1971 Plymouth roadrunner, 383 engine. Newer stock style alternator bought and has been sitting for 18-24 months while I have been getting it up and running again.

Long story short, I got the engine started yesterday.

The alternator spins free by hand with no belt tension. The belt tightened and it is locked solid.

Started the car and the belt was spinning but alternator would not turn. It squealed and smoked the belt.

Took off the very hot belt and the alternator spun with no problems by hand again. Water pump pulley is also free and spins effortlessly.

I have another alternator in my stash, but I was wondering if anyone has ever seen anything like this before. It's a first for me.

Thanks
eightlitermopar

bad bearing

Is the alternator charging?

How would it charge if it's locked up?

Yes, I've seen this very problem. Its quit simple but not obvious to everyone.

When you have the belt off, all the mounting bolts are loose and the alternator spins normally.

When you tighten the belt, you now have tightened the mounting bolts to hold the alternator in place and keep the belt tight.


Well, IF YOU DON'T HAVE ALL THE CORRECT SPACERS between the ears of the alternator and between the alternator and the head, you are now squeezing the ALTERNATOR CASE together enough that it locks up the rotor of the alternator.

I'd put money on the fact that you are missing one or more of the correct factory spacers for the alternator bolts.

^^^^^^^^^ yup

Sigh. If the OP is in the car, starts the engine, and from the driver's position hears the belt squealing and then shuts the engine off to investigate, how do we know it's not just an overloaded alternator? It's just a question, Christ, lighten up.

a BIG 10-4 ! i have had many experiences with this happening !

Also the single locking bolt on the adjustment arm, if it's too long it will hit the vanes on the rotor and lock up the alt....DAMHIK

Squiring the case is a pretty high possibility.
One other thing I've seen was the bolt that tightens the alt to the slotted adjusting bracket is too long. When its tight, it protrudes into the case too far and stops the armature from spinning. Often times, a second flat washer between the head of the bolt and the adjusting slot is enough to make it work, but a shorter bolt is a better option. Putting a couple washers under the bolt head would give a pretty quick answer as to how much shorter that bolt may need to be.

At any rate, without the belt in place, you need to tighten up both bolts and try to spin the alt. Then loosen one of the two bolts at a time to see which one, or if both bolts are the problem. if its not a bolt problem, the next bet would be a bad bearing, but you should be able to see the pully move in the case if a bearing was the problem. Gene

Thanks for the input guys! I was chasing an electrical short when I got the new alternator 2 years ago.

I may have messed up when putting the new alternator on. I will investigate tomorrow and let everyone know what I find.

Thanks for the heads up!

eightlitermopar

It sure will, thanks, I forgot about that little possibility. Done that in the past also.

I have seen when the power is on the field that it also locks up the alternator, try it all unplugged and disconnected from any power.

Alright, update:

It turned out it was the bolt squeezing the "ears" together, which was causing the alternator to lock up. My old alternator had no issues, but the new alternator casting was slightly wider, and the spacer was more "sloppy" with enough play to allow this to happen.

I placed a washer next to the spacer and it worked fine.

However, the long arm adjustment bolt hole was "wallered out". So instead of tapping for a bigger bolt, I put my old alternator back on and it runs great.

I did notice that the alternator is charging at 15 volts, but maybe it was just charging a slightly drained battery. I replaced my voltage regulator with one I had laying on the shelf, but that one only would increase from 12.6 up to 12.63 with the engine RPM's raised and the amp meter on the dash was jumping around.

Long story short (and a discussion for another thread most likely), I returned the original regulator to its proper place and its charging again, although a little high. I'll dig into that later.

It was the bolt squeezing the alternator case and locking up the belt. I have never had the privilege of experiencing that before.

Thanks for the help, I really appreciate it.

Eightlitermopar

Thank you for following up on the results and resolution.

That really helps others that might get in same situation or prevent it from happening.

Yes, thanks for closing the loop.

However, the long arm adjustment bolt hole was "wallered out". So instead of tapping for a bigger bolt, I put my old alternator back on and it runs great.

How about putting a heli-coil in it?

thanks for the follow up !
those case holes are easy to helicoil. i have done several.