Diode Problem - Any Ideas?

OK. My car was overcharging. I took it to my usual Auto Electrical place who are really good. They replaced the diodes as they said they'd blown - in the alternator. The car was fine then a week later the car started spluttering and the alternator wasn't charging - I've learnt that if a 911 isn't getting it's charge it lets you know by running like **** until it stops! So, they said the diodes had blown again, replaced it with stronger ones(?) and....it was OK for half a day and it's not charging again and stopped running!
I'll take it back on Monday. But, has anyone any idea why it would keep "blowing" diodes - I replace the regulator too BTW, before the last diode blow. Also it backfired twice, I hope I've not cracked the airbox....
Thanks,
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

Well, it sounds like a shop used to American alternators, and a 'generic' replacement was made, to me! And, the specs he looked up were probably for the earlier 770 Watt alternators was used, rather than the 840 or 980 Watt units! 1975 was an 'interim' year, with more than one possibility as to what came on the car!

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Warren Hall
1973 911S Targa

Thanks Warren, I'm in the UK but I can see how the "generic" assumptions can apply. So, if the wrong spec diodes are put on they can blow??
Thanks,
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

Yes, diodes can blow because of wrong specs, or just poor quality, or even 'infant mortality' (semiconductor industry statistical phenomenon) ... but your problem doesn't sound very random! Replacement diode arrays are available for most alterntors, and considering another repair shop might be a good idea!

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Warren Hall
1973 911S Targa

I would also have the shop check your battery.

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Robert Stoll
83 SC
83 944

An alternator in good condition can handle a bad battery or a short in an electrical system without blowing out, as they are INHERENTLY SELF-LIMITING IN CURRENT OUTPUT!!!

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Warren Hall
1973 911S Targa

[TSwede]

I know this is an old post but I'm reviving it as it's with regard to the same car (I'm the new owner). I asked around about a replacement alternator (diodes still blown - battery charging a bit but can't stand to be left) and a couple of fo;lks I spoke to said it sounded odd that the auto electrician who had seen it before thought the electrical system had a propensity to blowing diodes in the alternator. They thought that I should not replace the alternator until I am sure what the problem is. I spoke to the auto-electrician today (the one who saw the car when Saffs owned it) and he is convinced that if I buy a later (internal reg) alternator and he rewires the feeds to it that it will be OK. Fair enough... new wiring and a later alternator may well solve the problem.... but is it not better to find out why the blasted things keep blowing in the first place...

The battery is fine
The reg. is new (ish)
The fan belt is OK

Anyone any ideas... before I fork out money unnecessarily.

(also - re: an earlier posting - what exactly are the correct diodes for a euro '75 2.7 S)



Swede

[nhromyak]

Could it be the brushes and commutator???

Just throwing it out there...

[TSwede]

I assume the electrician would have checked (never assume too much)
However - could this blow diodes if it were the case?

[nhromyak]

I don't know if it would blow diodes. I would think the diodes would blow, if they are subjected to too much current going the right way for which they were designed, or still too much yet less current if they are trying to make the voltage go the wrong way.

It seems to me, (I am not an EE by any means) if the comm and brushes are making sparks thereby making surges, this could blow diodes.

I would check all connections through-out the car too.