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If you have blown one or more diodes then your options are
1 - identify and replace them
2 - replace the alternator with a good one
3 - live with reduced output

Alfa factory manuals show how to test and replace the diodes.
 

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1978 Spider
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Discussion Starter #22
commutator was smooth feeling when I checked. This is also allegedly not the correct alternator for the car (78 Spider). Notes left by the PO indicate 80 amp 92-92 Alfa Spider unit was installed.
 

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1978 Spider
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Discussion Starter #23
If you have blown one or more diodes then your options are
1 - identify and replace them
2 - replace the alternator with a good one
3 - live with reduced output

Alfa factory manuals show how to test and replace the diodes.
It's been 20+ years since I last rebuilt an alternator...are the diodes on a board inside or integral to the VR? I recall them being a separate component/board/card inside. It was a GM unit though.
 

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The diodes are not part of the regulator. You have to remove one end of the alternator to get to them.
 

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the diodes are generally on a bridge, the rectifier
like this
diode bridge.jpg
 

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Take a reading at the alternator and one at the junction box. My power cable between was corroded internally.
 

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1978 Spider
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Discussion Starter #27
Take a reading at the alternator and one at the junction box. My power cable between was corroded internally.
bingo!!! checked it again this weekend....13.5-13.8 at the junction box on the fender....14.1 right at the alternator terminal...at idle...

those cables are original for sure...40+ years old...I don't think they corrode "internally" but at the crimped connections and terminals...a good cleaning or new terminals is probably in order.
 

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It is very common for the voltage gauge to display low alternator output, and for the charge warning light to be continuously on, immediately after starting the car, and to then show normal/adequate charging levels after a quick rev of the engine. I'm not sure why this is the case, or what actually causes the regulator to "wake up", but a momentary throttle burst always seems to do the trick.
 
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