replace the alternator?

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radiofun

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Ok. I've been keeping track of the voltage in the battery for the last few weeks and it has been steadily dropping even though I've been limiting my headlight usage to just the pilot light. Started out with 12.2 volts and just got back from a ride and the voltage is 11.47. I put the volt meter on the battery and ran the bike at 3,000 rpm and the voltage went from 11.5 to 12.5. Am I correct in assuming the alternator should be putting out at least 13.5 volts at 3,000 rpm. Is it time for a new alternator.

PS. battery takes a charge just fine with the battery Tender.
 
Forgot the mention. On the last ride the bike was not idling very well with the headlight on, and when I pushed the horn button it didn't sound and sound and the bike would stall. Low electricity?
 
As I remember my old 69 with the original rotor and stator will produce over 12.7V at around 3K, that's about the break over to charging. Goes up to 13.5 above that, I'm still using the zener, so that is my limit. The battery is new and at rest after a few days is still at 12.7/8. Put it on the tender for a day or so, then let it sit a day, and see what it's doing then, not the charging circuit, but the battery itself, it should hold at least 12.4/5. You should be getting at least 13.5V when you get up to 4K or with the charging circuit. If the battery drops to 12.2 or so after a few days, it's pretty weak and probably doesn't have much capacity left.

Dave
69S
 
Your alternator sounds like it is ok because the volts go up when you raise the revs.

But I would first question the condition of the battery itself. I don't know how old it is, but I would get a new one because it is not holding a charge. You don't need to buy an expensive brute unless you have kick start.

You state it "starts" out at 12.2 volts (that assumes fully charged) then goes down to 11.5. Fully charged it should be more than 12.2 so it is shot, and your alternator is fine because it raises the voltage to the battery over a full volt.
 
I got 13.56 v on my factory '72 charger with good but not full charged battery, with head light on reving into upper 2000 rpm range. I vote new battery first effort to try. Mag rotor should be a good stick to metal easy holding its own mass and then some. Light dim and horn weak are traditional voltage meters and their brightening up with rpm was the test for charger working. Battery can get so low at low rpms a brake light on can stall out the spark. Poor connections don't help either.
 
I have it on the charger now to ensure it is fully charged and then will see if it holds a charge. Battery is 3 years old so that is likely the culprit.
 
If you can put an ammeter in the rectifier output wire you can get a better idea if the alternator is doing its job or not.
 
You really need an oscilloscope to check this problem, is the rectifier ( a bridge rectifier ) rectifying correctly, is the zener regulating the voltage properly.

The output of the alternator shold be a nice sign wave, the output of the bridge rectifier should be a nice rippled DC voltage etc etc, an electronics type person will best identify the problem.

An easy test would be to rev the bike to 3,000 RPM and disconnect the battery and see if the bike stops running.
 
radiofun said:
I have it on the charger now to ensure it is fully charged and then will see if it holds a charge. Battery is 3 years old so that is likely the culprit.
Click to expand...

Yes, it is. Put an AGM in there.
 
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