Bad Diode Stator means?

Sorry to keep bugging everyone about this, but I just don't know. 77 dodge van 318. On startup ammeter needle swings from full charge to midrange. Loud squealing when ammeter needle swings to full
charge. Noise goes away after engine warms up. I think I tried tightening the belt long time ago and made no difference. Visual inspection of belt appears ok. It is one of those kind that is serrated on the pulley side, but no glazing or obvious wear.
Looking at a shop diagnostic report sheet from long ago it is somewhat illegible but says, under "Alternator Output: Bad Dode stator" which I guess legibly written means: Bad Diode Stator? Would this cause the noise? The belt looks ok and I brought the alternator down to Autozone for them to test on their machine. Test said it was ok, but Mgr. there said it was probably a bad diode in the alternator. Should I buy a new alternator and see if the noise disappears?
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============Informative diagnostic post.... good for you.....
Addressing your LAST question in the post........
NO!!!!!!!!!!!!!!!! not yet.
The needle should raise on ANY vehicle, momentarily after startup as the generator recharges the battery. (fact, not fiction)
I would not condem the altenator yet, as long as the charging system seems to be doing it's primary job, and your main symptom is the noise.
Keep your money in your pocket for now.
now ..as to the topic, and your main concerns... being a tech....i would'nt suspect the stator making the noise, but that doesn't eliminate the FACT that it still could be the altenator making the noise, or the stator for that matter. Quite frankly....i don't know.
I defer to higher minds, which will be with you shortly if you're patient.
On closing..... You should be informed that I AM NOT..... a fan of AutoZonedOut parts.....especially electrical anything!!
with that said.......
:)
still..... jest to see........ you could ALWAYS....carry it back if the noise didn't go away, or..even if it did.
:)
and.....i wuddn't feel to baddly bout doin it if i was you...after all.....the Dealer techs do it all the time.
:)
~:~ MarshMonster ~takes a sip of his shroom juice...and tries to remember what the inside of an altenator looked like....20 years ago~ ~:~
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Bad diode can cause this noise:
"I had a funny noise under the hood of my 89 Taurus that no one could identify. It varied with engine speed but could not be localized, even by a Ford technician that I respect. Finally on a trip through Nebraska I had an occasion to take the car into a Ford garage in Scottsbluf . I asked if anyone there might know what I had been hearing, one of their young technicians came over and pulled off the alternator connector. The noise went away. He explained that when a diode fails the unbalance in the alternator magnetic circuit causes the whine. About 6 months later the alternator had to be replaced and of course the noise was gone."
http://www.misterfixit.com/altnoise.htm
wrote:

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wrote:

THis is true, usually in the form of a loud whine that varies with RPM. Alternators are 3 phase Delta wound and have 6 output diodes for full wave rectification of each phase. When you loose one output will drop some. If you have access to a scope you can find/verify a bad output diode(s) in a flash ----------------- TheSnoMan.com
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Why not just junk it and send it to the salvage yard, instead, and buy a another vehicle?
Larry
: wrote: : : drop some. If you have access to a scope you can find/verify a bad : output diode(s) in a flash : ----------------- : TheSnoMan.com
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And again from a third source confirms this:
"Whining
A whining noise can be caused by shorted diodes or stator, or by a dry rotor bearing. A quick way to test for the cause of a whining sound is to disconnect the wiring to the generator. Then start and run the engine. If the noise is not there, the cause of the noise is a magnetic whine due to shorted diodes or stator windings. Use a scope to verify the condition of the diodes and stator. If the noise remains, the cause is mechanical and probably due to worn bearings."
(don't have the url handy, it's from the autozone site)     

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Damn too much time in the sun and I missed this. I just gotta fix the Idiot's post

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You could be describing a polished or loose belt.
I test them by taking a cold off engine and seeing if I can hand spin the alternator pulley. If I can. it will for sure slip under load and make the noise you describe.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id !15147590 (More Off Road album links at bottom of the view page)
sorry wrote:

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The belt might be squealing because the alternator is dragging. Or, maybe the belt is just slipping.

This is a diagnostic report sheet from a different problem, though, isn't it? It's possible you have an intermittently bad diode. But you don't know for sure if the problem isn't repeatable.

I'd buy a new belt, and I'd buy it from the dealer and not from Autozone. I might consider looking at the alternator if that doesn't fix it. --scott
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"C'est un Nagra. C'est suisse, et tres, tres precis."

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Agree, Scott.
I dont think that anyone has mentioned, yet, that diodes can fail in either of two modes....shorted or open. And either can be permanent or intermittent.
The consequences of those failure modes can present different symptoms.
Either should be easy to diagnose if you have the ideal equipment (which most of us do not).
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wrote:

Take a ten dollar bill and go get a new belt and a piece of sandpaper. Use the sandpaper to lightly buff the groove the belt rides in. Just enough to breakup any glaze that might be present. Put on the new belt and tighten it properly. No more problem. Replace the third lugnut on the left rear tire at the same time.
Steve B.
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happening........... If it goes away, it is the belt.
Ed
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sorry wrote:

So what to do. Do you believe the test or do you believe the manager. It would seem to me that if they have a machine to test alternators it would be able to detect a bad diode.

What is the noise? Your description leads one to believe the noise is the belt slipping. If that is where the noise is coming from it is unlikely to be a bad diode. By now one would guess that you have probably overtightened the belt so the belt being too loose can be ruled out.     If your gauge truly measures amps then it is normal for the amp rate to go high after starting. It may be that it always did exactly what it is doing now - only you never noticed because there was no loud squeal to draw your attention to it. Or it could be that it is now charging more than it used to because of a bad starter or bad engine or some other unknown draw on the battery that requires a lot of amps to restore the battery. A weak diode could be draining the battery.     Another possible cause of the problem is a bad belt or the wrong belt for the application. I assume this is a v-belt so the cause could be a worn pulley given that the vehicle is 30 years old. If the pulleys and belt don't fit together as designed you can get the symptoms you describe.
-jim
-
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hmmmmmmm........
~:~ marsh ~takes a toke......grabs the remote..... turns the volume up on his favorite sitcom........good stuff~ ~:~
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LOL...
puff the magic dragon
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sorry wrote:

Alternator tests OK? Leave it alone (for now). The squealing is a symptom of a belt problem. Either worn, glazed, or loose.
How many belts does this vehicle have? It might not even be the alternator belt. Check them all.
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