Could use some help troubleshooting a 2004 XJ8 with 105k miles. I'm getting ABS, DSC, Transmission Failure lights - typical of low battery voltage. The battery is only a few months old. I just replaced the alternator - it was original and the bearing was whining. Car was fine for a day, now back to the dash lights. The battery reads 12.8 volts when the car is off, but only 9.6 when running. What else should I be looking at?

I had the original alternator rebuilt at a reputable local alternator shop. Can that be tested in the car to rule out faulty alternator? It's a pain in the ***** to remove! Thank you!

 

Check your ground connection

 

Looks like the alternator is not supplying charging current. If there is only 9.6V at the battery when the engine is running, you should measure again directly at the B+ output of the alternator to rule out a cable or connector problem.

There are about 30 groundpoints on these cars. Wiring diagrams can be downloaded from jagrepair.com

Fritz

 

If you are measuring right on the battery terminals then you have a bad cell in the battery. It should not drop to 9.6v as soon as you start the engine.