See Dana's post. ' In car' is too vague. The location of the probe connections and the load at time of measurement will make a difference.
Poor output at low rpm can happen when one of the windings open or one of the diodes is out. In that situation, 2/3 of the alternator is still working normally so at higher rpm it can supply moderate demands, but really struggles to do so at low rpm. A diode out is more likely to show ripple voltage and look like pulsing than an open winding connection. Only point I'm making is that it could be alternator.
