Contamination or Oil on Alternator Brushes

The harness ground wire connects to the frame up on one of the coil mounts.
 
Thank you Pamcopete. I was reluctant to trim the runout on the lathe because I had some bad experiences with copper and the way it flows. I have now tracked down the missing brushes so have taken your advice and fitted them and will see how they behave when the weather clears up.

TM the mounting brackets on the new brushes from Yambits UK were the correct dimension. They were just like the originals where it looks as if the pigtails are soldered the wrong way around - Didn't stump me though.

Test time probably tomorrow - Thank you.
 
5twins, thanks for the information. I was hoping that connector was not the only earth as it seems a bit inadequate to me. Consequently I have now added another fresh new earth tab down by the Rec/Reg and connected it directly to the rectifier -ve wire.
 
As far as I know, that's the only harness ground wire. You're right, it doesn't seem adequate for the whole bike, does it.
 
I don't think it matters whether the carbon is on the ring, or still attached to the brush :) Spring weakness and hills and valleys in the ring will come into play though if they make the brush skip. Time to break out 2M's pocket o-scope .
 
FWIW, I've never given a second of attention to my rings and they're doing fine. My brushes appear to be graphite though, not blacker, non-shiny carbon like some blanks I have.
 
Thanks Guys for all the ideas. I did notice the springs are much stronger on the new brushes than the old ones, so perhaps this is the real issue. I read an account on one of the XJ websites where they say that it is not uncommon to measure a value like 14 Ohms across the brushes and rotor. They say what is important is the resistance under operating conditions. They traced their alternator issue to epoxy contamination on a ring/brush.

I hope to test tomorrow but the weather is looking crap!!
 
I have tested my wiring today with a dummy load (10W 3.9 Ohm ceramic resister) and can confirm the following with the engine off:
  • At 3.9 Ohm the brown wire delivered 2.8A at 11.6V which shows it to be up to the job
  • With power applied direct to the rotor slip rings we have 1.95A at 11.8V when earthed rather than connected to the regulator. This indicates a rotor resistance of 6 Ohms which is acceptable considering the rotor and engine were warm.
  • With the brushes and wiring connected (regulator disconnected) we get 1.65A at 12V when the meter is connected at the green wire in the connector for the Rec/Reg or if the outer slip ring brush is wired direct to ground i.e. the green wire is up to the job - no faults
  • When my DIY Rec/Reg is connected the current drain is 1.65A and when the stock Rec/Reg is connected the current drain was 1.5A. So no obvious issue here
I have made two rides today, 20 and 10 miles. The voltages seem to be OK but not 100% convinced yet that all is well. The clutch adjustment was pissing me off a little, must do something about a bimetallic push rod. Will do another 20 miles test tomorrow and see how things go - Time will tell!!!
 
So, what are you thinking, just weak brush springs caused all this?

I'm a fan of O.E.M. brushes. They're not available from Yamaha anymore but pop up all the time on eBay, don't cost much more, if any, than the repops if you shop around. What's got me sold on them are the ones in mine. As far as I can tell, they're the originals, and after nearly 40 years and 30K+ miles, are barely worn at all.
 
Paul are your new brushes oem? If this points to the problem, I will be on the hunt for a set. Actually should just buy a set from what 5twins testifies!
 
The new brushes are not OEM Yamaha but come from Yambits in the UK. They look good and have good solid pigtails and the springs are strong. It may be the issue was all to do with brushes or contaminated slip rings - See Pamcopetes comments above about contamination from sand paper.
 
Last edited:
Confession Time: The issues with my bike appear to have been solely down to old brushes and weak springs. After thoroughly testing my rotor, electric wiring and and installing new brushes the voltage output from my alternator seemed much better but still poor at below 3500 rpm. This remained the situation even after 40 miles. Then yesterday while trawling through the recent thread Running badly; thoughts? I noticed something gggGary stated on Page 2:

"Charging break even is about 1200 RPM with everything in good nick and stock incandescent bulbs.".

It was the incandescent bulb part that got me thinking - Big thank you to gggGary! I checked the specification and the headlight was originally a sealed beam 50/40W. I have been running a 35/35W halogen headlight for the past 2 years but recently a filament blew. I replaced it with another bulb I had lying about, a 60/55W halogen. This was about the time I started to experience charging issues. I whipped the bulb out and put a new 35/35W in and started the bike. All was good and still good after a 10 mile ride. Today I ran up another 25 miles and the voltage was steady at 14.4V from 1800 rpm upwards with 12.5V at 1200rpm idle - Break even!

I have added up the load on my bike and with the 35/35W running I draw approximately 7.5A for the full system whereas with the 60/55W the load goes up to 9.5A. This extra 2A loading is just enough to pull the system down at below 3000rpm. The alternator is rated for 16A at 5000rpm.

Note: System load = Headlight +2x5W Tail lights + 2x4W Numberplate Light + 2x4W Instrument Lights + 1A Ignition + 1.6A Field Winding. I used 12.5V to calculate current from Watts.

So there you have - bad brushes with a high current drain overlaying it all.

Thank you very much for all the contributions.
 
I've been running a 55/60 H4 since I got my bike with no issues. Does it drag the charging rate down low at idle? Well, sure, but it doesn't seem to have much effect at higher engine speeds. I know because I have a headlight on-off switch and I have turned it off at speed just to see the effect on the charging output. I see little, if any, change in the voltmeter read-out. At idle is a different story. It drags it down a good half volt or so.
 
xjwmx, I see your point. Those resistance measurements had me "barking up the wrong tree". If there is a positive spin then I learnt hell of a lot about the electrics, testing and system loading. I also cut the harness open and replaced all the charging and ignition related wiring and connectors.

Thanks
 
There can be intermittent connections in the harness at the terminal crimps, especially when moving things around. The charging problem on my '81 for a long time was the green wire varying in resistance due to that. I'm a big proponent of buying a whole new harness...which I will probably do someday :)
 
5twins, it dragged it down to approximately 11.5V at idle. It had me wishing I had an ON/OFF switch for the headlight.
 
I used that 3.9 Ohm ceramic resistor in place of the rotor as a dummy load to enable me to test the wiring and connectors under load by measuring current flow. So I am very confident that the wiring at present is good. I also put another earth tab down by the Rec/Reg and must exam the original tab by the coils to see what condition it is in, that is why there is always something else to do.
 

Similar threads

I
Pre recommissioning electrical checks?
Replies
13
Views
460
Its been a long time
I
JNaw
Rotor melted itself 500km after rewind?
Replies
15
Views
360
JNaw
JNaw
ssjody
Overcharging Alternator on XS650SH
2
Replies
23
Views
866
Jim
Jim
N
Odd charging problems on '75 xs650
Replies
8
Views
214
Its been a long time
I
B
Alternator modification by ungrounding the brush and using a solid state regulator/rectifier.
2
Replies
26
Views
2K
funky
funky
Back
Top