CONTINUITY PROBLEM BETWEEN ALTERNATOR SLIP RINGS AND BRUSHES

With my bike's engine running at 2000 RPM and a voltmeter connceted across the battey terminals the alternator output will not go above 12Vdc.

• The voltage regulator/rectifier is brand new
• The battery is in good shape and takes and holds a full charge off of a charger.
• The alternator bushes are brand new (17mm overall length)
• The resistance between the three legs of the alternator stator are within spec
• The stator AC voltage output between all three legs are consistent
• No shorts between stator windings to ground
• No shorts between rotor windings to ground;
• The resistance across the rotor slip rings is within spec
• The rectifier diodes are all good
• Continuity is good on all the wires and connectors involved with the charging system
• The continuity from the tip of rotor brushes to the two pin connector is good
• No short to ground on any of the wiring.

Now here's the thing that got me. With the rotor cover (and brushes) on and the wiring connector for the rotor brushes disconnected I am measuring about 12 ohms across the two pins on the engine side of the connector.
However, the resistance right at the slip rings is about 4.5 ohms. Like I said; the brushes are new. Also, the slip rings are bright and shiney. Brush spring pressure is good. All terminal screws are clean and tight. Seems the brushes are not making good enough contact. Any suggestions on how to resolve this?

 

I don't think this is necessarily a problem. The best way to evaluate the field coil would be for current. With the key on (bike not running) and approximately 10V across the field coil you should get a reading of close to 2.5 amps. I just checked mine and it is a working system and I got 2.0 amps, even though the resistance measured was 12 ohms at the connector on my bike. In theory, that would be less than 1 amp, but the 10V applied to the brushes is going to produce a different result - in my case resistance calculates based on current to an actual value of approximately 5 ohms.



And this from when you posted earlier in a different thread:

Do you have a service manual? There is a detailed procedure for testing the charging system that is quite good with the exception that the V1 and V2 voltages are labeled incorrectly in some of the manuals. This is easier than checking current as above and should give a good indication of the regulator doing its job.

From that procedure, what kind of voltage do you have on the green wire with the bike not running? Should get 1 to 1.8V, which is a good indication the field coil excitation is correct. On a working system the green wire should go positive to around 9 to 11V with the bike running and the RPM raised to around 2K. The brown wire at the regulator, which receives power from the output of the ignition switch should be close to battery voltage.

If you are disconnecting the stator to do this note that the charging system is running open loop in this mode with the AC generator output disconnected - no voltage to the IC regulator so no feedback. That means maximum AC voltage should be outputted as the IC regulator supplies maximum current to the field coil trying to increase the output. I did not want to rev too high because of the open loop condition.

Idle of about 1100 RPM = 18.5 VAC
Rev to about 2100 RPM = 30.5 VAC

This is measuring phase to phase, and should be equal between any of the 3 wires.

Edit: Brown wire origination is output of key switch. Yamaha service manual circuit diagrams shows output of signal fuse, wiring diagram shows output of ignition switch.

 

Hello Rooster53. I appreciate you taking the time to provide all this good info. I am going to follow your advice and retest at the earlies opportunity. I only have a Haynes workshop manual. Is this one of those publications that has the V1 and V2 voltages labeled incorrectly? If so, is it just a matter of switching the labels? I haven't checked the voltage on the green wire but will be doing this next. I measured the stator voltages with the stator connected by inserting the meter probes in the back of the connector.

 

I have a copy of the Haynes "Yamaha XJ650 & 750 Fours" published in 1994, and V1 and V2 are labeled incorrectly. It's easier just to think of the green and the brown wire. The green wire is the control line. Its voltage level is a function of the IC regulator with the purpose of maintaining the correct system voltage by adjusting the excitation to the field coil. The brown wire is basically battery voltage from the output of the main switch and is used for power / feedback to the regulator and power to the brushes.

A somewhat simple test on the brushes / rotor / regulator to check for excitation and no open circuit:
Turn the key on while holding a thin feeler gauge about 1/4 inch from the AC Generator cover. If the circuit is complete and current is flowing through the rotor a magnetic field will occur pulling the feeler gauge toward the rotor cover. With the bike not running and no output from the stator the IC Regulator should supply near maximum excitation to the field coil (rotor).

 

It's 2018 and I just read this article. Was this problem ever solved? If so, please tell me how. I have a similar problem.

 

bunglejyme has not visited the site since 2016, so you are not likely to get a response from him.

Go back to your original thread and reply to the questions and testing advice given and hopefully we can get your bike charging.

https://www.xjbikes.com/forums/threads/battery-charging-problem-alternator-rotor.119858/#post-601806