Topic: 81' cb650 only charging when cold

[bochnak]

Hi Everyone.

I have an 81’ cb650 that will charge when cold (14.5V) and won’t once warm (12.5V).
Here is what I have done to try and diagnose the problem.

Rotor/Stator
The rotor winding plastic insulation material was starting to deteriorate, and plastic was everywhere inside the cover. Resistance between slip rings was 5ohms cold and slightly higher once warm at 6ohms. Never did I get a reading that it was shorted out. I replaced the rotor anyway since the insulation was damaged. Reading are similar on new rotor.
Stator checks out fine, 0.5ohms between each leg and no continuity to ground.
With 12V supplied to brushes from battery, I measured 50vac output from each yellow lead from stator at 5k rpm with no load on the system.

Reg/Rec
I had a known good RR laying around and swapped it in with no change.

Battery is new and I have cleaned all connections at solenoid…ect.

What else should I be looking for? TIA.

[bochnak]

I forgot to mention that I see about a 2V drop when I measure voltage at BLK & GRN wires @ RR. The BLK wire is the 12V+ from ignition switch.

I might try jumping this wire to batt and see if that helps.

[Mainerider]

It sounds as though you have heat-induced high resistance or even a heat-induced open circuit somewhere in the charging system; the problem doesn't appear until the components have warmed up and the resistance rises. I would double check all of your grounds and connections, especially those around the voltage rectifier/regulator.

Also, was the bike warmed up fully when you drew 50 AC volts per stator lead?  If memory serves, your bike should generate approximately 260 watts at 5,000 rpm, so you should be seeing roughly 65 volts per stator lead on your bike; that puts you roughly 30 percent down from spec.

[bochnak]

The 50vac measurement was taken @ 5k, when warm, and "not charging". I further tested the alternator with a 1ohm load @5k, warm, "not charging", yielding 15vac on all the legs.

I jumped batt voltage to ignition wire to increase voltage with no luck.

[bochnak]

If memory serves, your bike should generate approximately 260 watts at 5,000 rpm, so you should be seeing roughly 65 volts per stator lead on your bike; that puts you roughly 30 percent down from spec.

The manuals I found online did not include the "electrical" section that would contain the info you mentioned. Care to share where you found that info?

Thanks,
Matt

[Mainerider]

I have a hard copy of the 1979-1980 factory service manual, which also has the 1981 supplement (several changes in '81). I still think you have heat-related high resistance there somewhere; have you carefully checked all of your connections into and out of the r/r? I would start with a voltage-drop test from the r/r to batt cold and then when the bike is hot, work backwards until you find the source of high resistance.

By the way, all of your other electrical switches, lights, etc work okay? Any aftermarket electrical accessories on the bike? Have you checked the voltage when warmed up while pulling your headlight fuse? I ask these questions because perhaps your issue lies on the demand side and not on the supply side.

[bochnak]

I have a hard copy of the 1979-1980 factory service manual, which also has the 1981 supplement (several changes in '81). I still think you have heat-related high resistance there somewhere; have you carefully checked all of your connections into and out of the r/r? I would start with a voltage-drop test from the r/r to batt cold and then when the bike is hot, work backwards until you find the source of high resistance.

By the way, all of your other electrical switches, lights, etc work okay? Any aftermarket electrical accessories on the bike? Have you checked the voltage when warmed up while pulling your headlight fuse? I ask these questions because perhaps your issue lies on the demand side and not on the supply side.

I checked and cleaned all connections at solenoid and RR. The solenoid was corroded. I'll try the voltage drop test you mentioned.

All other electrical components work fine. Headlamp is STD wattage. No aftermarket electrical acc.

With the bike not running and ignition on, current draw is 11A, which seems pretty high to me.

[Mainerider]

That is high; you likely have a parasitic draw; essentially something staying on when it shouldn't or something drawing more current than it should. I think your charging system is probably ok, its a demand-side issue.  Turn on the ignition, then pull one fuse at a time and see if you can find which circuit is the culprit by checking for changes in your current draw testing. You can also pull the fuses one at a time  with the bike warmed up and running and see if charging voltage jumps up to acceptable levels.

 It's hard to diagnose these things remotely but this may help you start to poinpoint the source of your trouble. Good luck.

[bochnak]

Mainerider,

I think you are on to something.

There is no parasitic draw when bike is off. I'll start pulling some fuses and see what happens.

What is an acceptable amp draw when not running? It has to be at least 8A / 100W?

[Mainerider]

Not sure for your bike. With the ignition off  I would take a test light (if you don't have a series ammeter)  and disconnect the pos  batt cable and see if the tester lights when bridging the terminal terminal and cable. If yes, then you have at least 4 amps drawing. More than 4 amps is a definite problem. Less than 4 is still a prob but it won't light your tester. You will need to connect a series ammeter to the batt post and cable. If the draw is up to an amp or so, suspect a light is drawing it down. If it is closer to 4 amps or more it could be a screwy diode in your alternator or rectifier. Also poss a stuck-on relay.
While watching your tester, disconnect the wires to your alt and RR; if the draw drops you've found your issue. If it doesn't solve it do the same test by pulling fuses one by one.

[bochnak]

Think I found the culprit!

The rear brake light switch was not adjusted properly and brake light was on. This wattage of a 1157/2057 brake light can be 21-27W or 1.75-2.25A.

I started pulling fuses one by one and it charged better and better as expected. For whatever reason, I squeezed the front brake and noticed the light did not come on....it was already on!

The battery was not fully charged before I started, but here are the numbers:

With brake light on, voltage steadily drops to 12.4V @ 3k rpm. With brake light off, the voltage will climb to 13v @ 3k rpm. I expect this the be better with a fully charged battery. Idle voltage is 11.8V. Reaches 13.5-14V @5k rpm.

So am I in the clear?

[Mainerider]

Yes you should be all set. Spec is approx 14.5 volts @ 5K so you are right there.

[bochnak]

I know I'll get to that value with a fully charged battery.

Thanks for your help Mainerider! 

[Mainerider]

No problem, and good job tracking it down logically and not just replacing parts and hoping.

[Duke McDukiedook]

Keep an eye on that stator because those bikes are known to have stator problems that were inherent to that design.

[Mainerider]

Actually, the rotor is more often the issue.

[bochnak]

Actually, the rotor is more often the issue.

x2....from what I have read. The rotor is subject to heat and centrifugal force...not a good combo.

[bochnak]

I know I'll get to that value with a fully charged battery.

Thanks for your help Mainerider! 

Update:

With a fully charged batt, I'm getting 15V @ 5k RPM.