The VR should maintain a constant voltage to the system assuming the alternator can supply sufficient current.
Essentially the VR will increase or decrease the voltage/current to the field to maintain the correct output voltage from the alternator.
Think of it this way.
Your battery is a bucket.
It has a small hole in the bottom, (that is the load of the car, headlights, ignition, etc)
Your alternator is a pitcher,
when the engine is off no water is being poured from the pitcher into the bucket. So the water level in the bucket starts to drop.
With the engine started, you are pouring water from the pitcher into the bucket. At idle you are pouring a very small amount of water into the bucket, not enough to equal the amount going out of the hole in the bottom of the bucket.
Now you are at a higher rpm and you are pouring more water from the pitcher into the bucket faster than the water is leaking out the hole in the bottom, and the bucket is slowly filling up. to keep it from overflowing you start pouring a little less water into the bucket. Untill you are able to pour just enough water into the bucket to maintain the level in the bucket.
You are the VR!
By tipping the pitcher more or less you are changing the volume of water going into the bucket.
The fact that the pitcher is constantly pouring way more water into the bucket than is running out the bottom is the issue, nothing that the engine or the rest of the car is doing can cause that
EXCEPT...
If the voltage to the VR is low it will try to tip the pitcher more than it should and over charge.
To check that, check the voltage going into the VR when it is charging at 15 to 17 volts
I know that I am interchanging current and voltage when talking about the VR, but if the input voltage to the VR is low it will think that the alternator needs to increase in voltage so it will cause the field coil to be fully energized more of the time causing a higher output voltage.
I hope my wandering example is not too confusing
