I recently replaced the alternator on my XJR, and after the car sat for a month or so, on start up, the electrical voltage gauge goes from 13 to max out (17). Checked the voltage on start up, and with in a few seconds, the voltage does climb (12, 13, 14, etc). Why would this alternator just go bad? I would say it has less than 10 hours of useage..... Ideas, thoughts etc. It was a factory rebuilt unit.... Not sure if it has any sort of warranty......HELP!

Did you do a battery load test and cell check? Does the battery voltage read 12.75+ by itself, no cables? Does the ECM on the XJR monitor the regulation of the altenator as some of the newer Jags? If battery is low it may produce the spike in voltage of the altenator to compensate for the engine electrical and the ECM/BECM controls. Are the engine to body grounds tight?