a27 (4D61) Alternator Replacement Questions

[Don from Mystic]

My alternator is not charging correctly based on it's inability to charge two known good batteries fully and a 12.8 - 13 v output taken with two different meters. I believe the alternator is a Motorola, likely original. I have a couple of questions which will help me to decide which direction to take.

1) I'm considering physically modifying a 120 amp generic "hot rodders" GM one wire replacement (I have the stuff to accomplish this- mounting, pulley, etc) but Im wondering about the load it will place on this small engine and if there are any other considerations I'm missing.

2) I have found a replacement for the Peugeot auto - (50 amp). I see no reason why this also wont work fine - Am I missing something?

Thanks in advance for the advice

[don123]

A fully charged 12 volt battery measures about 12.6 volts, so if you're measuring 13 volts, likely the alternator is working . . . . at least somewhat. Not saying it doesn't need rebuilding, but if it wasn't working at all, you wouldn't see 13 volts anywhere

High amp alternators are a popular modification on many sailboats (even those with tiny 20 or 30 HP engines) because the boats are frequently used for long periods of time without the engine running, so when it is run for a short period, you want to recharge the large batteries as quickly as possible . . . . so you can shut the engine off as soon as possible

On a trawler which is usually operated with the engine running and which also likely has a smaller battery bank, a high output alternator would be of little use, unless of course you have some really high demand electrical accessories on board. If you have a pair of 100 AH battery banks which you routinely discharge to 50%, then you only need to replace about 100 AH of capacity and your 50 amp alternator can probably do this in 3 hours or so - Why would you need a 120 amp alternator which could do it in less than half the time? Keep in mind the fact that batteries last longer when they are not rapid charged

An alternator rated at about 20% of the capacity of the battery bank(s) should be more than sufficient - Any more is probably overkill. I have 400 AH worth of golf cart batteries which are kept charged by the stock 55 amp alternator on my Yanmar

Don

[Don from Mystic]

I appreciate your thoughts and good points. The resting voltage of the batteries is under 12v. I do realize the alternator is doing a bit, just not nearly what it should. Guess my next step will be to remove it and do a few tests, maybe have it tested at the local parts chain. Also going to photo it so If I do buy the replacement I can email the seller a pic before he ships. Even at 6kts, many of my rides are an hour or less, so the high output might do well for me.Thanks again-

Don