The blue wire does 2 important and related things. When the key is
turned on it sends 12 volts from the cluster to the alternator to
activate the alternator field windings so charging can start after the
alternator begins turning with the engine. Before the alternator starts
charging the alternator field acts as a 'ground' and this makes the
light be ON. After charging starts the alternator field windings gets 12
volts from the alternator itself and no longer acts as a 'ground' for
the blue wire so the light turns OFF. If the alternator should stop
turning like if the waterpump belt breaks then the field becomes a
'ground' again so you have a warning light to stop and check things
right now.

So if the alternator is charging, and the 13.5 volts says it is, then
the light should be OFF, not ON. Since the light is ON, there is likely
something else connected to the blue wire that is providing a 'ground'.
The relay under a Westy front driver seat is connected to the blue wire,
and so can be aftermarket aux battery relay setups. These can be messed
up, or have been wired wrongly. Also various engine conversions use
alternators that can't properly turn off the light, most notably Subaru
conversions.

Mark

kenneth wilford (Van-Again) wrote:
> .....  I guess what I need to know is exactly what the light is
> looking for as an input to turn the light out.  I have never been
> completely clear as to exactly what I should be seeing from the blue wire
> that goes to the alternator since this is also the wire that is controlling
> the light.  Should I see a voltage from this wire?  Is it a ground
> instead?  If someone could explain what this blue wire does that would help
> me out a lot.
>
>