Alternator Replacement for W50 Prestolight (Motorola)

Mike · **Joined:** 09 Jul 2008 12:51 **Posts:** 251

I need some advice navigating the wonderful world of alternators.....

Last year I had to replace my alternator. I went with a direct replacement, MA12N600 Prestolight (Motorola type),12V, 51A, 8MA Series
manufacturer Wilson Electronics because I just needed a fast replacement.

Wilson Part Specs: http://ws1.wilsonautoelectric.com/ec03/part.php?part_search=90-05-9057&part=[url]

I have a large house battery bank and need a larger alternator, say 100 amp

My problem is finding an alternator that will work with my tachometer. The tach needs a feed from one of the internal posts from the alternator as shown in the following picture.

Has anyone with this type of tach upgraded to a 100a alternator or larger ? if so what model or alternator and regulator and how is it connected ? THANKS

Tortuga's Lie · **Joined:** 27 Oct 2006 10:32 **Posts:** 827

Mike,
I am in the process of doing this upgrade currently and there's a a discussion here:
viewtopic.php?f=5&t=265

I went with a Balmar 7 series, 80 amp with the ARS-5 external regulator. I went with the 80 amp because I thought it was a good fit for my W-40 and I wouldn't need to upgrade the pulley system. Defender has these alternators on sale still for $299 which isn't all that bad. The regulator isn't cheap though.

Velera · **Joined:** 29 Dec 2006 09:38 **Posts:** 656

OK let's see if I can help here. I am sure others may disagree.
First, most alternators you buy off the shelf will have internal regulators in them because they are primarily truck or industrial vehicle derived and that is OK. They will need to be rewired for external regulation (I do not recommend that you do this yourself, but--if you are handy with a soldering gun and know alternator theory, it can be done--I am sure there are instructions on the internet somewhere) by a competent auto truck electrical shop. With just about any alternator there is a schematic about which wire goes where and one almost always is tapped for the tach or is a dedicated for the tach (one old Yanmar model is an exception, but this is the rule). The person who rewires your alternator to work with an external regulator will identify each of the wires for you: there will be a ground (obvious), a field wire, a power wire and a tach wire. Field and ground (and sometimes a 12v power lead) will go to your external regulator per their instructions. Hooking the regulator and alternator up is not complicated as most the three step regulators are designed for amateur installation.

You can pay a hefty premium for a special "marinized" alternator from someone like Balmar--but--this is MHO--- you are paying a very large premium for a simple rewiring and powder coating paint job and a more open case. I would suggest that you probably do not want to go above 105 or 120 amp alternator as when you get above this, you will likely need two belts on your crank to alternator or be ready to shred a bunch of single belts--it takes real horsepower to drive one that big under load. I know that some will claim that marinized alternators have better bearings, diodes, etc. Have not seen evidence of that actually--and in my years on the hook, I saw as many folks with Balmar problems as anyone else-actually more---maybe because there are so many out there. You need to get a model that has a "foot" where it attaches to the engine that matches the engine bracket. There are two common versions (single and double foot and some variants of size of foot) and any alternator with a proper foot that will fit your engine bracket will work.

External smart regulators are much the same in function. They vary in the number of technical adjustments you can make manually-- if you have bizarre batteries like LiPO or Glass Matt batteries (I fail to comprehend why folks want to add this level of complexity and expense to their cruising lives--I have used golf cart batteries for years and cannot imagine any thing working better or less expensively) you may want a three step that accommodates the special charging regimen required by exotic batteries. I think the smart regulators by AMPLE Power are more sophisticated and have a very good record of performance, but the simple ones by Balmar are OK too. I have used both on my boats. You want a regulator that has temperature sensors at least on the alternator and better if it senses battery temps as well.

Remember that your alternator rating is for a temperature that few engine compartments maintain for long--your output results will be far less when the heat increases. Also, a 120 amp alternator will not charge your batteries twice as fast as a 60 amp, because there a thing called acceptance rate of your batteries, which tapers off phenomenally as the alternator and batteries heat up while charging. I strongly suggest a battery monitor in the system (Like a Linklite or Link Pro--and there are others) so that you actually can measure and see what is going on with your charging system. if you are like me, the data will reframe your view of battery charging systems and add a whole new level of skepticism to the boat show battery and alternator guys. You may see that they are like the anchor salesmen ("third generation! you'll never drag with this one!!!). I confess--I am very interested in using proven technology, like smart regulators, but really really skeptical about brand claims of superiority and function in the real world. Some basic laws of physics are hard to transcend.

Those are my thoughts. I am sure others will have differing points of view.

Mike · **Joined:** 09 Jul 2008 12:51 **Posts:** 251

I appreciate the knowledge about alternator theory but I'm looking for someone that has done the replacement and what.. how specifically they did it with the tach that I have. I know there's a huge variation of choices and combinations out there. I soldiered the alternator that I have today for the tach wire lead. It was no big deal once I knew what to do.

So...... I ask for those that have done it....

What model of alternator, what regulator and how did you do the wiring for your tach ?

jvoelxen · **Joined:** 21 Oct 2006 15:36 **Posts:** 268

I just bought a 105amp alternator from Sailon Electric, which was modified to accept my Balmer AR-5 external voltage regulator. You can reach them at 516-921-3074. It is a Motorola clone with the same number of poles so your tach will work OK. I haven't installed it yet because my boat Is on the hard and can't verify its operation but suspect it will be fine this Spring when I do.

Maine Sail · **Joined:** 16 Oct 2011 13:33 **Posts:** 12

Mike wrote:

I need some advice navigating the wonderful world of alternators.....

Last year I had to replace my alternator. I went with a direct replacement, MA12N600 Prestolight (Motorola type),12V, 51A, 8MA Series
manufacturer Wilson Electronics because I just needed a fast replacement.

Wilson Part Specs: http://ws1.wilsonautoelectric.com/ec03/part.php?part_search=90-05-9057&part=[url]

I have a large house battery bank and need a larger alternator, say 100 amp

My problem is finding an alternator that will work with my tachometer. The tach needs a feed from one of the internal posts from the alternator as shown in the following picture.

Has anyone with this type of tach upgraded to a 100a alternator or larger ? if so what model or alternator and regulator and how is it connected ? THANKS

A Leece-Neville 90 or 105A alt will fit and can easily be converted to external regulation. Keep in mind that these alts will need to be dialed back some in Balmar's Belt Manager/Amp Manager and should utilize the Balmar alt temp sensor too.

Converting a Leece-Neville 8MR to External Regulation
http://www.pbase.com/mainecruising/alternator_conversion

A Balmar 6 series or Mark Grasser DC Solutions alt would be a better performer but for the money an 8MR is a decent little alt, IF you utilize "belt manager" and limit its current potential..

The Leece-Neville 8MR series and the Grasser and Balmar alts all have an AC Tap or "Stator Tap" to drive your tach. Even with the same number of poles the tach may still need some adjustment...

Mike · **Joined:** 09 Jul 2008 12:51 **Posts:** 251

Thanks for the advice everyone !

berwick · **Joined:** 21 Oct 2006 18:12 **Posts:** 327

I just upgraded (not tested til spring though) with an internally regulated alternator from Hamilton Ferris, it is a 100 amp model which should be good for my W40. They will have a hook up for the tach if you ask for it.

http://www.hamiltonferris.com/products/ ... egulated/9

Richard

Mike · **Joined:** 09 Jul 2008 12:51 **Posts:** 251

I upgraded my alternator to a Molorola 105amp 2 weeks ago. I had to change belts because the one that was on previous slipped a lot. When the new belt was installed I tightened it as tight as I could get it. All was ok until I noticed that after running the engine for more than an hour the engine temp rose to around 184-185; I didn't put two and two together at the time. Back at the dock I loosened the belt just to the point at which any more would cause it to slip. My engine warming seemed to disappear.

Anyone else experienced engine warming with belts too tight ?

Velera · **Joined:** 29 Dec 2006 09:38 **Posts:** 656

Can't think of anything a 105 would do that would increase temp. I ran one on my W50 for 2000 hrs at all stages of discharge with no problems. You are using only a fraction of the hp the engine produces to drive a 105, even at full tilt, so I would think it is likely a coincidence or just pointing out a latent issue with your cooling system.

Ray

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Alternator Replacement for W50 Prestolight (Motorola)

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