'80 CB750 Charging Issues

GroupBEnthusiast

New Member
Hello all,

It's been about a year since I originally posted a thread regarding my charging issues with my 750 Supersport. Long story short, I thought I had fixed the issue with a new charging system, rode it for another ~2 months before finally putting the bike away in early December. The bike started without hesitation the 2-3 times I went out and started it over the winter, though I finally found myself with a dead battery when I went to pull it out for a ride in late January (Cue new battery #2. Didn't have time to charge #1 before leaving). Rode it on and off (~400 miles total) until it finally lost battery power again on May 1st while on my way to work (got to the point of only having enough voltage to power one coil/2 cylinders). Threw another battery in it later that day and rode it home (new battery#3). :mad:

2 months later, the bike is still sitting. I thought it might be a bad connection between my new stator and regulator, so I fixed the pin that appeared to be not seating correctly, and put about ~50 miles on it with a freshly charged battery. Went out the following night and it was, again, dead. Now, I can only assume I have a massive current draw somewhere that's being affected by ambient temperature. A few days ago I read something about a bad solenoid switch/relay sending excess voltage to the spark units, heating them up, which may cause some of the black epoxy leaking out onto the battery (didn't think anything of it before).

So... Now my question is, before I spend the time and money to replace literally every electronic component and wiring harness in the bike, is it feasible that replacing the solenoid switch, solenoid relay, and spark units might fix the issue? The bike seems to run perfectly fine aside from this one (major) issue.

Thanks in advance!

What I've replaced so far: Stator, rotor, regulator, battery (AGM, x3), accessory fuses, main fuse.
 
Before throwing any more parts at this thing, I recommend taking voltage readings while the bike is running. Check at idle, 2500 RPM, and 5000 RPM and report back.
 
Thanks for the speedy reply, Sonreir.

I just went and re-checked the charging system, per https://www.denniskirk.com/learn/how-to-diagnose-motorcycle

My measurements are as follows:

Battery (fresh off the charger): 12.89V

Battery connected, key off: 12.89V

Battery connected, key on: 12.89V

Engine running (cold): 12.18V

2500RPM (cold): 12.18V

5000RPM (cold): 12.18V

*took the bike for a 10 minute warm-up ride*

Engine running (warm): 11.89V

2500RPM (warm): 11.89V

5000RPM (warm): 11.89V

Resistance across stator leads: 1ohm

Stator output across all leads: 0.5V (AC)

All other ground and resistance readings were within spec, according to the linked guide. Obviously I'm not getting any output from the system.
Hope that's enough to work with!
 
I agree; no output from the stator.

Now to find out why. :)

Coming from the R/R and headed to the alternator will be a black and a white wire. With everything turned on, but the engine not running, please measure DC voltage between the black wire and ground and then the white wire and ground. Next, measure resistance between the black wire and ground and then the white wire and ground.

All of the above measurements should be done with all of the plugs connected.
 
OK... one more check, please.

With the alternator plug disconnected, measure resistance across the white and black wires on the alternator side.
 
GroupBEnthusiast said:
No resistance reading across the black and white stator wires.

OK. That's what I thought.

That's an indication that there's something wrong with the field coil assembly.

Specifically, it's likely to be one of the following:
1.) Bad field coil itself
2.) Bad brushes
3.) Wiring to/from either of the first two

You can narrow the problem down by reading resistance between the two copper rails on the field coil. Image attached.

A good resistance reading means the problem is #2 or #3 on the list.
 

Attachments

  • 10141.jpg
    10141.jpg
    15.9 KB · Views: 427
Thanks a lot! Re-checking the rotor was next on my list. The brushes were replaced with the rotor, though I wasn't the one who physically installed them...

Edit: After pulling the stator cover and re-checking, I’m getting 0.3ohm when testing from the brushes to the black/white wires at the connector, and nothing across the rotor slip rings. Guess I’ll be ordering another, better-quality one. I’ll update the thread when I get the new one installed and tested.
 
Well, bad news... I installed the new rotor last thursday. Everything looked good, and the system measurements were perfectly within spec for the first two days. Took it out again on Sunday, and it died on the return trip. I was out of town this week, so I didn’t have a chance to dig into it until today. The brand new rotor (~95 miles) is partially shorted and reading 2.9ohm. When I installed it, it was at exactly 4.4ohm.

Does my bike just prefer the rotors a bit toasted? ???
 
Back
Top Bottom