Electrical meltdown/charging issues

Bernie Benz b.benz at charter.net
Fri Sep 28 21:11:05 PDT 2007

Good explanation Alan,

It ‘s been so long since I’ve had an alternator apart that I don’t  
remember the internal wiring detail of the blue wire, but it must be  
diode isolated from the + output side of the main full wave 3 phase  
diode bridge on the regulator side such that its current path to  
ground is only thru the regulated field coil, as you indicate.

Inasmuch as the rotor’s magnetic circuit is made of high magnetic  
residual cast iron, it usually retains ample energy to self excite  
the alternator without the help of the ¼ watt fault lamp current. To  
test this just unplug the blue wire on a functionlng system and see  
if the alternator comes up on restart.


On Sep 28, 2007, at 7:16 PM, <alancordeiro at att.net> wrote:

> 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 residual
> 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...
> Alan
> ----- 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
>> 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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