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Ignition Light

To: rogersj@worldnet.att.net, mgs@autox.team.net
Subject: Ignition Light
From: Simon.MATTHEWS@st.com
Date: Wed, 12 Jun 96 09:23:48 +0200
     John asks what this does, well I will attempt an explanation. 
     
     The generator on a modern car (alternator) has 2 sets of windings 
     (coils) and no magnets. Thus, in order for it to start producing 
     electricity, the field coils must be supplied with a small current 
     from the battery. The car is wired such that this current passes 
     through the ignition light. 
     
     Once the alternator is producing electricity, it supplies current for 
     the field coils by itself. At his time, the ignition light has 12 
     volts on both sides so no current flows and the light goes out.
     
     It is important to note that if this bulb (or the wiring to/from it) 
     fails, your alternator will not work!
     
     So why can the light be misleading? Fundamentally because it says 
     almost nothing about the state of your ignition system and only some 
     about the state of your charging system. 
     
     If you look at what is also connected to the ignition switch (and 
     hence the ignition light) -- many accessories are. Thus a fault in 
     another wire can cause current to flow from the generator BACKWARDS 
     through the ignition light and into the accessories. This will cause 
     the bulb to light, but because of the bulb's resistance will not 
     supply enouh current to operate the accessories. 
     
     This problems happened to me (but I hasten to add, no Lucas products 
     were involved: the make was Alfa Romeo).
     
     In John's case, I would look fo some kind of short -- but as to where 
     - well start the easy way -- take out fuses until it stops?
     
     Hope this clarifies things a little. Of course this does not describe 
     the situation on cars fitted with dynamos. Someone else will have to 
     fill that in.
     
     Regards,
           Simon


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