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
|