When the Alternator is spinning, It Always outputs battery voltage from the warning light terminal, So whether the Diode pack has failed or not, It makes no difference.

Think about it like this, If the alternator is spinning, The Warning light terminal is outputting battery voltage, The Warning light bulb then has battery voltage on both sides so the light goes out.

If you turn off the igntion, How does the alternator know that you have turned it off. ?? it doesn't do anything different, It still keeps outputting battery voltage up the warning light wire until the engine has stopped.

This is why on some diesels. ( remember the ford escorts ) They had to have a Diode installed in the Warning light wire, otherwise the Alternator did Backfeed and would keep the engine running by keeping the stop solenoid held open Simply due to the few milliamps that was flowing back through the warning light bulb back into the ignition circuit. ( Thats Because thats how an alternator works, not because it was faulty )

As Roy Says This does not apply to the Vectra C as the alternator warning light wire goes to the Ecu, Not the Dash, But I would think the same Scenario applies. The test method for the Alternator is Exactly the same as a Conventional alternator.

H.T.H. Tony.