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  #1   IP: 172.74.214.3
Old 06-09-2015, 10:55 PM
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Question Alternator swap

I finally pulled the trigger on a "new" (Rebuilt) A4 and among the handful of surprises was the fact the new engine came equipped with a 35 amp Motorola alternator. I was surprised by this because it's not even offered as an option in the parts catalogue from MMI. Its been pointed out that the only way to get a 35 amp alternator is when ordering a rebuilt engine.

My existing A4 already had the 55 amp version and it didn't occur to me to specifically request the 55 amp version when I ordered the new engine. I have since spoken to the parts folks that have told me I could keep my old 55 and consider it a wash, but after reading so many of the alternator threads I have to wonder now if my new engine configuration with it's 35 amp set up to include the pulley, belt, arm, rpm, etc., will directly swap with my 55 amp.

I'm weighing the pros and cons but not sure if I'm over thinking this.

The 35 amp should be a rebuilt with the expectancy of prolonged longevity. The 55 amp however was not original equipment and had therefore been replaced at some point before I purchased the boat bringing some parity to the longevity issue. While I prefer the increased charging capacity I have already invested a lot of time, money and engineering/mechanical prowess to make too many more changes, especially after the substantial investment in a re-manufactured engine. The existing 55 amp alternator should already be equipped with the larger pulley and internal regulator but I suppose I should check to be sure.

If the pulley on my 55 amp is sized properly is my only concern a new belt size and what size should the pulley be?
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  #2   IP: 76.179.157.47
Old 06-10-2015, 01:04 AM
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I swapped my OEM 35 amp with the MMI 55 amp, and it was about that simple. I used the pulley that came with it - no further calculations. No change to the accessory drive. Maybe a 26" belt, but the 25" works also. You'll need to make up a new end for the excitation - replace the wire from coil+ to the alternator ( the OEM has an inline spade lug IIRC).

For smaller house batteries (mine are 2X group 24) the size of the alternator is really more about ruggedness and reducing heat than real charging difference. Bigger house batteries will be a different story.
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Last edited by BunnyPlanet169; 06-10-2015 at 01:08 AM.
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  #3   IP: 107.0.6.150
Old 06-10-2015, 08:57 AM
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Talking

You might be over thinking this one. Just keep both alternators on board so you will have the security of having a spare when you are far from home.
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  #4   IP: 65.160.210.134
Old 06-10-2015, 11:20 AM
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Quote:
Originally Posted by Nauti Buoy View Post
I finally pulled the trigger on a "new" (Rebuilt) A4 and among the handful of surprises was the fact the new engine came equipped with a 35 amp Motorola alternator. I was surprised by this because it's not even offered as an option in the parts catalogue from MMI. Its been pointed out that the only way to get a 35 amp alternator is when ordering a rebuilt engine.
...I have since spoken to the parts folks that have told me I could keep my old 55 and consider it a wash
Who did you get the rebuilt from? MMI?
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Old 06-10-2015, 12:24 PM
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The 35 and 55 amp Motorola alternators are identical form factors. You can swap them back and forth easily.
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  #6   IP: 172.74.214.3
Old 06-10-2015, 07:14 PM
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Unhappy

Thanks everyone for all the input on this one.

The 55 amp from the old engine is an API part # 20054-UNI. The sticker reads "Fits Atomic 4 gas engines" 05/24/06. Can't get the photos to upload.

The rebuilt on the new engine is clearly a Motorola.

Still agonizing over do I keep the 9 yr old 55 amp alt or give it up and take the newly rewound 35 amp?

It's gonna be a coin toss.

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Old 06-10-2015, 09:26 PM
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I would NEVER toss a coin over a decision like this - Rock, Paper, Scissors all the way!

Electrically, if you don't have big battery banks, it's a wash. Keep the new one on the engine, and your 55 A API as a spare. Path of least resistance. Stop thinking about it!
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  #8   IP: 65.160.210.134
Old 06-11-2015, 10:53 AM
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Part TWO

Who rebuilt the engine?
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  #9   IP: 72.45.14.161
Old 06-11-2015, 02:37 PM
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Considering it is around 15-20 minutes to swap them, I wouldn't get too worked up over it. Try'em both and pick your favorite

Quote:
Originally Posted by Nauti Buoy View Post
Thanks everyone for all the input on this one.

The 55 amp from the old engine is an API part # 20054-UNI. The sticker reads "Fits Atomic 4 gas engines" 05/24/06. Can't get the photos to upload.

The rebuilt on the new engine is clearly a Motorola.

Still agonizing over do I keep the 9 yr old 55 amp alt or give it up and take the newly rewound 35 amp?

It's gonna be a coin toss.

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  #10   IP: 12.216.194.200
Old 06-12-2015, 06:29 AM
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I swapped out the 35 for the 55 amp and I did change the belt to 26 inch.
I did see a slight increase in charge rate from the 2, but the 55 amp
consumes more horsepower from the engine.

I ended up installing a solar panel and there's no advantage anymore.

Steve
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  #11   IP: 174.58.90.170
Old 06-12-2015, 11:44 PM
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The amount of hp used by an alternator should be proportional to the amount it is charging, for the most part.
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  #12   IP: 107.72.164.18
Old 06-13-2015, 06:59 AM
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Quote:
Originally Posted by joe_db View Post
Considering it is around 15-20 minutes to swap them, I wouldn't get too charged up over it. Try'em both and pick your favorite

There we go, fixed.
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  #13   IP: 66.249.83.252
Old 06-13-2015, 07:01 AM
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Quote:
Originally Posted by romantic comedy View Post
The amount of hp used by an alternator should be proportional to the amount it is charging, for the most part.
+1

Bill
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  #14   IP: 172.74.214.3
Old 06-13-2015, 10:51 PM
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Jerry,

I purchased a remanufactured engine from MMI. 35 amp comes standard on the A4. Can't get a 35 amp any other way from MMI is my understanding.

I did decide on the 55 amp version and will return the 35 amp. They expect a 1 for 1 exchange. Sea trial was today. Everything was ok electrically. Running 175 deg at idle.

Heading to cooling section thread
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alternator, alternator rpm, belt size, pulley size, rebuilt engine

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