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Re: Ignition Warning Light on with new alternator

To: tigers@autox.team.net
Subject: Re: Ignition Warning Light on with new alternator
From: "Steven L. Finberg" <w1gsl@MIT.EDU>
Date: Fri, 13 Sep 2002 18:50:13 -0400
Theo is ecentially correct but let me explain it a little further
to emphise the function of the idiot light in a modern automotive 
alternator system.

On start up the regulator (internal or external)  is connected to 
the battery only through the idiot light.   The initial current to 
get the alternator field powered flows through the light bulb.  As 
the alternator starts generating current, a separate set of diodes 
inside the alternator take over supplying the regulator and there 
by the field current, from the alternator not the battery.
When this separate diode bridge can supply all the necessary field 
current no current flows through the bulb and it goes out.   

You might think that if the bulb were burnt out, or disconnected,  
the alternator would never start  In reality the field has some 
residual magnitism and will eventually get the regulator powered 
but will require a much higher RPM to start up.   For this reason 
many factory installations parallel the bulb with a resistor to 
supply some starting current if the bulb fails. 

Finally for those of us still running the Lucas 10AC stock 
alternators, the warning light serves a couple purposes,  Yes it 
warns that you may not be charging the battery if it remains on 
while the engine is running But much more important  it warns you 
that the ignition and alternator field are powered when ever the 
ignition switch is on but the alternator is not turning.  If the 
10AC is not turning, the old design regulator is applying full 
current to the field,  because it thinks the output voltage is too 
low.  But no air is flowing to cool the field winding which will in 
short order burn it out.

The new design, which is quite a bit cheaper to build (no relays) 
solves this problem by only allowing the limited bulb current to 
flow until the alternator is turning.

                        Steve F


*******************************************************************************
Steve Finberg                        W1GSL                       w1gsl@mit.edu
PO Box 397082 MIT Br        Cambridge MA  02139-7082              617 258 3754
*******************************************************************************


Theo Smit wrote:                                                                
                                                                                
>I'll take a swing at it.                                                       
>                                                                              
>First off: Read Larry's post carefully: One wire to the battery/starter 
solenoid, and another wire to the light. The one-wire alternator has TWO 
wires. My Toyot alternator has three: the output wire (big bolt-on post) 
and two spade-lug connections, one for the ignition and the other for the 
light. The "one-wire" alternator combines the function of the last two 
connections, and Steve L's description of its operation is pretty much 
correct: When the ignition is on but the engine isn't running, the 
alternator doesn't put out any current, and its internal regulator 
pulls some current from the 'light' connection to turn it on.           
    
>When the engine is running the alt output is above some threshold, and the
internal regulator turns off its current switch for the 'light' output. 
When the ignition is off the voltage at the light is zero, and the 
alternator's regulator takes that as a cue to do nothing. If your charge 
light is on when the ignition is off then either the light is wired to 
the wrong side of the switch, or there is some internal fault with the 
alternator that is allowing the current flow in the wrong direction 
(some older cars had the diode Steve and Tim Ronak referred to on the 
instrument panel circuitry but this newfangled alternator shouldn't  
need it).                                                                      
                                                                               
>So we should have                                                             
>12V ---> ignition switch---> (run position) -->light--->alternator 'red' 
>wire->                                                                         
>     
>alternator.                                    
>Clear as mud? Let me know - I can try and draw you a schematic and email it.   
>                                                                              
>Theo                              

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