2007       09-24-2012, 11:40 AM Reply   
Dual batteries with Perko isolater/switch. I leave it set to "All" all the time. When it's back in the garage, I have a 1.5 amp maintenance charge that I plug it in to (still set in the All position to charge both batteries equally). Batteries were bought new in 2010.

Stereo amps are Orion HP-4600 (running tower and in-boat speakers) and Rockford Fosgate Punch 1505 running the subwoofer. These were professionally installed in 2008. We don't "party cove" nearly at all. Most of the time the stereo is running, the boat is too.
Old     (polarbill)      Join Date: Jun 2003       09-24-2012, 12:15 PM Reply   
Well, it looks like you have less than 500 watts although it is class A/B which isn't very efficient. Even if it was 500 watts/14v=35.71 amps your alternator shoudl be able to keep up with that if you are running above an idle. I am guessing you have a 70 amp 7si Delco unit. THat is a great little unit that we have seen put out around 90 amps in ideal conditions. When you say the boat is running are you talking about idling or actually runnign around cruising, wakeboard, surfing, etc??? If you are talking at an idle and you have your system cranked you might be pushing the alternator as it might only be putting out 40 amps or so. Seeing how you keep a maintanance charger on the batteries at night you should be workign with fully charged batteries every day assuming the batteries aren't completely drained when teh charger is put on. Have you ever checked the voltage of the batteries with an actual multimeter? I would check the voltage at the back of the alternator when running and the voltage at the actual batteries to see if you are seeing a significant amount of votage drop. Have you checked the belt/pulley? Is it tight and not slipping? I would check all connection points including grounds through the system and take a wire brush to the terminal side and where it connects. When you had the alternator replaced before what unit did they use? An OEM Delco/Mercuiser(same thing) or did they throw on an overseas clone, cheap rebuild or paint and pray?
Old     (ottog1979)      Join Date: Apr 2007       09-24-2012, 12:30 PM Reply   
Might have been a re-build but it would have been to OEM specs. Dealer did the install.

We do spend time now & then idling with the stereo on but not cranked. Before replacing I'll check belts, connections & voltage as you recommend. I checked the belt out on the water and it seemed tight enough and the alternator pulley was turning just fine.

Assuming I need to replace, would you recommend OEM new or is any upgrade cost effective?

A dumb question: Does using more amps than an alternator puts out (say at idle) over-stress and eventually (or immediately) kill it? Would too much of this shorten the useful life down to a couple of years?

Last edited by ottog1979; 09-24-2012 at 12:33 PM.
Old     (polarbill)      Join Date: Jun 2003       09-24-2012, 12:45 PM Reply   
Makign the alternator work to it's full capacity all the time can definately shorten it's life. Workign hard means more heat which leads to failure.
Old     (polarbill)      Join Date: Jun 2003       09-24-2012, 12:49 PM Reply   
I would generally stick with an actual OEM unit. You can get the OEM Delco unit for under $200. Unfortunately Mercruiser uses a weird dual lower foot mount usually and there aren't a lot of good options ot get higher output. What year is your boat and does your alternator look like this?

Old     (david_e_m)      Join Date: Jul 2008       09-24-2012, 1:05 PM Reply   
Music is transient in nature so it would be very unusual to be pulling the full 35 amps unless you played some really nasty compressed downloads and just hammered the system into hard clipping. Also, poor system set-up and poor tuning can really ruin the audio system efficiency.
Once this thing is put back together, it would be a good idea to check the voltage at the amplifier primary power terminals to check the voltage drop under the most adverse conditions. Pulling more current than the charging system can supply will result in a measureable drop in the voltage.
Also. check the battery voltage end of day and immediately as the boat is placed into storage and before attaching the charger. For example, if the voltage is resting at 12 volts then you may need more than a maintenance charger.
Taking measurements at the right spot and at the right time will provide valuable information and put an end to the 'what's wrong' speculation.
Back to Brett.

Earmark Marine
Old     (ottog1979)      Join Date: Apr 2007       09-24-2012, 3:36 PM Reply   
Thanks David & Brett. You guys are great. It will be until the weekend before I can do a little testing.
Old     (david_e_m)      Join Date: Jul 2008       09-24-2012, 4:01 PM Reply   
Like I said, Brett is the real alternator and charging expert here. I was just throwing out a few comments on the obvious.
Good luck. All that's left now is the hard work.

Earmark Marine
Old     (ottog1979)      Join Date: Apr 2007       09-30-2012, 1:05 PM Reply   
OK, I had some time today and took some measurements. Here's what I found:
With the boat running at 1,200-1,300 RPM's Alternator was kicking out about 12.4 volts. This is measured at the alternator. Drop to idle, about the same. Belts tight, pulley turning, connections tight. I measured at each battery with the boat running: 12.4-12.5 volts.

I then measured the voltage delivered to the main speaker amplifier with the boat idling. Stereo turned off it was 12.3. Stereo turned on mid-upper volume level, still 12.3.

I turned off the boat and measured each battery buy itself: both at 13 volts. (Remember I have a trickle charger that was plugged in all week.) Funny that this is above the measured voltage at the batteries with the boat running.

Didn't notice the voltmeter on the dash, unfortunately. Either way, it looks like the alternator is not kicking out the 13-14.4 volts it should be. Is it common for alternators to fail so slightly? Would the cumulative effect of 12.3-ish output cause the problems I'm having? Anything else this data is telling?

Last edited by ottog1979; 09-30-2012 at 1:08 PM.


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