A few years ago I got a 140amp alt. The engine had to be reved up to about 3-4k rpm before it "kicks" on and starts charging. The guy that built it said this is normal. One day it stopped working and I took it to the guy that I bought it from and he took it apart and found the rotor broke. He said it broke because the engine revs so fast. Well It broke AGAIN yesterday. The same way.

What is causing this and has anyone ever seen one break like this? I will get a pic of it when I pick it back up.

If it's a GM with a plastic internal fan, I've had that happen more than once too. Try to get one with a steel fan, and no way you should have to rev it up to get it to charge. It should start charging just above idle.

no way you should have to rev it up to get it to charge. It should start charging just above idle.He called this alt a "flash something". Said it was normal to have to do this. I do not like this. Has anyone else ever heard of this?

Here is a pic of what happened. He said this piece is just pressed on. Is there any way that I can fix it or is it junk? It only cost me $40 to have it fixed. If it does it again I will not let him fix it and I told him this. Any reason why it is doing this?
http://i90.photobucket.com/albums/k264/pnewell44/torino077.jpg

Patrick,

not sure why its breaking but my car did the same thing in not charging until I got to 3-4k RPM's. Mine was I had a burnt 24ga brn/wht resistance wire from the ignition sw to the solid bwn 16 ga wire going to the voltage regulator. This is what gets your alt charging slightly after idle. You can see the wire on top of your steering column from the main terminal pin A. It's a very small wire as easy to miss. This is assuming you have a factory dash harness. There is a diagram in the stickies at the top of this catagory.

Somethings wrong in wherever the heck you live. That looks like a 10 or 12 SI alt rotor. I have been rebuilding those for 35 years and have never seen that. They are pressed on and require a press to remove. and it isn't easy. I can't see how that could come off anyway since it is held together with the nut squeezing the pulley against the fan which is squeezing against a spacer which is squeezing against the bearing which is squeezing against another spacer which is squeezing against the part that is loose. There is no way that 1/2 of the pole could come off. Maybe the other 1/2. Which leads me to more problems I see there. It is obviously a rewound rotor, so maybe wherever he is getting them is doing poor work. If you look at the slip rings, where the brushes ride you will see that it is not in the correct location. The brushes are riding at the ends of the copper. They should be in the middle. the inner brush is overriding the copper by about 1/2 a brush and almost, if not touching the other ring. that would be a short. If you have a 1 wire alt it will need to be reved up to start charging, but this should not occur to the rotor. I would ask the rebuilder to install a factory rotor, then you should be ok. Something is definitively not right where you live.

I have rebuilt several over the years...and agree with Mrk above...
the only thing I can think of is miss matched second hand parts...

In the old days cars had generators...these would not charge unless rpms got up well above idle...this resulted in flat batteries on cold wet nights....and the reason crank handles where still used to start cars for many yrs after the inroduction of starter motors.
Generators improved in the 50s to elimate a common need for the crank handle but the issue still remained.
Then the alternator came along that put out reasonable charging current at very low rpms..
If it is not charging at idle/low rpms then I would be thinking there is something wrong with diodes, wiring, armitures, missmatched parts...
The use of alternator over generator IS TO BE ABLE to charge at very low rpm.

The reason you have to "rev" the engine to 3Krpm is because it is a 1 wire alternator. They are not self energizing. I HIGHLY suggest you ditch that unit and go with a standard 3 wire unit. Much better performance and reliability.

3 wire is much better. There is another problem with 1 wire alts not kicking in til very high RPM which is what is known as air gap. This is the distance between the rotor and stator. If you have a rewound rotor it may have been cut after assembly and a change in the air gap occurs. Even a small change will cause a 1 wire to be more difficult to energize, requiring a higher RPM to kick it in. That is one reason I told you to ask the rebuilder to use an OE rotor. If a 1 wire is properly built it can solve a lot of problems and work quite well but it has its limitations. A 3 wire will work better. Mark