• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Charging System Failure

ssrhinehart

CB750 New member
Messages
4
Reaction score
0
Points
1
Location
Saraland, AL
I have a 1978 CB750K. I recently did a complete engine rebuild. 3 days after the rebuild was complete, the battery went completely dead. I tried to charge the battery and it would not charge, failed. I cranked the bike and checked the voltage from the stator and it had no AC voltage on any of the wires. I pulled the stator and field coil and benched tested with a ohm meter. Results were: Stator 1.0 ohms on lead 1, .9 ohms on lead 2, .9 ohms on lead 3. That to me is fine (I think??) Field Coil test: 7.9 ohms. That is fine also.

Here are my questions. 1) Why do I not have AC voltage? 2) What roll is my rotor playing in this charging system as it had no brushes to create any type of magnetic field. Where is it getting its magnetic field? Could this be why I have no AC voltage. Any solutions? HELP!!!
 
The rotor there does nothing but provide the iron for magnetism. The field coil is what magnetizes it. You must have full battery power to power the field up or no magnet there to make power. Battery if it does not charge back up could be the only problem there. Like all alternators, it takes some power to make more power.

Checking for proper ohm through your parts is only half testing, you have to check for shorts to ground too. Does no good say for lead 1 to be one ohm if it is shorted to ground too.
 
Until you get a known good battery in there you are pissing in the wind. It MUST pass a loadtest at proper CCA.

I use a simple voltmeter for all my work there. The problems can be different, I fix all my own alternators whether on car or other. Never worked on SOHC alt but I know what to look for. Worked on DOHC alts plenty, the rotors are the common fails there followed by regulators that the rotors fry when people keep insisting on driving bike with bad rotor. SOHC can't fail rotor though other than maybe it just coming apart as there are no electrics in one.
 
Back
Top