Hi all.
Need some advice.
1984 XJ 12 series 3.
Car has only done 50.000 miles so expect the alternator to be the original (lucas 25 ACR)
The charging light remains lid (weak though) when the engine runs.
When I switch the headlights on it becomes brighter (Alternator charging harder due to load?)
Voltmeter in dash settles at 13 V in idle and driving.
Voltage level across battery is approx 13,2 V when not running and approx 14 Volt when running.
If I measure at the D+ of the alternator I get 2 volt with ignition on (exiting from ignition swith ?) but almost 18 volt when idling.
I Haven`t disconnected the leads from the alternator to the battery but will except it also to be higher than normal.
Is this a faulty voltage regulator in the alternator?

Regards
Thomas in Denmark

 

It could be the voltage regulator but I am suspicious that a diode in the alternator may be the problem.

 

Agreed there,

AND

Have look see at the Engine EARTH Strap Fiasco at the LH engine mount. Worked OK for the 1st 5 years, after that, good luck.

Clean it up, clean it again, and refit properly.

Run a separate dedicated earth strap, Engine to Chassis.

That alone has sorted many, many electrickery issues with most of mine.

Next, the +ve terminal posts each side of the engine bay firewal need dismantling and cleaning, DO NOT drop those nuts, they be some weirfUK thread.

The battery lead FROM the top post of the starter solenoid, thru the steel tube, TO the D+. Rarely an issue, but why not check it out.

Ignition Switch electrical section in dire need of a service. I have a PDF of that if you need it.

Good luck.

 

Thank you both for your quick replies.
What I can't get to match up is that meter reads 13V and voltage at battery is 14 when running and not 17-18 V like in this case
If the voltage regulator or diodes were bad I.d expect the voltage to be equal incorrect at both B+ & D+ but in this case it's only at the D+.

The voltmeter has ocasionally shown intermediant values in the past so somehow my suspecion is towards the ignition key switch or bad grounding.
I did more or less all of them when I got th ecar last year, but not the earth strap or contact surfaces between alternator and engine.

I'll let you know what I find as I get along.

 

And by the way Grant, yes I'd really like to have your pdf. conc. ignition key switch.Your experience has helped me in the past!

 

Here ya go.

I forgot earlier.

I ran a 2nd +ve wire from the Alternator direct to the RH +ve stud, AND, a dedicated Earth wire from the alternator casing to a nearby chassis bolt. I used, OZ speak, 6mm wire.

Slow and steady, one beer per 2 hours, problem solved, HA.