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Old 07-11-2019, 10:07 PM
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Default Alternator fan compatibility

Several years back, about the time I switched to efi, I added a 12si alternator. This one happens to be a 94 amp(rated). Because I exploded one of these plastic cooling fans (through my fiberglass hood incidentally) I made a pulley slightly larger to slow it down a bit. The good news is my fan has stayed together since. The bad news is I've added more electrical draw, and now the charging is marginal again due to the output being lower at normal driving speeds, and especially idle. This is not surprising to me. My question is, are these plastic fans standard(recommended) for the 12si alternators, or can they be changed to one of the metal fans? If I didn't have to worry about exploding the fan, I could go back to the correct diameter pulley, and probably have enough charging capacity. If not, I'm gonna need to make a change. Any input appreciated.
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Old 07-11-2019, 10:13 PM
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As an aside to my question, I don't think this plastic fan cools all that well, or at least not at slower speed. Charging seems to diminish as the engine bay temps get hotter. If the metal fan is ok to use, will it cool better as well?

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Old 07-12-2019, 02:20 AM
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Subscribing. I've wondered the same thing as I've had one of those plastic fans on my alt. for years. Figured I'd change the entire alt to the correct type eventually but it seems indestructible. It was on the car when I got it in 2005 and still charging fine. I've wondered if I can just put the correct appearing fan on it, which I have on a shelf already. For me, it's an appearance thing. Just doesn't look right.

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Last edited by Greg Reid; 07-12-2019 at 02:27 AM.
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Old 07-12-2019, 06:15 AM
stellar stellar is offline
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I have seen aftermarket plastic fans explode, but not the OE fans unless they had previous damage. This of course doesn't include foreign objects being in the mix. The fans draw air through the alt from the rear to the front, so drawing something through the alt would be a rare occurrence. The plastic fans do cool better. The extra cooling provided by the plastic fans on the 12SI units was needed because of the extra heat generated by the higher output of the 12SI alternators. The diode assembly design was also changed for better cooling. The front housing of the 12SI is also different. It has a lip around the top to work with the plastic fan. If you put a metal fan on it will need to be spaced away from the lip which would make it even less efficient than a 10SI cooling capability. You can change the front of the alt to a 10SI front to use a metal fan and have the correct spacing and pulley alignment but it won't cool as well as the plastic fan and 12SI front housing. The drawback being the alt won't last as long. It will cause the alt to overheat and burn up the stator and or diodes prematurley.

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Old 07-12-2019, 08:32 AM
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Not the news I was hoping for, but thanks for sharing. I guess I'll start looking for a different alternative.

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Old 07-13-2019, 03:39 AM
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And guess I will leave well enough alone. Thanks stellar.

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