Heya CB750 fam! I recently bought my first bike (a 1975 750F0 Supersport) and I'm loving it. It came with a few projects, and I found yet another one when my bike died about 30 minutes into a ride. I'd like your help on what I can do to get my bike running well again.
Tl;dr, I tested the stator coil, field coil, and battery and from what I can tell the only tests that failed were: voltage across the battery at high RPMs and the output voltage from the stator coil at high RPMs. This leads me to believe that the stator coil is at fault. I would love a second set of eyes on the steps I took before I possibly spend $$$ on replacing it.
This is the first bike I've ever tinkered on, and I'm relatively new to motorcycles in general. So I'm sure I'll get some terminology wrong, or overlook something obvious...
After doing a bunch of searching through this forum and sohc4.net I found a few guides that helped me narrow down my problem. I mainly used Mike Nixon's guide at the motorcycle project, augmented by a couple of other troubleshooting guides for charging systems slightly different than mine, including this one from Honda Chopper and the DOHC guide from cb750.com.
For starters, I'm working with a brushless three phase alternator, a combined regulator/rectifier, and a new battery.
I started off returning to my bike after a full night on the battery tender. I then took DC voltage measurements across the battery at various RPMs and expected the voltage to increase from ~12V at idle to 14-15V at 5k RPM. However, I only saw a constant 12.3V as I was cranking the throttle from idle through 5k RPMs:
In some of the guides, there was mention of a magnetism test. So I took a lightweight paperclip and dangled it by the alternator cover. It was attracted to the bolts fastening the field coil to the cover whether or not the electrical system was engaged (key turned off/on), or whether or not the engine was running. It was never attracted to the alternator cover (aside from the field coil bolts).
Stator Coil tests
On to some stator coil tests. The first two tests (resistance and voltage measurements) were taken with the stator coil disconnected from the main harness. The output voltage was the only failure here, both the resistances and insulation tests matched expectations.
I took the resistances between the three yellow alternator wires, expecting something like 0.2Ω. I got a consistent reading of 0.7-0.8Ω for each of the three pairings. This seems well within reason according to what I've been reading.
Next was measuring the AC voltage across the three yellow alternator wires. I was expecting all of them to be equal, and about 50V AC when at 5k RPM. I found all of them to be equal, but all with a very low voltage: 0.3V at idle and 1.5V at 5k RPM.
I wasn't able to find any continuity between the alternator wires and the cover, frame, or ground. So at least the insulation checks out.
Field Coil tests
The field coil tests were straightforward, and all agreed with the expectations for resistance and insulation.
The resistance between the white and green wires for the field coil was 7.8Ω, close enough to the expected 7.2Ω.
I wasn't able to find any continuity between either the green or white wires to the cover, frame, or ground, as expected.
Next steps?
So, is this data enough to confirm that the stator coil is at fault? Are there other tests I should run? Is there any way to diagnose the problem in the stator coil and repair/clean it?
Thanks for joining me down this rabbit hole, and happy riding
Tl;dr, I tested the stator coil, field coil, and battery and from what I can tell the only tests that failed were: voltage across the battery at high RPMs and the output voltage from the stator coil at high RPMs. This leads me to believe that the stator coil is at fault. I would love a second set of eyes on the steps I took before I possibly spend $$$ on replacing it.
This is the first bike I've ever tinkered on, and I'm relatively new to motorcycles in general. So I'm sure I'll get some terminology wrong, or overlook something obvious...
After doing a bunch of searching through this forum and sohc4.net I found a few guides that helped me narrow down my problem. I mainly used Mike Nixon's guide at the motorcycle project, augmented by a couple of other troubleshooting guides for charging systems slightly different than mine, including this one from Honda Chopper and the DOHC guide from cb750.com.
For starters, I'm working with a brushless three phase alternator, a combined regulator/rectifier, and a new battery.
I started off returning to my bike after a full night on the battery tender. I then took DC voltage measurements across the battery at various RPMs and expected the voltage to increase from ~12V at idle to 14-15V at 5k RPM. However, I only saw a constant 12.3V as I was cranking the throttle from idle through 5k RPMs:
- off - 13.3V
- idle - 12.3V
- 2.5k RPM - 12.3V
- 5k RPM - 12.3V
In some of the guides, there was mention of a magnetism test. So I took a lightweight paperclip and dangled it by the alternator cover. It was attracted to the bolts fastening the field coil to the cover whether or not the electrical system was engaged (key turned off/on), or whether or not the engine was running. It was never attracted to the alternator cover (aside from the field coil bolts).
Stator Coil tests
On to some stator coil tests. The first two tests (resistance and voltage measurements) were taken with the stator coil disconnected from the main harness. The output voltage was the only failure here, both the resistances and insulation tests matched expectations.
I took the resistances between the three yellow alternator wires, expecting something like 0.2Ω. I got a consistent reading of 0.7-0.8Ω for each of the three pairings. This seems well within reason according to what I've been reading.
Next was measuring the AC voltage across the three yellow alternator wires. I was expecting all of them to be equal, and about 50V AC when at 5k RPM. I found all of them to be equal, but all with a very low voltage: 0.3V at idle and 1.5V at 5k RPM.
I wasn't able to find any continuity between the alternator wires and the cover, frame, or ground. So at least the insulation checks out.
Field Coil tests
The field coil tests were straightforward, and all agreed with the expectations for resistance and insulation.
The resistance between the white and green wires for the field coil was 7.8Ω, close enough to the expected 7.2Ω.
I wasn't able to find any continuity between either the green or white wires to the cover, frame, or ground, as expected.
Next steps?
So, is this data enough to confirm that the stator coil is at fault? Are there other tests I should run? Is there any way to diagnose the problem in the stator coil and repair/clean it?
Thanks for joining me down this rabbit hole, and happy riding