Then riddle me this... Why does my ignition light never come on? The bulb
is good. The car runs great. But no light. Not at startup, not with the
key on (but the engine off). Never.
I have tried several times to find a solution, but have had no luck. Since
the alternator and battery are both working well, I haven't worried too
much; but I know I am testing the fates.
The car is a 1978 MGB with (I presume) standard wiring.
Any input would be greatly appreciated.
From: email@example.com [mailto:firstname.lastname@example.org]On
Behalf Of Paul Hunt
Sent: Tuesday, November 16, 1999 1:25 PM
To: Charles D. Sorkin; MG List
Subject: Re: A charging question
The ignition warning light (red, in sun-challenged climes) is indeed in
series with the alternator field winding and is used to give the initial
exitation to the field to start the charging process. Once the charge has
started it is self-sustaining.
It wasn't designed that way so that if the bulb failed it would strand the
car, but because it is used to monitor the field winding voltage. You can't
just 'connect them in parallel' because the ignition warning light would be
on all the time and would not be indicating what the alternator was doing.
I suppose you could use a system of relays, but that just would just
introduce more points of failure. Instead we have a very simple, reliable
circuit that, when the red light doesn't do today what it has always done
for the past 25 years, we investigate.
In practice alternators will start charging without the bulb albeit only at
about 3000 rpm or so. Once started charging continues as normal i.e. at
anything above 900 rpm or so. I have seen a logical diagram that shows a
resistor connected across the bulb that would stop bulb failure affecting
the alternator charging, but this does not appear on any full circuit
diagram. If you want to protect against bulb failure then this is the way
to go. But it still wouldn't protect against wiring breakage unless it had
its own wiring as well, and how far do you want to go?