A quick explanation of my situation before getting into the question... My Mom owns a '96 Coupe Deville that's having its fair share of problems lately, but the biggest problem is that she's a nine hour drive away! So, I have to get all my info through her when trying help diagnose things.

That said, the latest problem is a SERVICE CHARGING SYSTEM warning on the info readout. The car starts out with a strong battery that can sit for days and still start the car fine, but as she drives the voltage drops like the charging system isn't working. Last time out she said it was down to 8.5 volts by the time she made it home, and this was without using the a/c and any other power that wasn't needed. To make things worse, she 70 years old and lives in Vegas, so a/c is very important! Anyway, she took it to a reputable shop in town and they spent two days trying to figure out the problem. The altenator checked out fine and they traced wiring to look for a short somewhere, but can't figure it out.

I'm leaving Sunday to visit her and was hoping to learn as much as I can about her type of car before going. If anyone has any ideas at all about what I should look for please chime in.

I'm fairly mechanical and usually do my own work, but I know next to nothing about Cadillacs.

Thanks and regards,
Dan
Sacramento, CA

Maybe a bad tensioner allowing the belt to slip, but you'd think a reputable shop would have found that. Given the heat in Vegas and the toll it takes on batteries perhaps the battery simply won't take a charge any more and is headed south.

If the battery were going bad, wouldn't that show up when starting and not while driving? The warning comes on while driving and the voltage continues to drop (according to the display) the longer the car is driven.

I'm leaving in the morning, so hopefully I'll have an explanation when I get back next week...

Dan

You have to get the alternator tested under load, not just a voltage at idle test.