1982 Honda CB 900f low charge

VintageMBike

Been Around the Block
Picked up an immaculate 1982 Honda CB 900F with 8k original miles. The seller couldnt figure out why the battery kept dying. He changed the Stator, Rotor and Reg/Rec, still low charge, like 12.5 at 3k RPM.

Any ideas on what it could be? I enjoy riding, but with a wife and a few kids (one of them 3 months old) there's no way I can devote a lot of garage time to this. I had a quick look at the connections and all seemed well. If it comes to it, I will have to hand this bike off to my mechanic to fix, hes is kinda pricey but, I want to get this bike running right and have a decent budget (over $400) to devote to getting it hashed out.

Ive attached a pic of the bike.
 

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Wow what a beauty. I had a similar issue on my bike. Turned out it was the old wiring going from the r/r to that battery what was eating up the voltage.

My bike was charging at 14 volts at idle and 13.2 volts at 5000 which is below spec.

I went through and checked everything and it seemed to all check out. I cleaned all the connections, replaced the regulator, etc. Nothing seemed to fix the issue. So i ran new wires from the regulator to the battery. 12 gauge wire with a 25 amp fuse on the positive line. Boom, a steady 14.5.

Maybe that's all that's wrong with yours too. It sounds like everything else has already been replaced. Good luck.
 
B400 said:
Wow what a beauty. I had a similar issue on my bike. Turned out it was the old wiring going from the r/r to that battery what was eating up the voltage.

My bike was charging at 14 volts at idle and 13.2 volts at 5000 which is below spec.

I went through and checked everything and it seemed to all check out. I cleaned all the connections, replaced the regulator, etc. Nothing seemed to fix the issue. So i ran new wires from the regulator to the battery. 12 gauge wire with a 25 amp fuse on the positive line. Boom, a steady 14.5.

Maybe that's all that's wrong with yours too. It sounds like everything else has already been replaced. Good luck.

Thanks for taking the time out to respond. I dont take it lightly that youre trying to help out. Im really hoping its something as simple as that, a wire from the regulator to the battery!


I will let you know how it turns out!

Thanks again.

Really excited to have this bike. Its SOOOOOOO fast!
 
B400 said:
Wow what a beauty. I had a similar issue on my bike. Turned out it was the old wiring going from the r/r to that battery what was eating up the voltage.

My bike was charging at 14 volts at idle and 13.2 volts at 5000 which is below spec.

I went through and checked everything and it seemed to all check out. I cleaned all the connections, replaced the regulator, etc. Nothing seemed to fix the issue. So i ran new wires from the regulator to the battery. 12 gauge wire with a 25 amp fuse on the positive line. Boom, a steady 14.5.

Maybe that's all that's wrong with yours too. It sounds like everything else has already been replaced. Good luck.

Checked the gauge, etc on the positive line. Its 12 gauge and seems to be ok. Guess its something else. Here are more pics of the bike. Can barely wait to get it running:
 

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Check all the connections, sometimes corrosion can build up causing poor contact.

If the frame has been painted pay close attention to the earth connections as you may need to sand the frame to get good contact at the mount point.

Contact Sonrier of Sparck moto if you get stuck. He is the resident electrical god and will be able to help you sort it out.
 
Brodie said:
Check all the connections, sometimes corrosion can build up causing poor contact.

If the frame has been painted pay close attention to the earth connections as you may need to sand the frame to get good contact at the mount point.

Contact Sonrier of Sparck moto if you get stuck. He is the resident electrical god and will be able to help you sort it out.

Thanks for responding. I will PM him. When its resolved, I will let the forum know how it turned out, in case others need this info.
 
Been a busy weekend and I haven't been on the forums at all.

Pretty much what everyone else has said would be the way I'd proceed.

Grab a multimeter and check resistance on the field coil and go through the stator checks. Verify low resistance on all the associated wiring (yellow from stator to R/R, black and white from field coil to R/R, and red from R/R to battery/solenoid). We're looking for under one Ohm for each run.

Brushes are a common failure point for the DOHCs, but a resistance check between the white and black wires on the field coil should pick this up.

Another easy way to eliminate the field coil as the source of the problem is to turn the bike on (keep kill switch set to off) and remove the alternator cover. A 10mm wrench should stick to the alternator because of the magnetism. If it does, you have a stator, wiring, and/or battery issue. If it does not, you definitely need some work on the field coil (or wiring or brushes).
 
Sonreir said:
Been a busy weekend and I haven't been on the forums at all.

Pretty much what everyone else has said would be the way I'd proceed.

Grab a multimeter and check resistance on the field coil and go through the stator checks. Verify low resistance on all the associated wiring (yellow from stator to R/R, black and white from field coil to R/R, and red from R/R to battery/solenoid). We're looking for under one Ohm for each run.

Brushes are a common failure point for the DOHCs, but a resistance check between the white and black wires on the field coil should pick this up.

Another easy way to eliminate the field coil as the source of the problem is to turn the bike on (keep kill switch set to off) and remove the alternator cover. A 10mm wrench should stick to the alternator because of the magnetism. If it does, you have a stator, wiring, and/or battery issue. If it does not, you definitely need some work on the field coil (or wiring or brushes).
''

The battery is brand new. I took out the PO battery that was run dry.

By field coil do you mean ignition coil? The terminology is confusing to me. Main component parts are listed as Rotor, Stator, Battery, Alternator/generator. Where does the field coil come into this?When I google field coil all that comes up is the ignition coil. Forgive my ignorance. Im a finance guy who doesnt mind getting his hands dirty, but my 3 kids and the Mrs. have scrambled my brain.
 
On this bike, the rotor is made up of a couple of different parts. Inside the rotor is another winding of copper called the field coil. This is what the black and white wires connect to, when they pass through the alternator assembly. The field coil gets power from the regulator/rectifier and the way the R/R ensures you have good voltage is by varying the resistance on the white wire. When resistance is lower, more power passes into the field coil, resulting in stronger magnetism. Increased magnetism or increased revs means more electricity. As revs increase the R/R will increase resistance so that too much electricity is not pushed into the system.
 
Sonreir said:
On this bike, the rotor is made up of a couple of different parts. Inside the rotor is another winding of copper called the field coil. This is what the black and white wires connect to, when they pass through the alternator assembly. The field coil gets power from the regulator/rectifier and the way the R/R ensures you have good voltage is by varying the resistance on the white wire. When resistance is lower, more power passes into the field coil, resulting in stronger magnetism. Increased magnetism or increased revs means more electricity. As revs increase the R/R will increase resistance so that too much electricity is not pushed into the system.

Right

My question is, is this part sold outside of the rotor of will i have to buy a new rotor? for what its worth, the rotor in there is brand new. put in less than 3 months ago.
 
The field coil and rotor are one piece, but there's really no reason why they should be. If I were to make them, the field coil could be removed and repaired, But I digress...

Don't buy any new parts until you figure out where the issue is. Throwing new parts at the problem might work, but better to actually know what's wrong.
 
Sonreir said:
The field coil and rotor are one piece, but there's really no reason why they should be. If I were to make them, the field coil could be removed and repaired, But I digress...

Don't buy any new parts until you figure out where the issue is. Throwing new parts at the problem might work, but better to actually know what's wrong.

youre right. the only thing i did do was buy the brushes. I'll probably need them as some point and $20 for a pair is cheap. Im hoping its a ground or something. Could that be a possibility?
 
Possible, but unlikely. There's only one ground in the DOHC charging system and it's fairly low current. Even a shoddy ground is likely to be OK.

Attached is a picture of the DOHC charging system. As you can see, almost all of the wires are running between the alternator and the R/R, so that's likely where the problem exists (either with the components or the wiring). The field coil gets power from the black wire (it's a pass-through from the other black wire going to the main harness) and then regulates power by varying resistance on the white return wire. Electricity generator by the alternator comes back through the R/R on the three yellow wires. They're rectified (changed from AC power to DC power) and then it exits the charging system out through the red wire to the solenoid, where it's connected to main power for the rest of the bike. The remaining green wire from the R/R is present only for the voltage sensor (which is also run from the black wire coming into the unit). Unlike permanent magnet regulator/rectifiers, the ground wire isn't a major concern because so little power passes through it.

Anyway... here is how I would proceed with testing if the bike were in my shop:

1.) Unplug the regulator/rectifier and remove the alternator cover. Measure resistance between the white and black wire from the rotor. We're looking for about 3.5 to 5.0 Ohms. Lower or higher is both cause for concern and the further we drift the more concern it should cause. If we have a good reading, then measure between the black wire and ground. We should not be able to get any reading at all (aka Open Line). If the resistance checks are OK, run a jumper from the wires to the battery. Black to positive and white to negative. Make sure a wrench will stick to the rotor. If we've failed at this step, we have a problem with the rotor, the brushes, or the wiring. Use a multimeter to read resistance between key points in the circuit to identify the point of failure.

2.) Replace the alternator cover and repeat the resistance tests (including test to ground) from step one. They should remain the same.

3.) Measure resistance between each of the three yellow wires and then from one of the yellow wires and ground. We should get about .5 Ohms on each pair of yellow wires (three separate measurements) and Open Line when measuring from a yellow wire to ground. If we are unable to get a low enough resistance reading, then we might have corroded terminals or bad wires. An Open Line reading on any of the yellow-to-yellow measurements is a failure in the wiring or the stator. Open Line between any yellow and ground is a failure of the stator as well.

4.) Moving over to the other R/R plug (red, green, and black), measure resistance from the red wire to the battery positive terminal. We should have under an Ohm and lower is better. Turn the ignition key to the on position and measure between the black wire and the battery positive terminal. Also looking for under an Ohm on this one. Turn the key back off and measure between the green wire and the battery negative terminal. Also looking for under an Ohm on this measurement. Any failures here are probably an issue with wiring or terminals. This would be fairly easy to correct and we can run through the solution if this is where the problem lies.

5.) Finally, testing of the R/R. If your meter has the ability to do so, perform a diode check on each yellow wire and the red wire on the unit. Also do a resistance check between both black wires. Any other, more complicated, tests will require an adjustable power supply. If you have one, please let me know and I can cover those tests as well.

6.) Assuming all these tests pan out, your system should be charging. Any failure to achieve proper system voltage might still be related to the R/R, but it possibly a short within the system. At this point, it's usually easy to just replace the wiring (at least for me) than it would be to track down the possible short, but I can run you through that process as well, assuming we haven't already found the issue by now.
 

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Sonreir said:
Possible, but unlikely. There's only one ground in the DOHC charging system and it's fairly low current. Even a shoddy ground is likely to be OK.

Attached is a picture of the DOHC charging system. As you can see, almost all of the wires are running between the alternator and the R/R, so that's likely where the problem exists (either with the components or the wiring). The field coil gets power from the black wire (it's a pass-through from the other black wire going to the main harness) and then regulates power by varying resistance on the white return wire. Electricity generator by the alternator comes back through the R/R on the three yellow wires. They're rectified (changed from AC power to DC power) and then it exits the charging system out through the red wire to the solenoid, where it's connected to main power for the rest of the bike. The remaining green wire from the R/R is present only for the voltage sensor (which is also run from the black wire coming into the unit). Unlike permanent magnet regulator/rectifiers, the ground wire isn't a major concern because so little power passes through it.

Anyway... here is how I would proceed with testing if the bike were in my shop:

1.) Unplug the regulator/rectifier and remove the alternator cover. Measure resistance between the white and black wire from the rotor. We're looking for about 3.5 to 5.0 Ohms. Lower or higher is both cause for concern and the further we drift the more concern it should cause. If we have a good reading, then measure between the black wire and ground. We should not be able to get any reading at all (aka Open Line). If the resistance checks are OK, run a jumper from the wires to the battery. Black to positive and white to negative. Make sure a wrench will stick to the rotor. If we've failed at this step, we have a problem with the rotor, the brushes, or the wiring. Use a multimeter to read resistance between key points in the circuit to identify the point of failure.

2.) Replace the alternator cover and repeat the resistance tests (including test to ground) from step one. They should remain the same.

3.) Measure resistance between each of the three yellow wires and then from one of the yellow wires and ground. We should get about .5 Ohms on each pair of yellow wires (three separate measurements) and Open Line when measuring from a yellow wire to ground. If we are unable to get a low enough resistance reading, then we might have corroded terminals or bad wires. An Open Line reading on any of the yellow-to-yellow measurements is a failure in the wiring or the stator. Open Line between any yellow and ground is a failure of the stator as well.

4.) Moving over to the other R/R plug (red, green, and black), measure resistance from the red wire to the battery positive terminal. We should have under an Ohm and lower is better. Turn the ignition key to the on position and measure between the black wire and the battery positive terminal. Also looking for under an Ohm on this one. Turn the key back off and measure between the green wire and the battery negative terminal. Also looking for under an Ohm on this measurement. Any failures here are probably an issue with wiring or terminals. This would be fairly easy to correct and we can run through the solution if this is where the problem lies.

5.) Finally, testing of the R/R. If your meter has the ability to do so, perform a diode check on each yellow wire and the red wire on the unit. Also do a resistance check between both black wires. Any other, more complicated, tests will require an adjustable power supply. If you have one, please let me know and I can cover those tests as well.

6.) Assuming all these tests pan out, your system should be charging. Any failure to achieve proper system voltage might still be related to the R/R, but it possibly a short within the system. At this point, it's usually easy to just replace the wiring (at least for me) than it would be to track down the possible short, but I can run you through that process as well, assuming we haven't already found the issue by now.

My God. I feel like Im talking to HondaYoda! Thanks so much for this!!
 
Sonreir said:
On this bike, the rotor is made up of a couple of different parts. Inside the rotor is another winding of copper called the field coil. This is what the black and white wires connect to, when they pass through the alternator assembly. The field coil gets power from the regulator/rectifier and the way the R/R ensures you have good voltage is by varying the resistance on the white wire. When resistance is lower, more power passes into the field coil, resulting in stronger magnetism. Increased magnetism or increased revs means more electricity. As revs increase the R/R will increase resistance so that too much electricity is not pushed into the system.

Im going to upgrade to this system.
 

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That's not a Lucas 3 phase alternator is it? Looks just like the one I fitted to one of my Triumph twins back in time.
 
teazer said:
That's not a Lucas 3 phase alternator is it? Looks just like the one I fitted to one of my Triumph twins back in time.

Cycle X made it. Not sure of the specifics but man oh man i cant wait to install this stuff. Comes next week.
 
cxman said:
should have just put a rotor in it

that what the last two owners did and it didnt solve the problem. I chose this system because i'll have more amperage at low idle, a lighter rotor (stock is 4lbs, the new one is 1.7 lbs) no alternator brushes etc. Its like getting a new lease on life.
 
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