Topic: cb650c charging system question

[stuka151]

I searched through quite a few threads and was not able to find the answer to my question, not to say it doesn't exist, I just can't find it. I have been troubleshooting the charging system on my wife's '80 cb650c. I know, trouble with the cb650 charging system? I can hear the gasps of shock and amazement now. I am trying to determine where I should have continuity on the rotor brushes. The outer one (w/ the black wire) has full battery voltage. I assume this is correct. However, the inner brush (w/ the white wire) has no battery voltage and really lousy continuity to ground. The continuity is good from the brush to the reg/rect so I know it's not the wiring between them that is the source of my troubles. Should I be getting battery voltage or continuity to ground on this inner brush?

[Pinhead]

To test the rotor remove the left-side engine cover and test resistance between the copper slip rings. It should be around 6 ohms, as low as 4 and as high as 10. There should be no continuity to ground. If you try to test the rotor through the harness the readings will be skewed due to the resistance of the brushes.

Check the link in my sig if you need a regulator/rectifier.

[stuka151]

I had better clarify. I'm not trying to test the rotor. The rotor tests good. I believe the rotor may not be getting energized due to a faulty circuit which supplies the power to it. I am getting battery voltage measured at the outer brush and nothing at the inner brush but really crappy continuity to ground. Should this inner brush be getting battery voltage too or does it provide a ground for the circuit?

[Pinhead]

I'm not exactly sure how the electronic regulator works in that regard.

[cb550fcafe]

What is the rotor resistance across the slip rings? Not to derail your testing but Clymers has the spec written wrong. The regulator supplies the ground path on the white and regulates voltage on the black to regulate output. Rotor needs to ohm out in the 5 ohm area. Most bad rotors I've seen are lower than 1 ohm.

[TwoTired]

The rotor is, electrically, a coil of wire wrapped around a slug of metal to form an electromagnet.
 The electromagnet forms a magnetic field, the strength of which determines how much output the alternator stator coils can produce.  (it also needs to be spinning).
The strength of the magnetic field is determined by how much voltage is applied across the two terminals.  Usually, one terminal is applied to the battery NEG terminal and the other terminal gets various voltages depending on what the regulator determines is best for the battery.

That is why the rotor is usually tested for resistance as Pinhead pointed out.   If the rotor coil is broken you will see voltage on one terminal and not the other.  If the rotor coil is intact, you will see voltage on one terminal and not the other.   So, the voltage test does not give useful information without also measuring the current going through the rotor, which means adding another meter for the test.

It is a very good test to know what resistance your rotor has.  They usually fail by shorting the windings together.  This lowers the resistance and reduces the alternator output, and further draws far more power from the charging systems output, robbing from the battery.

[stuka151]

It is above 1 ohm, not high and no continuity to the rotor base. I didn't get my specs. from the Clymer. I got them from someone at Rick's Motorsport Electrics who rebuilds rotors.

The rotor is electromagnetic. I'm guessing that the current passing through the rotor windings is DC battery voltage. In order to induce the oscillating magnetic field that is needed to produce the alternators AC current, either the interlocking "fingers" of the rotor halves are creating this oscillation or the current to the rotor windings is being switched to energize and collapse the magnetic field. The later seems less likely to me. So I think that the the inner brush is supplying the ground to the rotor winding. Is this assumption correct? If so the cause of my charging problem is probably related either to a bad ground to the reg/rect or a faulty reg/rect as the resistance to ground on this brush is around 50k ohms.

[stuka151]

Usually, one terminal is applied to the battery NEG terminal and the other terminal gets various voltages depending on what the regulator determines is best for the battery.

Thanks. This is what I was trying to get at. My resistance to ground on the brush that I believe is the NEG is very high, around 50k ohms. So I guess I should first check my ground to the  reg/rect. All this was measured with the alternator cover off so the brushes had no contact with the rotor.

[Pinhead]

Usually, one terminal is applied to the battery NEG terminal and the other terminal gets various voltages depending on what the regulator determines is best for the battery.

Thanks. This is what I was trying to get at. My resistance to ground on the brush that I believe is the NEG is very high, around 50k ohms. So I guess I should first check my ground to the  reg/rect. All this was measured with the alternator cover off so the brushes had no contact with the rotor.

Note that the "ground" that you're searching for goes through the regulator/rectifier unit. It is entirely possible that the solid state rectifier "switches" the ground source on and off to regulate the current flowing through the rotor. If this is the case, your 50k ohm reading may, in fact, be correct.

If your rotor does in fact measure out correctly, I would suspect a bad regulator. Check the link in my sig if that's the case.

[stuka151]

Note that the "ground" that you're searching for goes through the regulator/rectifier unit. It is entirely possible that the solid state rectifier "switches" the ground source on and off to regulate the current flowing through the rotor. If this is the case, your 50k ohm reading may, in fact, be correct.

Crap that's what I was worried about. I did check the diode forward bias voltages on the reg/rect and it seemed to check out good. I guess maybe I'll have to swap it out to find out for sure.