How to tell the difference between 28 and 40 amp alternator

Joined
Dec 20, 2014
Messages
8
Location
Bend Oregon
Bike
1992 ST1100
I recently bought this 92 ST1100 for $500 with 77000 miles on it. I was told the original alternator died so they put a new one in then it leaked oil around the alternator so they pulled it out and reinstalled it only to crack the outer alternator case and thats how I bought it. The guy didn't know if they put in a 28 or 40 amp alternator. The bike runs and starts fine and I thought the charging voltage was a bit high at 15 volts at 4000 rpm but after reading a bit it seems that's not unusual for a 28 amp alternator so I assume that's what I have but I was curious if there was any way to definitively tell. I can't find a alternator case so I assume they don't make them anymore and I"m left to scour eBay and junkyards for one, is that correct?

Thanks,
Patrick
 
Joined
Aug 10, 2015
Messages
2,029
Location
Fort Worth, Texas
Bike
91 ST1100/06 ST1300
The 28 amp unit has no cooling vents, the power wires come out an obvious round, sealed passage near the top rear. New these came with a rubber cap that covers this area.
The 40 amp alternator has cooling slots in the back side. Loks more like an automotive style alternator...but no pulley, of course.
 
Joined
Jun 3, 2006
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3,513
Location
British Columbia
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2021 RE Meteor 350
I can't find a alternator case so I assume they don't make them anymore and I"m left to scour eBay and junkyards for one, is that correct?
Yes, that part is now considered obsolete by Honda and not available, unless used. You do have the option though of upgrading to the 40 amp alternator and John O. can get a kit to do the job to you. Requires pulling the swing arm and some choice words along the way.
 

kiltman

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Apr 27, 2013
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68
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Stratford, Ontario Canada
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2002,ST1100ABS
STOC #
8826
Yes the 28 amp alternator generates around 15 volts at 3k. You can upgrade to the 40 amp unit and there are ones on eBay for a good price. The 28 amp unit is no longer available.
 
Joined
Aug 10, 2015
Messages
2,029
Location
Fort Worth, Texas
Bike
91 ST1100/06 ST1300
I recently bought this 92 ST1100 for $500 with 77000 miles on it. I was told the original alternator died so they put a new one in then it leaked oil around the alternator so they pulled it out and reinstalled it only to crack the outer alternator case and thats how I bought it. The guy didn't know if they put in a 28 or 40 amp alternator. The bike runs and starts fine and I thought the charging voltage was a bit high at 15 volts at 4000 rpm but after reading a bit it seems that's not unusual for a 28 amp alternator so I assume that's what I have but I was curious if there was any way to definitively tell. I can't find a alternator case so I assume they don't make them anymore and I"m left to scour eBay and junkyards for one, is that correct?

Thanks,
Patrick
I found a 28 amp alternator from a wreck with a little over 5000 miles on it. Looks nearly new with no evidence of oil leaking around the wiring exit. Resistance check is good. So, this one is my spare in the event of an alternator failure. In the two years since, I haven't seen one I would consider taking a chance on either at wrecking yards or on Ebay. If the seller doesn't disclose the mileage on the bike it came from or has some nebulous statement like 'looks good to me', pass. Replacing a 28 amp with a new 40 amp alternator is usually the safe thing to do. There are how-to tutorials here, just use the search function, and it is covered in detail in the Clymer manual for the 1991-1994 St1100.
 
Last edited:
Joined
Apr 20, 2018
Messages
272
Location
Harrisburg, PA
Bike
1991 ST1100
I found a 28 amp alternator from a wreck with a little over 5000 miles on it. Looks nearly new with no evidence of oil leaking around the wiring exit. Resistance check is good. So, this one is my spare in the event of an alternator failure. In the two years since, I haven't seen one I would consider taking a chance on either at wrecking yards or on Ebay. If the seller doesn't disclose the mileage on the bike it came from or has some nebulous statement like 'looks good to me', pass. Replacing a 28 amp with a new 40 amp alternator is usually the safe thing to do. There are how-to tutorials here, just use the search function, and it is covered in detail in the Clymer manual for the 1991-1994 St1300.
Quick tips on buying one from eBay:

-Prefer one off a running bike, preferably with video of bike running.

-If not available that way, prefer one from a seller who sounds like he knows what he's talking about, e.g. "looks good to me" is a big ??? vs. things like "this is the original OEM 28 amp alternator, not the upgraded 40-amp version."

-I used to say prefer a 'top rated seller' on eBay, but they've mucked that up so much it's now meaningless. I now say it's immaterial (and certainly don't pay -extra- for it).
 
Joined
Sep 10, 2011
Messages
2,211
Location
West Michigan
Bike
'98 ST1100
STOC #
8470
Yes, that part is now considered obsolete by Honda and not available, unless used. You do have the option though of upgrading to the 40 amp alternator and John O. can get a kit to do the job to you. Requires pulling the swing arm and some choice words along the way.
Mr. Bush:

When swearing in Canada, should it be done in both English & French ?? Just curious.
 
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