[Ian Comerford]

The external VR on my 79 SC went last year and the red light came on as well as the rev counter waving around like a mad thing. Initially I thought it was the alternator but then heard the battery fizzing after a drive. Checked the voltage at the battery with the engine running and it was about 17.5 volts, way too high. I think the answer to your question may be that you have to test it on the car, it would only take a few seconds of the engine running and would not damage the battery for that length of time.

Hope this helps

Ian