When you measure voltage across the "I" terminal with the the blue wire from he discharge lamp connected, you are essentially measuring the voltage drop across the regulator to ground through the internal resistance of the relay within the unit.
You must check the voltage at the wire connected to terminal "A" on the regulator to verify you are getting battery there. With the engine running and a properly working regulator and alternator, the battery discharge indicator lamp in the dash will light up if it does not "see" the battery on terminal "A", which is connected to the positive terminal of the starter solenoid.
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