[urq] Voltage regulator and idiot light question

Louis-Alain Richard larichard at plguide.com
Wed Nov 19 13:10:20 PST 2008

I've asked :
> But why the ALT idiot light never lit ?   
> Isn't it suppose to show any problem, no charge or overcharge ?

Then Frank de Kat answered :
Not quite..  It only shows 'not charging'. In essence, when the ign is on
(but the motor not running), the alternator regulator circuit is powered via
the warning lamp. No alternator output, so current flows through light,
supplying regulator circuit power. Think of it as the regulator circuit is
'sort of' suppling ground for the warning lamp.
Once the alternator starts spinning, it 'self powers' the regulator circuit,
and puts it's own 12V on the 'small blue wire' terminal of the alternator.
Warning light now sees 12v from ign on one side, and 12V from alternator on
the other side, (same voltage on both sides of the light) so light goes out.
That's about all the light will do. That's also why, if the warning light is
burnt out (or the blue wire breaks) that you may not get charging to start
in the first place. 

No, I ask :

Thanks Frank, I understand now.

OK then, why is the ALT light still very lightly lit, as it was since I've
own this car ? Could it be the blue wire that has some resistance ? Or is it
the other way, not enough voltage from IGN ?


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