Alternator Question

With the bike showing some symptoms of alternator problems i.e. turn signals failing to blink, losing power and rolling to stop and electronic tach fluctuations I measured the stator and rotor resistance.

Not good across each of the white pairs on the stator where resistance was 0.8 - 0.9 ohms and across the rotor wires was 36.7 ohms (this seemed very high and measured a couple of times to confirm it).

I unscrewed the bolts from the alternator cover to check the brushes but could not get the cover to budge at all. I have read of others problems with removal where the grommet is a problem but this cover seems stuck fast.

Gave it a couple of taps with a rubber mallet but nothing. I don't want to go hammer and tongs on the thing for fear of damaging it and would be open to people's suggestions.

 

Re: Stubborn Alternator Cover

I'm betting your stator is just fine. Most of our meters just aren't precise enough to discern between .5 and .0 ohms. You do need to check it for a short to ground, that is a sure sign of failure.

You're just going to have to worry that cover off.

 

Re: Stubborn Alternator Cover

Get an impact driver. That will take it off right now.

 

Re: Stubborn Alternator Cover

Make sure you short your probes together, before taking a measurement, so you can see the inherent resistance of your meter and subtract that from your readings. That might bring your white wires into spec.

Also, make sure you bump the starter a couple of times as you take your rotor field measurements. I recently changed my brushes which brought my readings from about 34 ohms to the high 4s. Then the next time I went to read them, they were indicating about 26. I ran the bike to check the regulator side and rechecked the white wires and they were back down to spec.

I know what you mean about the cover not coming off. It seems that any fuel that has dripped from your petcock, or the carbutetors, dribbles right down on that cover and acts like glue on the gasket. All you can really do is keep tapping it with the mallet. It will eventually come off.

 

Re: Stubborn Alternator Cover

Impact drivers are for screws. He already took the screws out, the cover is just solidly stuck in place.

 

Re: Stubborn Alternator Cover

Know what you mean I had to borrow my mate's electrician grade multimeter (that could still only read in the tenths of ohms) to get my readings as the $30 unit I have couldn't cut it. Not sure how to check for a short to ground. Are you able to point me in the right direction there?

Thanks for that bit of advice, I'll double check the readings.

Looks like I'll have to work a bit harder then to remove the cover and see what's going on

 

Re: Stubborn Alternator Cover

For checking short to ground set the meter on it's lowest resistance setting (typically 100-200 ohms) and ground one lead to the engine case. Check the resistance to each of the white alternator wires. You should read an "open" or no change from an unconnected meter (often an open shows as -1 on a digital meter).

If you get changes to the reading on the rotor by bumping the starter then you need to open the alternator, either the rings are dirty or the brushes are too short.

 

Re: Stubborn Alternator Cover & other questions

Thanks MiCarl.
Checked for a short to ground on the Stator and all good.

So pulled the alternator cover off and no oil just grime - an old English steel knife and a couple of taps with the mallet worked a treat.

So cleaned the rings with an eraser and isopropyl alcohol. Rechecked the resistance across the rings - a constant 4.2 ohms so within spec.
Brushes are a total of 10mm in the holder and 7mm from the line on the brushes so seem OK.

Reassembled for further testing.

Cleaned the contacts for the stator and rotor where they go into the harness. Stator now reads 0.5 ohms each but rotor reads 10.2 ohms.

Does this sound like I need to replace the brushes or should I be checking elsewhere (I don't like the idea of replacing the connector as there isn't much cable length to play with)?

 

Sounds like the brushes have lots of life left.

 

Crow:
I have the same bike and had the same problem. Everything checked out fine but I would get an intermittent problem. Finally I mounted a voltage meter inside the fairing so I could keep and eye on things while I was riding. Turns out it was the plug in connector from the alternator to the main harness. It looked good and I had cleaned it with spray contact cleaner and put dielectric grease in it, but when I wiggled it the voltage would drop right off. I just cut it out, soldered the wires and used shrink sleeve. It has been working great ever since.