Alternator works but...

[wolff at turboquattro.com]

Ok, it turns out the alternator is working. I get 13.8 volts at the battery with the car running, however the dash indiactor lamp operates in reverse. It is off when the ignition is on without the engine running. After I start the engine it remains off untill I rev the engine, then the light sticks on. Also the tach does not work. Any suggestions on that now?
Thanks,
Wolff