Subject: Alternator is taking a while to kick in

[Dupree, Jim]

Your close. With the key on engine off,  battery power is supplied from the
ignition switch to the charging/alternator/battery light and the D+ circuit
provides the ground for this light. The current flows through the light (the
light illuminates)  and grounds through the windings in the alternator and
initiates the magnetic field in the alternator. When the engine is started
and the alternator starts charging the D+ becomes a 12 v circuit (via power
generated by the alternator). The voltage on the D+ (alternator voltage)
circuit should be the same as the voltage  (battery voltage) to the
charging/alternator/battery light from the ignition switch and with the
voltage on both sides of the light no current can flow and the light goes
out.
If there is a break in the D+ circuit then the alternator will take longer
to start charging because there was no current to start the magnetic field.
It will generate enough current on its own to start the magnetic field but
it usually takes higher rpm than idle.


Thanks
Jim

 -----Original Message-----
From: 	Ben Swann [mailto:bswann at worldnet.att.net]
Sent:	Tuesday, October 08, 2002 10:40 AM
To:	'passat TS'
Cc:	'quattro at audifans.com'; 'Comcast'
Subject:	RE: Subject: Alternator is taking a while to kick in

Carlos,

Study the Bently diagram for your car carefully.

We just went through a similar ordeal with Sean Shoemakers car.  There is
much discussion recently regarding how the alt works and uses the field
wire.  Basically, it need to pass a small voltage through the alternator
light when you start the engine, which activates the field current.  The
voltages then "equalize" and the alt light goes out, letting you know there
is no problem and the alt is charging.

This is an oversimplification, I know.  The Bently diagram for all this
seems rather complex, as there are current track from D+ going all over the
place, since D+ can also provide the bulb current for the other warning
lights as well.  Essentially the D+ is the voltage feed for all of the
warning indicator lights.

Once a field current is established, it becomes self generating, and the D+
field current is no longer needed (except to power the warning bulbs).  In
some alternators, they do away with this altogether, but this is how it is
used on the '80's generation of the AUDI charging circuits AFAIK.

It does sound by your description below, that you have things wired
incorrectly for the field current to be established. Be careful, as
improper connection of the D+ wire either to ground or to a high amp source
like battery + can blow the alternator diodes.  Also, running the alt
without a load can also blow the diodes.

Ben


[On Tuesday, October 08, 2002 10:45 AM, passat TS
[SMTP:passat_ts at hotmail.com] wrote:
>
Ben,

I didn't think about current.

This is a custom panel (used to be a digital crap) that I wired to the male
connector of the digital dash (I desoldered from a
broken one). Everything else works fine.

This D+ is the only thing giving me trouble.

1. I use 12V small bulbs for all the idiot lights.

2. The Bentley says the D+ should show 12V.

3. What I understood by reading the arquives is that I should provide 12V
to the bulb, when ignition is on, and wired the D+ to
the other lead. So, when the ignition is on, but the car is not running it
should come on, then when the car starts the D+ switches
from ground to 12V, and the light turns off.

4. The behave of the light isn't like that. When I turn the ignition, it
stays off, when I start the engine it blinks, then stays off.

5. The alternator seems not to be working for a period of time (voltimeter
shows less than 12V) then it jumps to the normal
14V.

6. Once it starts to work (14V reading) it charges the battery, since the
voltimeter shows 12V when the ignition is on but the
engine is off after a little engine running.

Does it help to give a better idea?

Thanks for any help,

Carlos.]