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[Ken Juul]

boat is for sale again.  only used it 5 times last year because of weather and family issues.  It needs a new loving home.  Anyway, moving it to the broker's yard yesterday the tach fell to 0 and it stopped charging.  Took the alternator apart, diode pack appears to test ok and brushes are good.  Anything else I can check before I get it bench tested tomorrow.  Numbers are too faded to read.  If it tests bad, what part number should I replace it with?  Prestolite 55 amp.  If it tests ok, where is the next place to look.  All the wires are in good shape and connections tight at the generator.  I have not checked behind the engine instruments yet.

[Ron Hill]

Ken : You haven't said what alternator you have? 
It is usually the white wire going into the tac that gives the signal to the tac instrument.
 
I've seldom heard of an instrument going bad!  Usually it's a bad connect at the Tac!

A few thoughts

[awesome34]

Ken,

FWIW, the link below is the alternator I have on my boat as I was considering buying a new one recently. Instead, I had it rebuilt because the housing had cracked due to too much vibration on the bolt through the mounting bracket. The electrical was fine. I bought a replacement housing online and a better bolt to limit vibration that I found in a post on c34.org somewhere. I didn't have the tools to do the alternator rebuild on my own. So I found a local alternator shop, that did the repair for much cheaper than a new alternator. Could be an option for you too.

Part No: 8MR2198L
http://www.prestolite.com/pgs_products/specs.php?pf=true&item_detail_id=26470&item=8MR2198L&product=ALTERNATOR

[KWKloeber]

Ken

The alt model number is STAMPED into the side of the case, behind the fan.  You may be looking at the faded regulator model number on the back, not the Alt p/n.
if you have an m25 or XP, it's likely an 8MR2049K alt, actually a 51-a motorola/prestolite.
http://www.prestolite.com/pgs_products/specs.php?pf=true&item_detail_id=16374&item=8MR2049K

I've typically supplied/recommended a Wilson remanufactured 2049K (Wilson p/n 90-05-9141) -- top quality and reasonable cost and a direct drop-in - about $150.  If you wanted to upgrade a bit, there's a 72-a reman prestolite that is a drop-in (the one that was optional on the M25/xp and it also boasts an external voltage sense - the 2049K is an internal voltage sense.)
note that the Wilson remains list as not including a pulley but I have not seen one that does NOT come with the pulley installed.

You can find on amazon etc - an "8MR2049K" replacement but it's an offshore knock-off, not a prestolite.

Anything more info you need on above jus lemme know.

-ken also

[Patches]

Ken:

I went through the the long process of trying to find a drop-in replacement 8MR2049K for my 1990 M25xp.  I ended up purchasing one from a forum member before getting a second opinion on mine, and having it rebuilt.  I had the replacement bench tested and it is putting out the 51 amps.  You're welcome to it for what I paid ($75) + shipping.

Patches 

[mainesail]

boat is for sale again.  only used it 5 times last year because of weather and family issues.  It needs a new loving home.  Anyway, moving it to the broker's yard yesterday the tach fell to 0 and it stopped charging.  Took the alternator apart, diode pack appears to test ok and brushes are good.  Anything else I can check before I get it bench tested tomorrow.  Numbers are too faded to read.  If it tests bad, what part number should I replace it with?  Prestolite 55 amp.  If it tests ok, where is the next place to look.  All the wires are in good shape and connections tight at the generator.  I have not checked behind the engine instruments yet.

Ken,

Do you have solar? This can cause the regulator to shut down. If you turn on some loads, that exceed the solar capability, the alt will fire back up, if this was the cause?

We have numerous options for a replacement alternators, both internally and externally regulated, for Universal diesels, but the CMI-72-IR can be custom set for voltage before we ship it out. We build all of our CMI/AMP-IT alternators in-house here in Maine...

https://shop.marinehowto.com/t/alternators--regulators

[Ken Juul]

Tested bad.  Advanced Auto is one of the few places that will do off vehicle testing.  New on order from ebay.  $123 free shipping.

[Ken Juul]

Fedex delivered a day early   The old alt had a small red wire to the neg terminal, large red wire on the plus, purple wire on the excite and orange wire on the ac tap.  New alt only has the + and - posts marked.  I've got a yellow wire coming out of the voltage regulater to a plug, a red wire from the VR going to a post and another post with nothing on it.

Romaine Electric model 7495N  It does not come with a pulley.

[KWKloeber]

Rod

Do you recognize this Alt/ model no.?
It doesn't look like a L-N.  It's Ken's original alt.

-K

[mainesail]

That's a Prestolite 8EM series. Long ago discontinued...

[mainesail]

It does not come with a pulley.

https://shop.marinehowto.com/products/8mr-leece-nevillemotorola-billet-machined-58-id-x-38-belt-pulley