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Motorcycle Repair/1981 cb 900 charging problem


I have a 1981 CB 900 its been having problems charging. The rotor that was in the bike tested bad and I replaced it. I also replace the regulator rectifier, after I replaced the rotor. I drove the bike for about a week and everything seems to be working correctly. the rotor I bought was an o.e.m. remanufactured part that was suppose to correct the problem in the rotor that make it go out. Unlike my old rotor the new one had a material in the gaps on the rotor where you can normally see the windings. Well that material looks like it got very hot and started  to crumble apart. when I test it im gettin OL on my multimeter set on ohms 0-20. I used a diffrent multimeter and it just keeps gaining until it maxes out. Im very confused was wondering if you had any suggestions?


Hi Troy

The charging system on these is a bit different in that it
uses brushes on the rotor slip rings. The rotor is basically just
a big electromagnet or a coil of wire surrounded by alternating
north and south metal poles. The original rotor had 3.6 to 4.4 ohms reading
between the two copper slip rings that surround the outer
face of the rotor.

If you get either an open (broken wires) or no ohms resistance
readings it might be the rotor is shorted inside.  This means that the wires
could lose their insulation and short to one another causing
a shorter path of no or little resistance in the coil of wire.

The other thing is the brush contact and condition.

The stator black to white wire is 10-12 ohms.
Yellow to yellow is  0.4 to 0.5 ohms.
Yellow to ground, no connection, infinite resistance.

When everything is connected as usual for running
and the ignition key is turned on you should have battery
voltage on the regulators red/white wire to ground.

The stators black and white wires should also read battery
voltage between them (keep them connected when testing.

Disconnect the stators six pole connector and check
the voltage output between any two yellow wires.
They should read about 10 AC volts.

Total output to a good battery should read 14-15 volts DC
at 3000 rpm. These only put out about 1.4 amps so
they do not charge very much until the engine is speeded up.

All the connectors should be deoxidized with a good electronics
cleaner. Connectors are often the cause of current problems.

If you cannot get readings close to what I listed then
you may have to determine if the rotor or stator
or brushes have a fault.

Once normal  output is gained from the stator then the next culprit
is likely the rectifier/regulator.

Don't expect a large output from these charging systems,
they are very minimal at best and work best above 2000 rpm.
They may even lose battery power at idle.

Hope this helps a bit, they can be frustrating to fix if the new parts act up too.

Wayne S

Motorcycle Repair

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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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