Testing the Feild Coil

Does anybody know how to test the field coil on a 1981 Yamaha xj550 Seca?

Josh

 

Quick and dirty test #1.
Start the bike. Hold a 0.10 mm feeler gauge (steel) next to the alternator cover. The field will pull the feeler gauge toward the cover.

 

It is actually better not to start it and just switch the ignition on. With the motor not running the field coil should produce the maximum magnetic field.

Quick test number 2:

Disconnect the two pin connector to the field coil (green and brown wire). Check the resistance at the connector and ideally you should get close to 4.5 ohms. Typically, it will read a bit higher as there is a bit of resistance added from the brushes and condition of the field coil copper rings.

If you get a high reading - say above 12 to 15 ohms, then time to pull the cover and check the condition of the brushes and copper rings on the field coil.

And, better yet would be to report what kind of charging voltages you are getting when the bike is running. Good numbers are 14.5V +/- .3V at 2000 rpm and up.

 

Thanks guys. My original reading was about 20 ohms. I opened up the stator cover and cleaned and polished EVERYTHING with sand paper.The reading is now 5.3 ohms but still not low enough. The field coil was fine with a direct reading from the copper rings at about 4.8 ohms. I'm going to replace the brushes as they're pretty worn down and all the connections to the brushes look bad. We'll see. Thanks again.

 

Sand paper?! EEK!
Please let you have used 400 grit finishing paper or finer. The traces need to be as smooth as possible so the brushes aren't bouncing around.

The prefered method is to clean them with a hard eraser.

 

Yes, I did use 400. I actually did use an eraser first. I noticed after using the sand paper though that it got shinier and the reading on the multimeter went down from 5.6 to 5.3. But thanks for the heads up. Josh

 

Just curious. I noticed there was quite a bit of rust on the outside of the coil where it hits the magnets and the magnets on the inside of the stator were all covered with a black soot. I cleaned that up but it didn't seem to have an effect on the reading. Why is that?

 

That's jast carbon dust from the brushes. The windings are insulated and then covered with shellack (the rust looking stuff), so the dust won't effect any readings.

 

post a photo of the rings if you can