Alternator, White Wire, Where Does It Go?

[Carography]

I have rebuilt and installed the head on our 1979 2.3L Mustang
Starts and runs great but we are having trouble with the charging system.

The alternator tests good at the auto parts store. But it tests BAD on the car.

After further investigation, I found this loose plug (and white wire) coming from the alternator. where does it go?
thanks

[ashley roachclip]

I believe that goes to the choke.

[Carography]

I believe that goes to the choke.

if that is the case, would that be the reason the alternator test bad in the car (below 12v) and good at the auto parts store?
Is it a ground wire?

[Walking-Tall]

That wouldn't be a reason for the alternator to "test bad in the car". Approximately 9-volts comes out of that white wire, and it's for going to an electric choke, as mentioned earlier.
If you're getting below 12-volts at the battery positive and negative when the car is running, then yes, there's a problem with the alternator or voltage regulator...

[Carography]

If the alternator tested good on the bench at the autoparts store, then it must be the voltage regulator?