The red ignition warning light on my 1.5 litre 1948 MK IV has started to come on. Initially I thought it would be the fan belt - but this seems to be at the correct tension and the dashboard ampmeter shows a nice positive charge and a meter put across the battery the correct expected voltage. This only occurs above 1,500 RPM. The red lamp is wired correctly to Jaguar drawing - so any suggestions please of why this fault should now happen and what to test or check?
Below 1,500 RPM the red ignition light is off.
Above 1,500 RPM the red light now comes on.