I was driving with an old battery (I know not a good idea) but the battery light just came on so thinking I knew the problem I got a new one. The light is still on, it doesn’t start to dim after a while like an alternator so my question is: are there any common places that s3 jags drew power? I’m running around 11.5 volts and the car starts right up and behaves normally apart from the light. Also is there an obd port anywhere on this thing? I know it was made before they were standard but there must be some way to read the codes the car throws right?

 

Hi,
No codes here, and calling the fuel injection control unit ( ECU ) or the dash trip "computer" a computer is a BIG stretch ! From what I have read, they are hardwired logic circuits, timers, counters etc that can only be modified by changing physical components IE no software, programs or data in memory ( other than counters for mileage etc. )

The battery light only comes on to tell you the system voltage is low, NOT that the battery is bad.
It can be a bad battery ( which you have already changed ), but most likely there is a bad connection on the alternator wiring.
The next possibility is a bad alternator or the belt is slipping under load ( should be able to tell easily, belt really loose or squealing ).

First I would check the voltage stopped and running at the battery with a volt meter. It should go up to 13-15 volts while running. If it does but the battery light stays on / dash gauge still reads low, then the problem is in the wiring to or inside the dash instead of in the engine bay

Gotta run, I'll be back later if you need more info !
Cheers,
Brian

One final thing, if the battery OR alternator goes bad it can soon kill the other one by overloading / overworking it, so always try to "nip it in the bud" !

 

yes, a bad battery can damage a good alternator and a bad alternator can damage a good battery.

Assuming the problem just started happening, I doubt wiring is the reason. In the Series 3 XJ-6, the warning light comes ON only when there are battery or alternator problems. The alternator's voltage regulator might have been damaged

Solution is to remove alternator and bench test it.