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Old 07-08-2008, 08:15 PM   #1
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alternator- fixable?

I just finished diagnosing my red battery/generator light which started to glow yesterday. The alternator is not charging the battery.

It turns out that upon engine shut-down, when the serpentine belt stops, the alternator impeller (?) keeps spinning and then, a second or two later, decides to whine/screech to a halt.

Three questions:
Which part of the alternator has failed?
Is the failed part consistent with not providing a charge?
Can I remove and rebuild on my own?

Thanks. Hoping to get back on the road sooner rather than later...

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Old 07-09-2008, 01:23 AM   #2
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Quote:
Originally Posted by wanna986
I just finished diagnosing my red battery/generator light which started to glow yesterday. The alternator is not charging the battery.

It turns out that upon engine shut-down, when the serpentine belt stops, the alternator impeller (?) keeps spinning and then, a second or two later, decides to whine/screech to a halt.

Three questions:
Which part of the alternator has failed?
Is the failed part consistent with not providing a charge?
Can I remove and rebuild on my own?

Thanks. Hoping to get back on the road sooner rather than later...
Two things:
1. It may just be a false warning, have you confirmed indeed the alternator is not charging?
From Wikipedia:
Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed.

Not sure what year your boxster is. How many wires do you see coming from the alternator? I haven't even looked at mine. 2 wires = self-exciting, which i highly doubt, but who knows.


2. It could be the rectifier in the Alternator that goes bad. I highly doubt you can buy the rectifier alone without the alternator, they make them specific to the alternator. This is probably 95% of the reason alternators stop charging.

Alternators are really just generators, And Generators only produce AC current. The rectifier in the alternator converts AC to DC.



I highly suggest you don't service the alternator, just buy a re-built one, they're fairly inexpensive, plus you can get an aftermarket one that produces more current and maybe you can add a kick ass sound system

That's just my suggestio
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Old 07-09-2008, 05:20 AM   #3
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Thanks for the reply.

Indeed, the alternator's not charging as verified by a voltmeter across the battery terminals with engine running.

The cause of the failure, if indeed we have a cause and effect situation, is whatever's happening that's not allowing the alternator to stop when the serpentine belt stops. Knowing this, I think I'm close to knowing what's wrong and (perhaps) having a good chance at rectifying (no pun intended) the situation.

Anybody else?
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Old 07-09-2008, 06:01 AM   #4
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Hold on a minute there cowboy.

Your alternator has a clutch drive, and is doing what it is supposed to do. This design is made as a gas saving measure, allowing the engine to sometimes not drive the alternator as hard as if it were hard-coupled. Go to a shop and inquire - you'll see.

More likely your problem is the regulator which is attached to the rear of the unit. You can replace that for about $75.
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Old 07-09-2008, 06:38 AM   #5
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The alternator does NOT have a clutch drive !! Unlike an electric motor, an alternator lacks permanent magnets and so no permanent magnetic field exists. This means that the pulley 'freewheels' whenever there is no field current going through the windings, such as when not charging. The presence of a field current creates a magnetic field, and it is this magnetic field which causes resistance to the pulley. This is overcome by the mechanical energy of the belt turning the pulley and generates the running/charging current in the process.

It is most likely the regulator, a series of 3 diodes. This is what regulates whether a field current is supplied to the windings or not. A replacement of the regulator will likely solve the issue.

But, there are concievably many miles on the bearings and so these too should be swapped if doing any maintenance to the unit at all.

Most cities in the US will have an auto electric shop which rebuilds alternators/starters. They can rewind the unit to produce more Amps, usually quite easily and the rebuild should come in under $100 in most cases. Shop locally and compare with off-the-shelf rebuilt units and go with the best deal.
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Last edited by Lil bastard; 07-09-2008 at 10:14 AM.
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Old 07-09-2008, 06:31 PM   #6
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o.k.
So there doesn't seem to be any agreement on clutch/clutchless but are we agreed that what I am seeing at the pulley is normal behavior for this unit?

Is the regulator internal or external?
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Old 07-09-2008, 07:53 PM   #7
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I suspect that you have a total alternator failure - bearings and diodes.

Either have yours rebuilt... or buy a rebuilt unit off of eBay or a source supplier.
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Old 07-19-2008, 07:33 PM   #8
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Ran across this and remembered this post.

http://mike.focke.googlepages.com/replacingyouralternator

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