Fix one thing, get two more problems...

I've had vibrations at highway speed preventing me from driving much above 50mph for a while. Finally sprung for a set of OEM wheels and high-quality tires, fixed that. Took the XJR for several miles of highway blasts, no issues up to 70mph. Awesome.

Get home, notice dash lights seem dim. Revving seems to improve them, but only sometimes and only momentarily. Put a multimeter on the battery, 11.5V. Park the car and pull the battery to stick it on a charger (It's a new battery, and wasn't much discharged, but I figured it couldn't hurt). Next day I lifted it up and looked around the alt wiring connections. Alt has some gunk on the outside from an oil leak from the PO. Connections seemed good, main positive lead was shiny and clean. Pulled the smaller connector and contact cleaner'd it, then put a dab of dielectric grease on just to be safe. Gave the engine to frame ground strap the deluxe treatment as well while I was under there, pulled it out, sanding, cleaning, and greasing both lugs and mount points thoroughly.

Started it up, voltage read 14V. Good. Drove a few miles to get it warmed up, same issue happened when I stopped/idled. This was all 30mph local street driving too. Got home and ran through all the connector troubleshooting, all read good. However, battery metered very low (around 11V) but would randomly jump up to 13.5-14V and then fluctuate between there and 11V. This was noticeable through the trunk lights flickering in brightness as well.

Is this classic dead voltage regulator symptoms, or could there be more going on internally? It seems to be a standard Denso regulator that I can buy online for 30 bucks, and easy to install. If it's not that, or if it sounds like an internals issue, I'll go and try and pull and alt from the junkyard later on, take a chance on that.

 

There are easy tests -- load test the battery and the alternator. The system is simple.

If you need parts make sure they are real Denso .... If you have a good yard and bring doing this yourself .. low miles use is a reasonable way to go. The OE alternator is high quality -- IMO used is much better vs cheap rebuild (and most are cheap rebuild)

 

I know the battery is good, and believe rest of the charging system wiring is good, so I'm pretty sure it's the alt itself. Just wondering if anyone who's done alt rebuild/diagnostic has seen behavior like this before common to a certain part of the alt, or something worse like bad/dying windings. I'll probably just roll the dice with the junkyard alt tomorrow, worst case I'll have a spare alt to pull apart and practice a rebuild on. I'll look through the JTIS electrical schematic for the alt guts again too, see if something jumps out at me...

 

I think not. It is a Denso, but somewhat unusual. If you do find the cross
please post. I have never seen an exact confirmed match.

Try measuring the output voltage at the alternator output or at the
bulkhead connector using a DVM on AC volts range. If you see AC,
it is ripple caused by a dead diode bridge. I have had a shop press
out and replace the diodes with uprated diodes for just a few dollars.