I have an 87 XJS V12 with a unique problem.
With the car turned off the Overvoltage light is on. With the car running the light is off. Charging rate is 12.55 volts, a little low but no light. The power drain with the car off is 1.2 amps. The battary goes dead in a few hours. I have unplugged the overvoltage relay and the light is still on. According to the wiring diagram in the shop manual this can't happen, but obviously it can. Does anyone have any thoughts how to isolate the cause of this problem?

PS: I have downloaded and read most of "The Book"

GDK