Even with the blue wire unplugged from it, it is common for an
alternator that has been recently run to have enough residual magnetism
to self excite once the alternator is spinning fast enough.

The blue wire circuit light behavior varies somewhat depending on
options installed on a given van. In the simplest case the dash led will
not come on when the key is turned on unless the blue wire is connected
to the alternator. The field coil in the alternator acts as ground path
for the dash charging indicator led when the alternator is not spinning.
Once the alternator spins up the field coil becomes powered by the
alternator itself and this removes the 'ground' and turns off the dash led.

However in many Vanagons a fridge relay, or aux battery relay, or cruise
control relay is connected to the blue wire  near the dash. This acts as
a ground path for the led to light even when the blue wire is not
connected at the alternator.

Mark

Poppie Jagersand wrote:
>
>
> On the blue wire at the battery.
>
>
>> On Friday, February 7, 2014 4:35:45 PM, Fuzzy :philippe <fuzzmeister@GMAIL.COM> wrote:
>
>> Something similar happened before and it was the small blue wire that
>
>> was disconnected on the alternator.
>   This is for an '82 Diesel Westy
>
> So I had also mistakenly had the blue wire near the battery come off. Alt light was on. However surprisingly to me the alternator still charged. (This was even after stopping and waiting several days and starting again (before finding the problem))
>
> I measured voltage on the alternator field coil terminal. With ignition key off I had 0.15V on it. Now when disconnecting the blue wire from the battery this voltage increased to 0.5V. Very puzzling to me. It stayed 0.5V regardless if ignition was on or off (with engine not running). So I thought maybe there is an accidental  connection from always on 12V to someplace in the alternator/fridge relay circuit.
>
> Then I checked current from the battery with ignition off: 10mA. This seems reasonably low. What currents do others measure?
>
> How is the field coil circuit supposed to function?
>
> thanks,
> Martin (and '82 Westy 1.9TD)
>