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Which alternator for Merc BBC?

Old 01-24-2021, 03:54 AM
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Default Which alternator for Merc BBC?

Kind of a newbie question but as I cannot return anything I want to be sure before I order.
Which is the standard alternator for a Mercruiser Gen IV Big Block, which OEM part number?

And, why my alternator has 4 cables but only 3 connected? (and works fine)

Any recommendation which to order? Looking for one with some chrome.



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Old 01-24-2021, 04:31 AM
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​​​​​
I went with this company. No idea if the have anything chromed. But it works great. Direct bolt on and plug in.
I'd have to think you've got another wire hangout ng loose in there someplace to plug into the one that is not currently used.
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Old 01-24-2021, 09:15 AM
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You have an old mando Alternator but I can't say what the differences were off hand (AMP output or mounting bolt pattern)

Do you have the engine serial number (riveted on a small metal plate behind the starter mount of the engine on the flywheel housing - or more modern engines are on the spark arrestor cover)?

If yes, you can use a website like the following to find the exact alternator that was on you engine and work from there...

https://www.mercruiserparts.com/v-8-inboard-engines

Last edited by speicher lane; 01-24-2021 at 09:19 AM.
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Old 01-24-2021, 09:19 AM
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Old 01-24-2021, 02:33 PM
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Do not get anything chrome. It will rust in the engine compartment over time!
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Old 01-24-2021, 03:25 PM
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You have Mando ALT p/n 817119A4
alternate p/n's
One listed above
Wilson 90-31-7008
SIERRA 18-5966
MALLORY 9-19101 and many others

The ALT has 4 connections but will work with 3 do to the case is ground.
Its working but don't think its working correctly with it being wired the way it is, but would need to put a meter on the bat when running to verify

Orange - output to stater post (battery)
Purple - excite (turns ALT ON with key)
Red/Purple - Sense lead
Black - case Ground


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Old 01-24-2021, 07:38 PM
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Originally Posted by AllDodge View Post
You have Mando ALT p/n 817119A4
alternate p/n's
One listed above
Wilson 90-31-7008
SIERRA 18-5966
MALLORY 9-19101 and many others

The ALT has 4 connections but will work with 3 do to the case is ground.
Its working but don't think its working correctly with it being wired the way it is, but would need to put a meter on the bat when running to verify

Orange - output to stater post (battery)
Purple - excite (turns ALT ON with key)
Red/Purple - Sense lead
Black - case Ground
Thanks AllDodge, that‘s exactly what I was looking for, with this I can go through all the alternatives.

Still not completely sure about the cabling. There was not even a fourth cable around to connect the open black cable.
But somehow it’s working, at least it’s charging the battery with 13.3 V output when I run the engine on my test stand.


speicher lane: It’s from an ‘87 Top Gun, maybe the alternator is really 34 years old and was swapped from the original engine to at least two following ones, the last being a 582 Shafiroff without serial number. Which is why I sometimes struggle to find the right parts.
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Old 01-24-2021, 08:04 PM
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The ALT should be doing closer to 14.5 at 1500 to 2000 rpm
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Old 01-24-2021, 08:17 PM
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AllDodge has you covered for info that you need.
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Old 01-24-2021, 08:42 PM
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Originally Posted by AllDodge View Post
The ALT should be doing closer to 14.5 at 1500 to 2000 rpm
I think I was mistaken, the boat’s voltmeter is probably showing the battery’s charge, not the alternator output. Anyway I will test it when the new alternator arrives - which will take a while.
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