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Author Topic: New alternator - 51a up close comparison w original Motorola  (Read 2991 times)

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jonathanc34

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New alternator - 51a up close comparison w original Motorola
« on: August 18, 2021, 04:36:59 PM »

I blew the diodes on my 1988 M25XP 51a Motorola, which is fine: it had a wobble in the shaft and has done its duty.

(I mistook the Reg terminal for a Voltage Sense terminal, and it didn't like that. If someone could explain what that Reg terminal is actually for, I'd appreciate it.)

I ordered a 51a replacement on Amazon from RareElectrical for $165, needed to get one quickly for vacation next week.
I also ordered the 105a replacement and a new pulley from catalinadirect for $210 because it comes with the 2" adapter, but it won't arrive until Monday. I could not find a direct-fitting replacement 90a or 105a on amazon. I expect I will install the 105a and return the 51a.

The new one pictured below has the same connections as the factory with the addition of a sense wire. The one from CD does not have a sense wire, but does have a voltage adjustment on the regulator to compensate for longer cable runs.

Here are some pictures of the two side-by-side.

My house bank is 4x 6v 230ah. We have a 12v fridge, 1500w inverter for a 700w microwave that we use frequently, 12v fans that run many nights on the hook. We also have inflatable paddle boards that pump to 15 psi using a 120w pump. There have been mornings where the house bank did not have enough power to start the engine.

Any thoughts or observations? Should I wait on the 105a from CD and use it?
« Last Edit: August 18, 2021, 04:53:16 PM by jonathanc34 »
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She'll make .5 past 6 knots. She may not look like much, but she's got it where it counts. (I've added some special modifications myself.)

KWKloeber

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Re: New alternator - 51a up close comparison w original Motorola
« Reply #1 on: August 18, 2021, 06:59:30 PM »

@J34

The REG terminal is for the red wire from the internal regulator (looks like it's hanging down inside the case???)
The 12v+ (key switch) Excite goes on the yellow wire with the (funky looking) Packard 56 terminal.
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jonathanc34

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Re: New alternator - 51a up close comparison w original Motorola
« Reply #2 on: August 19, 2021, 06:41:05 AM »

@J34 The REG terminal is for the red wire from the internal regulator (looks like it's hanging down inside the case???)

Ahh the old alternator is not wired properly: the red wire from the regulator should be on the Reg terminal. I had taken it apart to see if I could find the problem, saw the diodes visibly cracked, but when I put it back together, I didn't refer closely to my original pictures because I wasn't going to install it again. Sorry if those pictures confuse anyone.
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'Owenoke' 1988 C34 Hull 548 Tall/Wing M-25XP
Worton Creek Marina, MD

She'll make .5 past 6 knots. She may not look like much, but she's got it where it counts. (I've added some special modifications myself.)

Stu Jackson

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Re: New alternator - 51a up close comparison w original Motorola
« Reply #3 on: August 19, 2021, 05:39:47 PM »

>>>>>>>>>>>>>>>>>

I ordered a 51a replacement on Amazon from RareElectrical for $165, needed to get one quickly for vacation next week.
I also ordered the 105a replacement and a new pulley from catalinadirect for $210 because it comes with the 2" adapter, but it won't arrive until Monday. I could not find a direct-fitting replacement 90a or 105a on amazon. I expect I will install the 105a and return the 51a.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

My house bank is 4x 6v 230ah. We have a 12v fridge, 1500w inverter for a 700w microwave that we use frequently, 12v fans that run many nights on the hook. We also have inflatable paddle boards that pump to 15 psi using a 120w pump. There have been mornings where the house bank did not have enough power to start the engine.

Any thoughts or observations? Should I wait on the 105a from CD and use it?

I'm confused.  You ordered the 105 but it won't come in time for the vacation?  Or will you delay your departure?

You have a pretty gigundous electrical load there.

Nor do you mention if you have an external regulator either.

Could you explain a bit more, please?

Given your loads and described use, I am pretty sure you're not marina hopping.  The small internally regulated alternator will provide only half the output of the larger one, and that only in the bulk stage.  Maine Sail has done that comparison, in the Electrical 101 Topic.

Good luck.
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pablosgirl

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Re: New alternator - 51a up close comparison w original Motorola
« Reply #4 on: September 03, 2021, 07:41:11 AM »

Hi,
You will definitely want an external regulator on the 105a.  The internal regulator that comes with it is dumb and will eventually burn out the alternator due to over heating.  I would give it 4-6 months of daily use.  Donít ask me how I know this.  Be sure the external regulator you choose has a temp sensor for the alternator and battery.  There is quite a bit of discussion in the 101 topics on this site and on mainsails site about this.
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Ron Hill

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Re: New alternator - 51a up close comparison w original Motorola
« Reply #5 on: September 03, 2021, 03:54:21 PM »

Guys : The dumb internal regulator usually only gives a higher charge for a nano second - then goes back to charging at approx. 13.5V - 14.0V that it is set for!!

A thought
« Last Edit: September 04, 2021, 10:20:47 AM by Ron Hill »
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Re: New alternator - 51a up close comparison w original Motorola
« Reply #6 on: September 03, 2021, 04:54:52 PM »

Hi,
You will definitely want an external regulator on the 105a.  The internal regulator that comes with it is dumb and will eventually burn out the alternator due to over heating.  I would give it 4-6 months of daily use.  Donít ask me how I know this.  Be sure the external regulator you choose has a temp sensor for the alternator and battery.  There is quite a bit of discussion in the 101 topics on this site and on mainsails site about this.

Paul Iím confused how a 105 would burn itself up.  Weíre saying buy a larger alt but dial it back so that it wonít output what youíre buying it for?  Seems odd.  The alt will ONLY put out what the battery lets into it. Regardless of the alt amperage.
So Iíd think a 51a OEM has the better chance of burning up itself, because the battery will more often let the alt come closer to its rating.  The same batt will have the 105 rise to a fraction of its rating.
Thatís for the same bank of course, one alt vs. another.

What am I misreading?
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Explore.  Dream.  Discover.   -Mark Twain

Stu Jackson

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Re: New alternator - 51a up close comparison w original Motorola
« Reply #7 on: September 03, 2021, 06:16:10 PM »

.......
........
My house bank is 4x 6v 230ah. We have a 12v fridge, 1500w inverter for a 700w microwave that we use frequently, 12v fans that run many nights on the hook. We also have inflatable paddle boards that pump to 15 psi using a 120w pump. There have been mornings where the house bank did not have enough power to start the engine.

.........................................


Guys : The dumb internal regulator usually only gives a hi charge for a nano second - then goes back to charging at approx. 13.5V volts!!


Ron, that's only partially how it works, and the "hi charge for a nanosecond" is just not true.  It starts and continues at whatever the voltage is that the regulator is set for.  It's dumb because there is only ONE voltage it can produce.

........................
.........................

Paul Iím confused how a 105 would burn itself up.  Weíre saying buy a larger alt but dial it back so that it wonít output what youíre buying it for?  Seems odd.  The alt will ONLY put out what the battery lets into it. Regardless of the alt amperage.
So Iíd think a 51a OEM has the better chance of burning up itself, because the battery will more often let the alt come closer to its rating.  The same batt will have the 105 rise to a fraction of its rating.
Thatís for the same bank of course, one alt vs. another.

What am I misreading?

I don't think you're misreading anything.  Perhaps another way to say it is how we discussed it in the Small Engine Mode topic in Electrical Systems 101 threads. 

The larger the alternator for a given load will work cooler.  Period.

Maine Sail has shown us that both internal and external regulators, if set to the same voltage, will produce the same output for a given load in the bulk phase of three stage charging.  That is discussed in the External vs. Internal Regulation thread in ES101.

The OP, in the top quote above, tells us he has 4 6V batteries.  He incorrectly states that as 230 ah, where it is actually 400-425 ah depending on which 6V batteries he bought.

The GIVEN LOAD the should be considered to be half of his house bank capacity, or 230 which is probably what he meant.

From personal experience, I have a 400 ah house bank and when discharged halfway, when charging starts I see as much as 75A going in.  I see this on my Link 2000 which I know how to use and read.  Of course, with either internal or external regulation, this tapers off as the voltage of the batteries rise.

Therefore, I agree with Ken, that the small alternator will be overtaxed and the larger will do fine.
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