Re: dim alternator light while running

[Barry Hodgetts <arkendale@xxxxxxxxxxxxxxxxxxxxx>]

If you were to follow the wiring from one of the 3 phase windings of the
alternator it would go through the output diode and to the battery. From
there via the ignition switch it would end up on say side one of the
alternator warning light.

The other path from the same point on the winding would be through the
diodes that supply the field winding and onto side two of the warning
lamp.


So under normal conditions the lamp has the same voltage across it. i.e.
it should be off.

As Jim suggests if there is a bad joint. there will be a small voltage
generated which will account for the  dim light.

I believe I have got the basics correct. It has been over 10 years since
the last time I pulled an alternator apart. On that occasion it was off
a VW van and the dim light was caused by the poor joint where one of the
diode packs was bolted down. Basically the same principal a bad joint
but this time in the alternator.


(by the way when the engine is not running side one has 12v on it and
the diodes are not supplying any power. The side two of the lamp is
grounded via the field control electronics, via the two carbon brushes
via the coil that normally spins when the engine is running.) So the
lamp is illuminated brightly in this condition. If the brushes
completely fail there is no circuit for the light to come on at all.

Hope I have not confused you....





In message , Jim Cash <j.cash@domain.elided> writes
>Ray
>
>Check your battery connections and then check the smaller of the + battery
>cables - the one going to the fuse box.
>
>I had the same trouble and it was corrosion inside the cable at the battery
>end - causing a poor connection between the connector and the cable - thus a
>high resistance in the cable.
>
>What is happening in this case is the alternator is feeding some power to
>the fuse panel by the "back door" method through the charge light - because
>the front door route (that cable) has too high a resistance and is causing
>too great a voltage drop across it's length- the voltage at the lamp is
>higher and therefore flows that way.
>
>Another possibility is that the cable is loose at the connection point under
>that fuse box - be careful working there - remove the battery cable from the
>battery first.
>
>Also check the battery ground  connections at the body.
>
>Cheers
>
>Jim Cash
>

barry@domain.elided 
Barry Hodgetts

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