I have an odd idle voltage issue with my 1992 XJ. I don't think it's the alternator, I'll go into more detail as to why:
When I'm idling (headlights on or off, fan on or off - doesn't seem to matter, just affects how low it goes) my voltmeter drops like a rock. I'm steadily at 14v at startup, and if I drive it's fine, but when I'll come to a stop, my volts drop to around 12 (sometimes 10-11 with everything on). If I give it a little bit of gas and let it drop back to idle, the volts are fine at idle again, even if my headlights, blower etc are all on. I think it may be the voltage regulator or something else in the charging system - the diode or whatever that attaches to the alternator maybe? If so how do I replace it?
It was doing this before I replaced the alternator (which was buzzing/whining) and seems to be doing it just as much now, so I don't think it's the alt (especially since giving a touch of gas to get the volts up again, then dropping back to idle, will let it run just fine). My lights dim visibly and it runs a little rough when the voltage drops way down, not surprisingly.
Does anyone know where the voltage regulator is, if it's not that piece attached to the alt wire? And what is that piece, and how does one replace it?
The voltage regulator is part of the ECU , the engines brain. I would first check all battery and ground connections, including all connections at the fuse block. Check the plug that goes into the ECU.
Did you check your block to firewall ground? How about the grounds at the oil dipstick bolt. It also wouldn't hurt to place an additional ground off the battery (-) to fender. Shining up and refreshing my grounds (ala Cruiser54's write-up) fixed my voltage problems. My gauge is now straight up 14 unlesss I'm running everything at once, even then it just drops a blip.
1990 XJ Laredo - White & Chrome
1988 MJ Pioneer - Metallic Green and Black