[urq] Voltage regulator and idiot light question

[Tony Lum]

LA,

I solved this problem years ago on my urq.  Careful voltage measurements showed that the voltage from the alternator was actually higher than at the instrument cluster.  So current was flowing backwards causing the the lamp to light dimly.  Your eye of course can't see which way the current is going.  I solved the problem by placing a 3A diode in series with the blue wire from the ALT light.  Anode towards the light and cathode towards the VR.  This allows current from the light to fire up the VR, but prevents current flowing back from alternator.  The slight voltage drop across the diode didn't affect normal operation.  I used to hate seeing that light glowing dimly at night.

HTH,

Tony
#DA900302


--- On Wed, 11/19/08, Louis-Alain Richard <larichard at plguide.com> wrote:
From: Louis-Alain Richard <larichard at plguide.com>
Subject: Re: [urq] Voltage regulator and idiot light question
To: "'fdekat'" <fdekat at sentex.net>, urq at audifans.com
Cc: quattro at audifans.com
Date: Wednesday, November 19, 2008, 1:10 PM

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 ?

Louis-Alain

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