Jaguar Repair: Jaguar XJ6 4.2 series 3, voltage drop test, diesel battery

Question
Hello Howard. I recently had a new alternator and battery fitted into my XJ6 4.2. Sometimes the battery indicator light illuminates on the dashboard. However, the battery condition indicator/guage stays at or just above the 13 volt mark at all times, so I think it is still charging ok. It appears to happen when the car is stationary and idling or driving slowly but when I get up to speed the light goes out. It does not always happen, perhaps a couple of times a week (if that) and I use the car as my everyday vehicle. Any ideas on what the problem might be? Perhaps the battery is not the correct specification? ....it's a 12V 69A 520Ah battery. Any help would be much appreciated. Thank you.

Regards,
Martin

Answer
Hi Martin,
To start with, the ser III Jag had a odd problem from birth and that is that the car needs a "Deep charge" battery (diesel battery)to operate correctly. This may or may not be related to your current problem.

If this car was brought to me with this complaint in the dealership, I would have to run the following tests.
First check that the newly installed alt had the belt tight.
Then I would put a volt meter across the battery terminals (not the cable ends) and read battery voltage and then start the engine and set the idle at a fast idle (1200 to 1500 RPM) and read the volt meter. (13.8 to 14.5) normal (you can't depend on the dash volt meter for diagnosis)

When the red light is on at any time it means more current/voltage is being used by the car than is being produced by the alternator or at least getting to the battery, for what ever reason.

If the idle is set to the correct RPM and all tests so far show "OK". I would do voltage drop tests on the positive side and the negative side of the system. (battery POST (+) to Alternator (+) large spade terminal on the alt. (Not an easy task)
Then run a voltage drop test on the negative POST of the battery to the alternator case. If you are not familiar with voltage drop tests, You just connect the two leads of your volt meter on each end of the same cable being tested. Put your volt meter on a low voltage scale and put the (-) lead on the (+) POST of the battery and the (+) lead on the large flat spade terminal on the alternator while the engine is running. (this seems backwards but when the alt is charging it is producing more voltage than the battery.) You should see very little voltage (less than .5v)

Do the same test on the negative side (with the leads reversed)
Somewhere in these tests you should find your problem.

Let me know,
Howard