Alternator fault

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vwjim
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Joined: Thu Jul 04, 2002 2:01 am

Alternator fault

Post by vwjim » Mon Oct 26, 2020 10:15 am

I've got an alternator where the output voltage measures over 14v, but the idiot light doesn't go out, so I'm thinking a diode or something has failed.

Would putting a new regulator solve the issue?


Thanks.

Regards,

Jim

Bruce.m
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Joined: Sun Aug 11, 2013 3:44 pm

Re: Alternator fault

Post by Bruce.m » Mon Oct 26, 2020 2:20 pm

Why?

The idiot light has the alternator on one side and the battery on the other.
If the voltages are the same, the light goes out.
If the alternator / battery voltages are different (either way) it lights up.
Nothing fancy or digital about it.

That might be a broken wire or more serious.

The light is the start of the investigation process not the end.

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vwjim
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Re: Alternator fault

Post by vwjim » Tue Oct 27, 2020 2:47 am

Thanks for the reply.

I know all of that, what I didn't know if the trio diodes are part of the regulator or not.

I've now removed the alternator. The trio diode (usually 3 in one package) is on the rectifier plate. Not the 6 larger power diodes on the rectifier plate. The trio is made up of 3 small wire ended plastic packaged diodes connecting to the 3 phases and a single connection to the D+ terminal.

These have failed, so the idiot light circuit stopped working.

telforddorr
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Re: Alternator fault

Post by telforddorr » Mon Nov 16, 2020 4:23 pm

Point to consider: when the alternator is operating correctly, the output of the diode trio is just like the main diode output, with one exception - the diode trio is rated around 5 amps maximum load. The alternator inductance will not limit the diode trio current, as it will with the main diodes. Thus, any excessive load on the diode trio, especially a short of some kind, will pop the diode trio diodes like flashbulbs. Any time you have a diode trio failure, it would be wise to do a thorough check of everything connected to the alternator D+ terminal blue wire, looking for issues.
For more VW electrical, see http://flowcon.us/td/ (available 9:00am to 9:00pm, PST)

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vwjim
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Re: Alternator fault

Post by vwjim » Mon Nov 16, 2020 4:29 pm

telforddorr wrote:
Mon Nov 16, 2020 4:23 pm
Point to consider: when the alternator is operating correctly, the output of the diode trio is just like the main diode output, with one exception - the diode trio is rated around 5 amps maximum load. The alternator inductance will not limit the diode trio current, as it will with the main diodes. Thus, any excessive load on the diode trio, especially a short of some kind, will pop the diode trio diodes like flashbulbs. Any time you have a diode trio failure, it would be wise to do a thorough check of everything connected to the alternator D+ terminal blue wire, looking for issues.
That's probably why it has a fault then, as the vehicle it came off has horrendous electrics.

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