Is my alternator going bad?

I installed a new blower motor for the heater and air conditioner system on my 1991 Chevrolet Blazer 2wd with a 4.3 v6. Now when I turn it on it pulls the voltage way down. It is grounded like it is supposed to be. The headlights do not affect it when I turn them on and the voltage stays around 12 volts when the truck is running. But when I turn the new fan motor on it drops the voltage down to like 8 or 9. Is that a sign the alternator is going bad or some other problem?