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Discussion Starter #1 (Edited)
Hi Guys,
Last night my Spider would not start. When I ran the alternator test using my charger last night it told me alternator was bad (even running at 3-4k RPM). I ran a smart charger overnight and had it at full power this morning.

This afternoon I was checking the connections around the alternator, and I noticed that bolt that holds the alternator pulley and fan was hot to the touch. After 15 hours, I expected the car to be completely cold. I had to double check to make sure I wasn't hallucinating. The battery was dead again.

I managed to start the car and got these readings from output of the alternator: 11.98V - 12.5V when running at 3k RPM. I'm also getting less than 10 ohms of resistance between the B+ terminal, and the pulley. This alternator is externally regulated...

What do you guys think, is this an obvious internal fault? I'm looking at the BME E30 90amp Bosch alternator, as they are the same price as the Alfa low amp units. Does anyone have direct experience and could give me a model # (i.e. BMW 325i 1989) that I can cross reference to make sure it fits, and the belt fits?
 

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is this an obvious internal fault?
Yes, this is an obvious internal fault. Or an example of cold fusion, but I'd guess the former.

I'm looking at the BME E30 90amp Bosch alternator, as they are the same price as the Alfa low amp units. Does anyone have direct experience and could give me a model # (i.e. BMW 325i 1989) that I can cross reference to make sure it fits, and the belt fits?
I have no idea whether a BME E30 90amp Bosch will fit. But if you want a larger capacity, internally-regulated, Bosch alternator, why not use the model that fits a late spider & Milano? It comes with a V belt pulley and will fit mechanically.
 

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Discussion Starter #3
Indeed. More testing revealed that the diodes are faulty.

I took the whole thing apart. I was thinking to myself, how can current flow back from the battery through the alternator and realized that it must be that the internal diodes are broken. A quick check with my multimeter confirmed this. It makes sense....

The AL49X model alternator from the 86-93 BMW 3 series is 90 amp and is $40 less than the Milano version (80 amps). My research shows that rockauto is selling the Alfa 164 alternator for $40, but that requires more alterations to fit the spider.
 

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The AL49X model alternator from the 86-93 BMW 3 series is 90 amp and is $40 less than the Milano version (80 amps)
The difference between 80 amps and 90 amps is meaningless unless your car happens to draw >80 but <90 amps. Unless your '78 spider has air conditioning, heated seats, xenon headlights, mega stereo, etc. it probably draws ~40. A higher capacity alternator doesn't buy you anything unless you have added high power accessories.

Going from an old-fashioned alternator, with an external regulator, to a modern model, with an internal regulator, is significant: less stuff to go wrong.

A $40 price difference is significant. However, if the AL49X comes with a flat belt pulley and you need to source a V belt pulley, that would offset some of the savings.
 

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Discussion Starter #5 (Edited)
Alfajay, you touch on some very good points. Moreover, you are correct on all of them. I do have an aftermarket stereo, and when the volume is loud enough - it will turn itself off due to a lack of current. Moving to any of the non-stock alternators would likely fix this issue.

It would be nice to get a comparison between the AL49X and the AL16X, mainly because I'm really bothered by threads that use anecdotal information with no concrete part numbers. I've seen 3 or 4 threads that reference this "BMW alternator" as a direct fit. Its almost mythical =). We'll find out! For example, the aftermarket the cowl hose alternative was something that was anecdotally referenced to on the BB until I wrote it up with pictures and part numbers last year.

I'd like to pursue the AL49X as an alternative, mainly for curiosity's sake. The pulley is a V-groove style, and is similar in shape. I'll be more than happy to produce a write up if all goes well (and even if it doesn't).
 
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