Be sure you check WARM. VRs are "temperature compensated."
Be sure you allow enough run time for the battery to become normalized.
In rare cases the battery itself can cause this
In some cases the VR can be "out of spec"
But MOST of the time it is a bad ground or MORE probable,
"voltage drop" in the positive harness
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Ground. It matters not how well sanded the firewall and VR are. (Actually it does) But what DOES matter after the VR is mounted clean and tight, is whether the firewall is "actually" the "same as" the battery negative. Essentially you want a nice big bonding cable from the battery neg (or the block) to the firewall.
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Voltage drop in the harness: The VR center pin is not only power to the VR, it is the voltage sensing. So the voltage the VR "sees" between the center pin and it's case (ground) MUST be "same as battery." Typically, "it is not."
As this voltage drop increases, providing less voltage to the VR, the VR "says to itself" "HEY the voltage is LOW let's kick it up"
So the VR regulates so the center pin is the set point...........13.8----14.2 ..........and because there is voltage drop in the harness, this equates to the V at the center pin PLUS the voltage drop = the V at the battery
IE if there is .5V drop in the harness and .3 drop in the ground, you ADD .8V to the charging voltages for perhaps 15V at 14.2
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How to check?
EASY
Warm and charge the engine / battery get everything "normal." Recheck charging voltage to be sure the problem still exists
With engine running at good fast (cold) idle or to simulate low speed cruise, make the following test with all accessories off, and again with heater, lights etc on
Stab one meter probe into the battery NEG post. Stab the second probe into the flange of the VR. Be sure to "stab" through any pain, chrome, rust. With the meter on low DC volts, you are hoping to read VERY little voltage, the lower the better, and zero is perfect. More than .3V (three tenths of one volt) is too much
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Harness drop. This can be done a bit different.
Access a point "same as" the VR voltage supply, "switched ignition." This can be the "key" side of the ballast, the blue field wire, etc
Turn the key to "run" with engine off. Hook one probe to battery POS post, the other to your switched ignition. As before you hope for a very low reading, and more than .3V is too much.