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Eric
Eric, Technician
Category: Motorcycle
Satisfied Customers: 1506
Experience:  MMI grad. 7 years experience in Honda, Suzuki, Yamaha, as service manager and tech.
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I HAVE A 81 HONDA CB 750. HAVE A CHARGING PROBLEM. BIKE WONT

Customer Question

I HAVE A 81 HONDA CB 750. HAVE A CHARGING PROBLEM. BIKE WON'T HOLD 13/14 VOLTS. TESTED ALTERNATOR SEEMS TO BE WORKING RIGHT. I THINK THE VOLTAGE/RECTIFIER IS NOT KEEPING CHARGE. IM NOT SURE OF MY TEST RESULTS. I KNOW OVER CHARGING IS MORE COMMON WITH VOLTAGE/REC. BUT WHAT ABOUT UNDERCHARGING. HIGH BEAM ON. I ONLY HAVE 17,000 miles. I HAVE HAD THIS BIKE 25 YEARS. VERY LITTLE PROBLEMS, UNTIL NOW.
Submitted: 4 years ago.
Category: Motorcycle
Expert:  Eric replied 4 years ago.

hello

is this a good tested battery? how was the alternator tested?

Customer: replied 4 years ago.
I tried two different battries, and tested altenator with ohm meter per directions in my manual,and appeared to tet good per manual. Wasn't coffident with my ohm testing on voltage-rectifier per manual
Expert:  Eric replied 4 years ago.

i dont know how well the manual explains out how this all works. but on your particular bike this has an electromagnetic alternator system. this means there is the stator coil, this coil puts out ac voltage to the rectifier/regulator to break down into a charging dc voltage. this is regulated by a field coil. the field coil is what creates magnetism for the stator to get its energy from. the regulator regulates how much magnetism is created by controlling the ground side of the field coil. thus varying the voltage as it needs to. so to be sure if this is all working, you can do what is known as a full fielding test. this will tell you if the stator is putting out adequate energy for the rectifier to break down to charging voltage. a good stator output voltage is anything above 30 volts AC. on that connector to the alternator, there should be the three yellow stator wires, and two other wires. those to other wires are to the field coil. so keep that unplugged and do the following. hook up you meter to read AC voltage, and connect it to any two of those three yellow wires. then hook up jumper wires from those other two wires to a comparable motorcycle battery source. the one on the bike can work just as well. this send all battery power to that field coil, so this stator should but out some good voltage. so start the bike and observe the voltage output of the stator. while giving some throttle, it should go well over that 30 volts AC mark. if not then the problem is with the field coil, its brushes, or the stator. and that is when you need to rely on the resistance checks. but if this does put out enough voltage the regulator/rectifier is not doing its job, so it would be at fault.

Eric, Technician
Category: Motorcycle
Satisfied Customers: 1506
Experience: MMI grad. 7 years experience in Honda, Suzuki, Yamaha, as service manager and tech.
Eric and 2 other Motorcycle Specialists are ready to help you
Customer: replied 4 years ago.
thank you for the infor. i did not check voltage output before checking resistance. did a visual inspect of brushes,coil,stator,brushes looked to still be good. i will check voltage as you explained. don't know when i'll get to it, as i have busy work load, and a boat project i'm working on also. thanks again. howie d.
Expert:  Eric replied 4 years ago.
Thank you for the question, just let me Know if anything comes up that doesn't seem right. Thanks

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Eric
Eric
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MMI grad. 7 years experience in Honda, Suzuki, Yamaha, as service manager and tech.