So me old jag is acting a might strange. Has a new battery, all seemed fine, but I don't drive it much. It all started when I was driving around and noticed the voltage gauge seemed a bit low (12-13) and the needle did not seem to move much. a couple weeks later I drove to a friends place and noticed nothing until I was about to drive home, started the car just fine, halfway down the driveway it just instantly died. It would turn over but would not fire, until I fully removed the key and reinserted, then it started right up. Then on the 20 minute drive home I noticed a belt kept squeaking and the battery voltage kept cycling from 13 to 16. Once home I checked the alternator belt and sure enough it was too loose. I tightened it and took a quick test drive, everything seemed great. Yesterday I was out and now notice, at first when I start the car the needle is about 13.5V, then at some point it climbs up to 16 and just sits there. sometimes at a stop sign it goes back down to 13.5, but then steadily rises to 16.


I need to get out there with my volt meter and do some checking. But until then, what do you think....bad gauge? bad alternator? bad battery? bad voltage regulator? ballast resistor? I can't find my repair manual.

 

In my opinion, you need some data from which to draw conclusions. Real time, real life stuff.


I made a patch cord for my VOM. To connect via the cigar lighter. Just plug it in, drive around and see volts under various loads and RPM's.


In your case, it is possibly the fascia "indicator" or the internal regulator in the alternator.


I think the key thing is another unrelated anomaly. Ignition switch needing attention.


Gremlins in these cars abound...


Carl

 

Ok Carl, I did a voltage test with my digital multi meter connected directly to the battery. Not running the batt was at 12.8 volts. I started the car and let it idle, the DMM climbed to 16 volts much more quickly than the gauge but it also went to 16. Then I held the throttle at 1500 rpm and watched my DMM go up to almost 18 volts!


I am surprised I did not catch the car on fire driving it like that yesterday!


so it seems to me this thing is running open loop for some reason, I guess a bad voltage regulator? Yikes the only one I see online is over 300USD


I need to get it to a car show this weekend so maybe I need to disconnect the alternator for now.

 

Sounds as if your Alternator is overcharging. It could be a faulty voltage regulator.
A funny thing happened to me a while back. I was testing some circuits with my DVOM and was getting high voltage (more than it should be). Turned out I had a bad battery in my DVOM. I replaced the DVOM's battery and all voltage readings returned to normal.
As per my manual if you have a BUTEC type alternator, you should have an external BUTEC regulator.
If you have a LUCAS 20ACR type alternator (1971-1974) the regulator is internal, which would mean alternator removal to replace the regulator.
There is also a LUCAS model 18ACR (69-71) that uses an external regulator.

 

It has the 20ACR.


Looks like this is what I need


Jaguar XKE V12 XJ6 XJ12 & XJS Lucas 20ACR 14TR Internal Alt Voltage Regulator


or this


https://www.amscovf.com/REGULATOR-12..._p_310899.html


OR...I wonder if I can just remove the lucas reg, run some wires out of the back of the alt, and attach an "off the shelf" external reg.

 

Naah no fire. But, tis odd that you've not fried a battery or had a plethora of bulb failures!


Unless as mentioned, the meter is also nutty.


At twenty bucks. that is where I would go. replace the external regulator. Why mess with a "jury rig'.


For the show, disconnect it and start only twice. Or merely increase the load. Turn on the head lights and all else electric that comes to mind.


Carl