I Hate Charging Problems!

[Justin B.]

Have had the '81 R100RT mothballed for a while due to a charging issue.  Multiple "spare" bikes makes it not that big of a deal and I kept putting off troubleshooting but decided it was time to figure it out today.

The symptome was no rise in voltage when revved and the charge light stayed on all the time.  I put a jumper between the DF and D leads going to the regulator and voltage remained flat.  Great, just what I wanted to do, tear into the front of an Airhead with full fairing...

I pulled out the diode board and all diodes checked good so ohmed out the rotor and found it had about 56 ohms resistance!  A bit more than it should but I had never had one go bad that didn't just open up and cause charge light to go out.  I had also checked this last since I had put in a new one (EME replacement)  about 15 years ago and probably only have 20k miles on it.  Oh well, ordered another one from EME along with a new brush hopder/brush assembly so hopefully in a few days it'll be back on the road.

Has anybody else had a rotor go bad that didn't either read a dead short or open?

[georgesgiralt]

Hello Justin,
You bike has been mothballed for a long time. So corrosion has taken it's toll.
I would try to clean the brush area with smooth sandpaper and check that the brushes are making good contact.
Also, check that the 3 stator winding give around the same voltage (alternate current here) by checking the wires two at a time.
You should get high voltage if the system is running fine : maybe 25 to 30 V~
Of course, if the rotor is dead you won't get any voltage at the stator...
Hope this helps.

[BPT]

When I had the charging problems right after I bought my bike all of the simple testing I could do came out fine.

I told a guy at the local shop that the rotor tested OK and he told me that they could sometimes do that sitting still but could have a break in a connection/wire that could open up once it got to spinning and heating up.

He wasn't trying to sell me a new one so I don't think he was BS'ing me.

[Bob_Roller]

Swap rotors with the junkyard dawg LS .

[mrclubike]

I suspect there is nothing wrong with the rotor
If the rotor was open  the light would not come unless it is
shorted in the winding or shorted to ground
 
Did you check the rotor for shorts to ground
If you ohm from either slip ring to the claws you should get infinite With the brushes NOT touching the slip rings

The brush that has the BROWN wire connected to it is Grounded
Make sure the Brush with the Black wire connected to it is NOT grounded
 
More than likely you have not checked all of the diodes in the diode board or have not done it    correctly
Did you check the 3 smaller diodes that feed power to  the rotor
Their are a total of eleven diodes in the diode board did you get them all

If you are sure the diode board is good then make sure the large red wire that feeds the charging voltage to the battery cable on the starter is good    

[mrclubike]

When I had the charging problems right after I bought my bike all of the simple testing I could do came out fine.

I told a guy at the local shop that the rotor tested OK and he told me that they could sometimes do that sitting still but could have a break in a connection/wire that could open up once it got to spinning and heating up.

He wasn't trying to sell me a new one so I don't think he was BS'ing me.

If this was the case the GEN light would go out as soon as it opened up

[mrclubike]

Hello Justin,
You bike has been mothballed for a long time. So corrosion has taken it's toll.
I would try to clean the brush area with smooth sandpaper and check that the brushes are making good contact.
Also, check that the 3 stator winding give around the same voltage (alternate current here) by checking the wires two at a time.
You should get high voltage if the system is running fine : maybe 25 to 30 V~
Of course, if the rotor is dead you won't get any voltage at the stator...
Hope this helps.

 Check the stator for shorts to ground also
Opens are rare

[Justin B.]

All diodes (including small ones) were checked and the rotor is not shorted or open but has around 56 ohms instead of the ~2.6 it is supposed to have.  The bike has been sitting BECAUSE of the charging issue, it's not something that developed while sidelined.

My only head-scratcher is that I've never personally had a rotor winding fail with this reading before.

[Justin B.]

Bob, the 'Dawg has went to live with somebody else!

[Justin B.]

Pulled rotor and now reads 36 ohms!  I think the solder joint from the windings to outer slip ring is flaky as I can probe the wire just behind the joint and get right at 3 ohms.  I will resolder and if it checks out keep as a spare since the new one is on it's way.

[skippyc]

Dry joint is what i would have said and that is what you have found. Make sure it is cleaned up well before re-soldering

[Justin B.]

Yeah, just kinda laying there with solder on top of it.  I cleaned everything up really good, soldered, and have the proper resistance between slip rings now.  I guess I should have investigated this before ordering a new one but EME has them on sale sale so I now will have an emergency spare.

[Tony Smith]

I pulled out the diode board and all diodes checked good

I have hard personal experience of a diode board that tests just fine with a meter, but fails as soon as a real world load is placed on it. Near drove me mad finding this out.

If all else fails, borrow a diode board and swap it in and see what happens.

[Bob_Roller]

yes, it's nice having a spare parts bike for trouble shooting !!

[Justin B.]

Tony, I'm 100% positive about the rotor failure and it is now repaired.  However, the new rotor will be here tomorrow so it will go into the bike and the repaired one into "stock," so to speak.

Bob - no way I'm going to tear wife.gov's bike apart to donate a diagnostic part!