1996 Twin Mercury 225 efi bad alternators

Took the boat out the other day for a ride, motors were running poor, looked down and I was at 9 -10ish volts. Luckily I had a spare battery that i tossed in and was able to get home just fine. After a little diagnostic work i noticed that both alternators were dead.

What could possibly kill both alternators? could it be a short or power loss somewhere? Batteries are not losing voltage when switch is left on so I am guessing there is no short but who knows.

I found the alternators I need on amazon part number 821663a1 for 80 bucks which is a quarter the price I got quoted from a merc dealer. I just don't want to put these things in to have them blow up again.

What kind of batteries are you using?

Are these the small, Nipon Denso alts?

I have used DB electrical ones i got online with no issues. Unfortunately they are all made in china...One alt. may have been bad for a while and the other just went bad and you had low volts. If your batts were hooked together one motor would charge them both.

If you turn the battery switch's from one to the other and back again as a test or for whatever reason while the motors are running , it will spike the regulators and kill them .... dead .

Moral of the story , find an old guy generator shop . He will fix them for $35.00 each. Everyone else want's $350.00 to fix yours or to sell you new ones .
After that , use a battery load tester and a VOM for any further experiments .

Batteries are a little small, they are 24 series. One passed a load test while the other failed. Think I am going to upgrade to 27's since the alternators charge at 100 amps total. They are the stock alts from the factory, not sure if they are Nipondenso

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Originally Posted by Chaz

If you turn the battery switch's from one to the other and back again as a test or for whatever reason while the motors are running , it will spike the regulators and kill them .... dead .

Moral of the story , find an old guy generator shop . He will fix them for $35.00 each. Everyone else want's $350.00 to fix yours or to sell you new ones .
After that , use a battery load tester and a VOM for any further experiments .

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Both are selected to all and I believe they have never moved but I cant really remember... There are 2 batteries with 2 switches, not sure how it is wired up, is this normal for twins?

New alts and new batteries and check all the connections and switches for proper operation. Then your good to go for a while.

The switches should not be left in the ALL position....this probably is part of the problem. Each switch should be selected to the battery designated for that engine ( stbd engine to battery 1 & port engine to battery 2). The ALL position cross connects the batteries and alternator outputs, and when a problem occurrs, it affects the entire electrical system and defeats the advantages of having redundant systems.

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Originally Posted by Go Time

The switches should not be left in the ALL position....this probably is part of the problem. Each switch should be selected to the battery designated for that engine ( stbd engine to battery 1 & port engine to battery 2). The ALL position cross connects the batteries and alternator outputs, and when a problem occurrs, it affects the entire electrical system and defeats the advantages of having redundant systems.

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Agreed, I went extra measures to separate my motors into two separate systems so they do not affect each other.
I also ran a bat 1/bat 2 leg to a left/right switch for all my accessories not controlled by their respective motor.
Should I have to shut down a side to come in on one, I can choose which battery my accessories run off of.

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Originally Posted by Seanj85

Batteries are a little small, they are 24 series. One passed a load test while the other failed. Think I am going to upgrade to 27's since the alternators charge at 100 amps total. They are the stock alts from the factory, not sure if they are Nipondenso

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I had run some of the mini batteries in the past and they didn't like the 50 amp charge rate of the Nipon-Denso Chinese copies.
After loosing 2 sets of alts and 2 sets of batteries, I went back to wet cell group 24 batteries and had no more problems.
It sounds like you took out the systems by having them tied together @ all times.( the bad side takes out the good side)

Nice Pair, I added a "hotel loads" switch as well...when trolling in my fishing boat with one engine, it allows me to run the boat off the operating engine and maintain segregation.

Alright, put everything in last night after work. The 75 dollar amazon alternators are working great and charging at speed just over 14 volts. I traded in the old 24 series for 27 deep cycles from costco. They are interstates and were only 75 each, at 650 Cold cranking amps each they are a little light but seem to do the job fine. I replaced the belts and switched each battery to their respective engine. If I had bought everything from the marine store here in LI I would have spent over 1000 bucks, but shopping around I did it for 340.
One old alternator was definitely burnt, and the other looked ok but probably blew the rectifier.
Thanks for the help guys, this is a great forum.

We have local shops than can replace the regulators pretty cheap. Look around, you may be able to repair it.