Testing the stator/rotor

[dforster]

I've been combing the forum for an answer to my question in some other threads, but haven't found it, so here goes:

I'm trying to test the AC output of the stator on my '66 Dream 305. With the engine running and my digital voltmeter set to measure AC voltage, I'm touching one lead from the voltmeter to where the yellow wire from the stator connects to the rectifier, and the other lead to where the brown wire from the stator connects to the rectifier.

When I do that, it measures about 5 volts at idle. When I rev the engine it goes up to a maximum of about 13 volts. I tried this with another voltmeter and got the same results.

The stator output seems low to me. So my first question is whether I'm using the proper procedure to measure the stator output.

Also, if the stator output is too low as I suspect, I'm wondering if my rotor may be the source of the problem. How strong should the magnetism of the rotor be? When I slip a screwdriver between the stator coils and touch the side of the rotor (with the engine off and the key turned off), the magnetic "grab" is very weak -- barely noticeable. Am I doing this right? Is there a better way to test the rotor's magnetism? (By the way, the screwdriver I'm using is not stainless steel; I touched it against another magnet and it stuck just fine.)

Thanks in advance for any advice.

[brewsky]

Have you checked dc across the battery terminals while reved up?
Should get 13.5 or so

[dforster]

brewsky,

Thanks for the quick reply. I'm getting about 12 to 12.5 DC volts across the battery terminals with the engine revved up, which seems low.

Again, it seems to me that there's too little juice coming in from the stator. Assuming I'm measuring the stator output correctly, is 5 volts AC at idle, and 13 volts at high rev too little?

And if so could this be the result of what appears to be weak magnetism at the rotor?

Thanks.

[brewsky]

What is your battery voltage with engine off? Should be 12.5 or better to begin with.

The brown to yellow checks only one of the three sets of coils in the stator.

Brown to pink checks the other two sets, which are activated when the key switch is turned to the "lights on" position. (Check wiring diagram sticky and switch matrix)

A weak battery, wiring problem or short, or bad rectifier are usually more common than stator or rotor problems.

[dforster]

Battery voltage with the engine off is around 12.8 after a charge, but slowly drops by the day until it reaches about 11.8 or so, which seems to be the average rectifier output (from idle to high rev). This bike is my daily commuter, so I spend a fair amount of time each day sitting at idle.

I understand that a bad rectifier is often the culprit, but my rectifier is new, and I've tested it following the instructions I found on this forum, and also on the Oregon Motorcycle Parts website. It appears to be OK.

I'll check the other stator coils by testing brown to pink. What AC voltage output should I expect to see, both from yellow to brown and from brown to pink? Again, my concern is that my stator may not be suppying the rectifier with enough juice to feed the battery.

Thanks.

[brewsky]

You should definately get more than 13 while reved up.

I would stop by a shop or auto parts place and have the battery load tested to make sure.

If you are spending a lot of time idling, the battery will discharge.

If you have a center "off" position on the dimmer switch (some do, some don't), you can use that to boost the output by having the ignition switch in "lights on position" and the headlight "off" via the dimmer.

You can unhook one battery terminal and check for voltage or current between the unhooked wire and the battery terminal it came from while the switch is OFF.

If you get a reading then there is a short somewhere.