81 Cb750c Charging System Trouble Shooting

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81 Cb750c Charging System Trouble Shooting

locust17
Hello this here is my bike




Now to get to the point. Bike doesn't hold the charge after charging it. I charge the bike, great, it works fine charges hold. When I turn it off, bam! Doesn't wanna turn back on right after I switch it off. The voltage slowly starts going down to 11.5. I don't know really how the charging system works I'm just testing all major components. I don't know wether I need to literally take out for a spin for it to charge because it reaches 14.4 volts so maybe I thought riding it will charge it.  Here are my tests.

I checked the battery voltage it was a 11.5 turned off. I try to turn it on nothing. This time I charge it, it turns on the bike and holds charge at 12.8 idle and 3-5k rpm it shoots up to 14.4. To me this looks good. I check my rotor and use the two leads on the copper slips and reads 4.5 ohms. So that's check off right?

Next, I check my stator, continuity seem good and readings are consistent using the connector. I check the brushes and they look good to with visible slit as people tend to look for that detail.

NOW the Regulator/Rectifier. So this took me a while to figure it out because I didn't know whether I was testing it right.  "0L" read for all my tests. I switched my leads and everything. I assume this is my problem because from looking at my tests it showed no readings.


What else should I check in the mean time while my R/R comes in?
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Re: 81 Cb750c Charging System Trouble Shooting

Truck
Sounds like the battery isn't holding the charge, how old is it?
It's only illegal if you get caught.

If at first you don't succeed, use more lighter fluid

95% of Harley Davidsons ever made are still on the road... The other 5% made it home.

New Baltimore, Va '82 CB900c, 1980 CB985F/K 'Mutt"
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Re: 81 Cb750c Charging System Trouble Shooting

shinyribs
Administrator
You're seeing 14.4v at the battery directly after a ride? If so, your bike is charging fantastically well. Your battery is just kaput and won't hold it. But just to make sure that it's not over-charging, read voltage at the battery terminals while revving the bike. Make sure it's not putting out more than 15v. If your battery is dead/has low voltage to begin with then the bike is gonna try extra hard to charge. It'll toss full charging power at it. If you see more than 15v while revving the bike you probably have a bad voltage regulator that is allowing it to over-charge, which will heat up and kill a battery. Good thing to check, but you probably just need a battery.

 Pretty bike, man. Tell me more about that good looking seat you got there.
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Re: 81 Cb750c Charging System Trouble Shooting

locust17
In reply to this post by Truck
The battery is maybe 5 months old but turned un often until this last few weeks. I went to go test it and it passed.
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Re: 81 Cb750c Charging System Trouble Shooting

locust17
In reply to this post by shinyribs
No I see 12.8 idling. But after ride and I turn it off it reads lower. like 11.5 and slowly drops to 9.5. The battery is 5 months old but turned on often these passed two weeks. It tested to be a good battery. It doesnt over charge. I ordered a 25$ R/R, *yeah i know cheap, expect what I pay for* but maybe it'll fix it. Im losing my mind.