Testing the rotor

[kuthe64]

Having some trouble using my multimeter to test the ohms of my rotor. I touch the negative and positive together and get the .3 that i would subtract from the total ohms that i get when i test the rotor. I have an 81 special I remove the stator completely so that I have full access to the rotor. Once i place the multimeter leads on the rotor I get absolutely nothing except when I place them both on the same slip ring the reading jumps all over the place anywhere from 2-180.
I have cleaned the rotor so that I am getting a good connection.
Also there are very distinctive circles in the copper where the bushes are, is this a sign of a bad rotor?
Does the rotor have to be off the bike to test it?
Does the key have to be turned on?
Should the leads be placed on the same or different slip rings?

 

[retiredgentleman]

Having some trouble using my multimeter to test the ohms of my rotor. I touch the negative and positive together and get the .3 that i would subtract from the total ohms that i get when i test the rotor. I have an 81 special I remove the stator completely so that I have full access to the rotor. Once i place the multimeter leads on the rotor I get absolutely nothing except when I place them both on the same slip ring the reading jumps all over the place anywhere from 2-180.
I have cleaned the rotor so that I am getting a good connection.
Also there are very distinctive circles in the copper where the bushes are, is this a sign of a bad rotor?
Does the rotor have to be off the bike to test it?
Does the key have to be turned on?
Should the leads be placed on the same or different slip rings?

Rotor stays on the engine. Key stays off.

You should remove both brushes, and measure their length (3/8" minimum). While the brushes are out, you put one meter probe on each slip ring. Should be around 5 or 6 ohms. Use the lowest ohms scale on the meter.

What type of meter is it? Post a picture so we can see it. Some cheap meters come with really crappy leads/probes. They may be so bad that they are useless for measuring low resistance. Another possibility is that your meter's batteries are worn out.

 

[kuthe64]

I tried the yellow one but just got the black one today and I am going to give it a try

 

[gggGary]

Yes this low of an ohm reading is somewhat of a meter battery stress test. I'll often have the meter start by showing 6 ohm and watch as it slowly drops to the low 5s over 5 or 10 seconds. But from your description you have an open in the rotor. An autoranging digital VOM will often bounce random numbers around like that. Look on the back side of the phenolic that the slip rings are on, there are a couple of solder connections, wiggle and pry them a bit see if a solder joint has failed, seen that a couple of times. I have not succesfully resoldered one but that doesn't mean it can't be done.

 

[retiredgentleman]

The black meter appears to be unsuitable for low resistance measurements. The yellow meter should be OK, used on the 200 ohm scale. The meter leads look to be the real cheap type, which can give intermittant readings.

A simple test is to go to an electronics store and buy a 5 ohm or 10 ohm resistor (or any low value resistor) Measure the resistance of the resistors to get familiar with your meter.