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Old 11-03-2010, 10:17 AM   #1 (permalink)
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Alternator case keeps cracking

I have a 1991 mustang gt that is a drag strip only car and I keep having my alternator case cracking on the top. I run a serpentine 4 rib belt from the alternator to the crank pulley. The pulleys are underdrive and the bracket is on the passenger side. any ideas. Thanks

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Old 11-03-2010, 12:50 PM   #2 (permalink)
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Re: Alternator case keeps cracking

Can you post a pic?
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Old 11-03-2010, 07:00 PM   #3 (permalink)
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Re: Alternator case keeps cracking

You got two choices.

Stop buying piece of crap alternators, or get the dynamics of the mounting done up correctly because there is undue stress on the case from improper alignment.
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Old 11-04-2010, 04:52 AM   #4 (permalink)
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Re: Alternator case keeps cracking

The alignment is correct on the motor I guess it must be the alternators they break at the same spot right on top bt the ears that the bolt goes to the block. I wonder if the belt could be to tight
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Old 11-04-2010, 09:39 AM   #5 (permalink)
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Re: Alternator case keeps cracking

I have NEVER seen a broken alternator the way you described.

I HAVE seen belts that were too tight many times and resulted in broken brackets, but never an alternator case.

I am telling you that you either got crap alternators or you have a mounting error that you are not seeing.
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Old 11-04-2010, 11:48 AM   #6 (permalink)
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Re: Alternator case keeps cracking

You can always test your belt tension with a tension gauge (cheap), but I developed a little trick to do it by hand. Use your little (pinky) finger to push on the center of the belt span. Push until the pressure is uncomfortable (not painful) and you should see about 1/64" deflection per inch of free span. So, on a two-foot free span, you should see about 3/8" deflection. Though it isn't a true gauge, it is surprisingly reliable and will at least ensure you're not grossly over or under-tightened. Hope that helps.

David

Example of belt tension gauge by Carlisle:

EDIT: BTW - new belts should be run 10 minutes to get initial stretch and always re-tensioned at that time. My directions only apply to typical automotive belt applications - every application and belt combination has different spec's.
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Old 11-04-2010, 05:43 PM   #7 (permalink)
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Re: Alternator case keeps cracking

The case is breaking where the bolt goes into the top of the alternator on the passenger side. I noticed there is like a seam there maybe that is week That is where the alternators is breaking. I just put another one on and took my level out and ran it from the alternator pulley to the crank pulley and there are no gaps at all. will take a picture and post it tomorrow
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Old 11-05-2010, 03:19 AM   #8 (permalink)
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Re: Alternator case keeps cracking

I think your upper alternator bolt is bent which would cause the misalignment and subsequent cracked cases. Alternatively you may consider adding an additional support bracket to the lower mounting point on the alternator so as to keep things from flexing when under load.
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Old 11-05-2010, 08:05 AM   #9 (permalink)
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Re: Alternator case keeps cracking

This is a very curious happening. How damn tight is the belt?

Pics of the bracketry will help a lot in the general dynamics of this conundrum.
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Old 11-05-2010, 08:23 AM   #10 (permalink)
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Re: Alternator case keeps cracking

I would think the bearing would fail before cracking a case. I'd bet your missing a washer or something in mounting putting strain on the case, then vibration of the motor is making is snap.

If it's aligned right you may want to fabricate an outer plate for the alternator that spans the two mounting holes and pulley area to put stress on the bolts instead of the case.

Also keep in mind that large 2inch serpentine belts have a lot of weight and are subject to centrifugal force at high rpms.
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Old 11-05-2010, 08:50 AM   #11 (permalink)
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Re: Alternator case keeps cracking

alignment problem will show on thw side of the belt
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Old 11-05-2010, 09:00 AM   #12 (permalink)
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Re: Alternator case keeps cracking

You're right Dan, but what thoughts do you have for a cracked case two times?

I read it as a broken case when I first replied and thought how ridiculous, and I think something is NOT straight...

Hey alternator guy, try this. Tighten the bottom bolt and leave the top bolt loose and see what attitude the alt sits on from that point.

something is amiss and you're not catching it.
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Old 11-05-2010, 09:06 AM   #13 (permalink)
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Re: Alternator case keeps cracking

My guess would be your belt is torqueing the alternator when your on the throttle. A picture of your setup would help. It might be that the belt is just to tight. At work I have seen alot of equipment damage from misaligned and improper tensioned belts.
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Old 11-05-2010, 09:12 AM   #14 (permalink)
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Re: Alternator case keeps cracking

I have seen Thousands of overtightened belts blow through Bearings in alternators and water pumps and PS pumps, but I've never seen a case on TWO alternators CRACK due to belt tension.
Nor have I seen it on ONE, so I am of the opinion that it's the Mounting Dynamics that are to blame. Something is off and he's just not seeing it.

If he tests the straightness with only the bottom bolt tight, then re-tests with ONLY the top bolt tight, it's possible that there will be shown an inconsistency in the mounting system that stresses the case and breaks it.
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Old 11-05-2010, 09:22 AM   #15 (permalink)
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Re: Alternator case keeps cracking

over tighted the alt. bolt ,starts a cracks in the case , then load breaks it , or no support bracket and alt. is bolted direct to block/head.

Last edited by DanH; 11-05-2010 at 09:55 AM.
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