Electrical meltdown/charging issues

alancordeiro at att.net alancordeiro at att.net
Fri Sep 28 19:16:07 PDT 2007

The alternator needs an electric field in the rotor to generate electricity 
(EMF) in the stator. The field is provided by an electromagnet, which gets 
its power from the alternator itself.

The current in the field is controlled by the regulator, thereby controlling 
the output voltage. To keep the battery from draining into the field, the 
excitation current is provided by a separate set of rectifier diodes, which 
only produce a voltage when the alternator is working.

When the alternator first starts turning, there may be a slight amount of 
magnetism left in the rotor. Usually it is insufficient to generate enough 
voltage to overcome the rectifier diodes forward voltage drop (1.4 volts) 
and to begin to boost
the magnetic field. Here is where a small "bootstrap" current allows that 
tiny bit extra field to bring up the voltage where the alternator can wake 
itself up from slumber.

The ignition circuit feeds one side of the fault lamp. The current flows 
through the lamp, into the alternator field circuit, (which at this point is 
close to ground). As the field circuit fires up, it reaches 12 volts, at 
which point the lamp turns off, since each
side of the lamp is at battery voltage.

Three methods can be used to overcome this problem.

1) fix the bootstrap current circuit in the lamp in the cluster
2) build a temporary circuit externally, feed the blue wire a small current 
through a 2 watt lamp right after you start the car, disconnect when it 
comes up.

3) rev up the car real high briefly, at very high speeds, even the small 
residual magnetism may fire up the alternator....this last method is 
unreliable.....but may be a good test that your bootstrap field circuit is 
defective and the alternator is fine...


----- Original Message ----- 
From: "Bernie Benz" <b.benz at charter.net>
To: "Kneale Brownson" <knealeski at sbcglobal.net>
Cc: "200q20V mailing list" <200q20v at audifans.com>
Sent: Friday, September 28, 2007 2:18 PM
Subject: Re: Electrical meltdown/charging issues

On Sep 28, 2007, at 9:10 AM, Kneale Brownson wrote:

> But doesn't the wire still have to supply that ground and light the
> bulb for the alternator to produce?
No. The blue wire doesn’t supply anything to the alternator, the
alternator supplies the ground or bucking voltage to the bottom side
of the idot lite, + is always on the other side of the lite with ign
on. Look at the schematic.

> Bernie Benz <b.benz at charter.net> wrote:
> The blue wire from the alternator is not an exciter circuit. It only
> supplies a ground path and a charge bucking voltage to light and
> extinguish the no charge idiot lite.

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