I went to test an alternator that I suspect is defunct. The diode trio tested OK as did the Rectifier diodes. I went to test the rotor which (according to the book) has 2 tests. The test performed touching the meter to the slip rings tested OK. The other test whereby you touch one probe to a slip ring and the other to the shaft gave me no reading (each slip ring tested individually to the shaft) –the book said a low reading means NG. Just for the fun of it, I had the original alternator to my 1969 which was performing OK until 2002 but I took it out and put in a Rebuilt unit. With nothing better to do, I took that unit apart. A similar test gave the same result. Am I doing the test wrong or are both units bad?

 

A low reading on an ohm meter means a good (low resistance) path. A high reading means a very poor path or open.

The slip rings on the rotor should have a very low reading between them, but individually to the shaft should be open.

TO maybe help get an idea of how this all works, a simple test. When you touch the meter leads together (meter on ohms) the meter (digital) will read .05 or somewhere close. Apart, the meter reads 0. or has a dash. Lots of different ones out there and rarely manufacturer's use the same indication.
OK not hold one lead in each hand between your thumb and forefinger- What's the meter reading? Remember that number. It'll probably be jumping around a bit so maybe get a rough average- right here this is not a real accurate test- Just a simple rough idea to give you a idea of how the meter reads- Got your average number? Good, now wet the fingers you were holding the meter leads with and grab them again just like you had them. DOn't worry, not enough voltage or current here to even feel it. Now look at the numbers on the meter again- they will be lower- the water on your fingers makes you a slightly better conductor.
Not a real scientific test, but it may help shed a little light on how a meter reads.

If you already know- sorry, I am not trying to make anyone look bad- but someone else may be trying to work thru a meter and this might help them too. And the way things are worded sometimes make a ohm meter as confusing as it can be and when you tie that with some guys are (rightfully) afraid of anything dealing with electricity ------

 

You should have continuity between each slip ring.
You should have an open circuit from the slip rings to the shaft.

 

Thanks to you both for your advise and explanations. I have wasted already too much time on taking this alternator apart and putting it back together with no definative explanation as to why it is not working. I'm going to pick up a new unit. thanks