Alternator AC Output Spec - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 17 (permalink) Old Apr 29th, 2019, 5:22 pm Thread Starter
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Alternator AC Output Spec

Before I install a new regulator/rectifier on my 1993 Ducati 900 Superlight, I ran a few quick tests on my alternator. The reading for the AC output across the 2 yellow wires was 30VAC @3,000 rpms & 40VAC @4,000 rpms. Can anyone tell me if this output is consistent with factory specs?

Thank you so much.
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post #2 of 17 (permalink) Old Apr 29th, 2019, 9:41 pm
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sounds ok to me. i usually go a bit higher rpm wise, where you should see 70 or so.

if you get a little rectifier and hook that up you can test the loaded potential of the alternator, but it's not regulated so you don't hold it at 7,000 and send the battery to 25v sort of thing. you should see 16v with that test pretty quick (and then shut it down).

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post #3 of 17 (permalink) Old Apr 29th, 2019, 9:49 pm Thread Starter
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Quote:
Originally Posted by belter View Post
sounds ok to me. i usually go a bit higher rpm wise, where you should see 70 or so.

if you get a little rectifier and hook that up you can test the loaded potential of the alternator, but it's not regulated so you don't hold it at 7,000 and send the battery to 25v sort of thing. you should see 16v with that test pretty quick (and then shut it down).
Well, I don't know if it's relevant, but the reason that I am replacing the reg./rec. is because the voltage at the battery was 16VDC @ 4,000 rpms (as well as a partially melted headlight connector). So that seems to tell me that the alternator is cranking out adequate voltage. Does that make sense?
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post #4 of 17 (permalink) Old Apr 30th, 2019, 6:37 am
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was it an original ducati or "correct" replacement reg, they have a voltage reference from the battery that goes through the loom and if it is low due to age and all that crap it will give a misleadingly low voltage at the reg and so the reg puts out enough voltage to make that reference what it wants.

it's not a reg issue if that is the case. fitting a reg without the reference wire does bypass it though, like the later fitment sh579b.

from my charging system write up:

If you use an original style or Electrex or other single phase regulator that uses a reference voltage input (the two wire flat connector has one wire to run the light, the other is a reference) then you need to make sure the reference is accurate. This reference is switched through the ignition switch and runs through the loom. If the reference is low, due to the resistance that naturally builds up in an old loom, the regulator will think the battery voltage is lower than it actually is and it will overcharge the battery. I think itís the white/red wire (or maybe pink?).

You can either check the voltage in this wire or (better) run a jump wire from the battery positive to the connector. I pull the connector apart enough that itís still connected, but you can touch the terminal. Then, with the engine running and while checking the battery voltage, touch the jump wire to the terminal. If the battery voltage drops instantly (and it will if itís overcharging) the easiest fix is to cut the reference wire on the loom side of the two wire flat connector, fit a relay running battery power direct to this wire into the regulator and switch the relay on with the loom side of the reference wire coming from the ignition switch. 906 Paso have done this for years, and Iíve seen a few Monsters and SS do it.

The SH579 3 phase regulator doesnít use a reference, so fitting one does away with this potential issue.

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post #5 of 17 (permalink) Old Apr 30th, 2019, 8:39 am
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Not a good image but it is from my 1993 superlight manual. I would ditch the Ducati electronica rectifier and only run one if you are trying to keep this rare bike 100% original. I think I would adapt a later rectifier and keep a good Ducati electronica in a box for the next owner.
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post #6 of 17 (permalink) Old Apr 30th, 2019, 8:41 am Thread Starter
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The faulty reg/rec is a Electrex unit. By "reference" wire are you referring to the "no charge" indicator light?
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post #7 of 17 (permalink) Old Apr 30th, 2019, 8:44 am Thread Starter
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Originally Posted by ducvet View Post
Not a good image but it is from my 1993 superlight manual. I would ditch the Ducati electronica rectifier and only run one if you are trying to keep this rare bike 100% original. I think I would adapt a later rectifier and keep a good Ducati electronica in a box for the next owner.
Thank you. If these specs are correct, then the VAC I am getting is quite low. Is this indicative of a faulty stator?
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post #8 of 17 (permalink) Old Apr 30th, 2019, 10:01 pm
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there are two wires that go into the loom - one is the charge light, the other the reference voltage.

if it was knocking out 16 i'm guessing the alt is ok.

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post #9 of 17 (permalink) Old May 1st, 2019, 8:01 am Thread Starter
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Originally Posted by belter View Post
if it was knocking out 16 i'm guessing the alt is ok.
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Is the fact that the alternator is generating 35 VAC @3,000 rpms an indication that there might be an issue if the factory spec is 70 VAC?
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post #10 of 17 (permalink) Old May 1st, 2019, 8:23 am Thread Starter
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Originally Posted by belter View Post
there are two wires that go into the loom - one is the charge light, the other the reference voltage.
Forgive me if I'm a bit dense when it comes to electrical issues. If there is considerable resistance in the reference wire (which leads to overcharging), shouldn't I be able to disconnect the reference wire & then check the battery voltage? If the indicated battery voltage is now within specs, that would seem to suggest that the reference wire was indeed the culprit. Does that make sense?

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