Diode in the alternator blown (if it's an alternator)
Voltage Regulator deceased (if it's a generator)
This isn't an exhaustive list, but it's where I'd start investigating.
Guy Weller wrote:
> 
> Most times the ignition warning light starts coming on it does it
> stealthily - like a bit of a flicker, a glow or glimmer.  Maybe it
> plays games after you have had some of the wiring circuits changed.
> This time, suddenly, cruising along at 3500 rpm and it just came on;
> bright and red, and stays on.
> What would cause that?
> 
> Guy
> (other than broken fan belt or angry Lucas LoD ?)
 
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