Intermittent and low voltage on alternator exciter wire

When the temperature outside is in the low 30's I get 0 volts on the exciter wire with it plugged into the alternator with the key on and with the engine running, this of course causes the alternator to not charge. After the temperature warms up to 50 degrees F, I get 2.1 volts on the exciter wire plugged in and battery voltage on the exciter wire with engine running. The no charge light on the dash comes on with key in on position, and goes off when engine is started with exciter wire having 2.1 v.
I don't know if the dash light goes off or not when there is 0 voltage on exciter wire. Will check that next time it doesn't charge. Am I supposed to have battery voltage at the exciter wire, 2.1 volts? Seems odd. Thanks.