Hello
Have ongoing issues with 1997 XJR and starting issues. First off I have replaced alternator and battery, still unable to keep battery charged. Sent to mechanic and he suspects electrical draw somewhere but cannot find. He gets a good charge on battery and it runs fine. I put a on/off switch at the battery to kill the electrical demon sucking the life out of my battery. Start the car every week and it starts fine, it's not a daily driver. I let 2 weeks go by (bad mistake) and now unable to start. Doesn't even turn over. The voltage at the battery is 12.4V with 675 CCA out of 825. The battery light is on and the voltage gauge in the car shows about 10V. No matter how much charging I do or jump starting it will not turn over. When I have it connected via jumper cables to another car the voltage in the car will show 12V and not start. When I have it connected to a battery charger it will show 10V. I have checked all the pertinent fuses and relays. I am stumped at this point. Any suggestions would be welcomed.

 

Does the starter motor engage?

 

No

 

Find a mechanic who's got a better understanding of electrical systems (or who will spend the time to diagnose properly). Standby draw can be measured, start at the battery and work backwards from there, dividing into fuse boxes, then individual fuses can be checked for draw. There's tons of resources on these forums, especially the "large files section", as well as jagrepair.com (there's electrical diagrams there), and many others.

Here's the first result on google that comes up with some principles of testing if you're interested in doing it yourself: https://axleaddict.com/auto-repair/car-batterydrain

The fact that it's reading 12.4v at the battery, but only 10v in the car (assuming a cigarette lighter gauge or OBD reader?) could mean an issue in the main power wiring, bad ground, etc. It's worth checking the mega fuse (right near the battery along the positive wire in the trunk), bulkhead terminal (under the hood, RH side) and grounds. Measure voltage at the battery and compare to the reading at the bulkhead terminal.

Hard reset is always an easy quick thing to try before all else (disconnect battery, touch positive wire to ground to drain/reset circuitry)

EDIT: if your car is the inline-6 XJR, you'll have better luck in the x300 subforums

 

Thanks for the advice.