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My car has a Lucas 18ACR alternator. When I switch off the ignition the ignition light burns. It goes out when I switch on the ignition. With the engine running it is out and I can see I am getting a small charge, but with the headlights on I get a discharge.

Is this an alternator issue or should I look somewhere else first before I change the alternator

Neil

Most likely cause is a failed diode in the alternator. A replacement should fix both problems.