My newly purchased '89 300CE isn't charging. 12.5V at the battery with engine off, 12.0 with it on so the alternator isn't supplying juice. I replaced the voltage regulator because the brushes were worn to nubs, but that didn't help. The battery indicator light in the dash isn't coming on with the ignition - in fact none of the lights in the instrument cluster are coming on except SRS. I pulled the cluster and checked the battery bulb and it's fine (6 ohms across contacts and it lit up when I touched it with a 9V battery).

I'm about ready to buy an alternator, but I wonder if something else is going on since the idiot lights in the cluster aren't coming on. Any suggestions?

Just FYI.
The original alternator on my 1988 300CE failed on me a few months ago. I have 135,000 miles on the odo. Nothing lasts forever. Not an expensive repair. Get it checked out before you get stuck on the road at night.
The night mine failed, the first bulb to light was the SRS. About a mile later another came on, then the radio died & I knew it was gonna be a AAA ride home.

Somehow I managed to create 2 threads.

It was the alternator. The reason I didn't swap it out right off the bat is because the charging system schematic had me confused and I couldn't understand why the idiot lights wouldn't come on with a dead alternator. After tracing all the wires from both sides of the battery light and some help from this site I finally realized that with the car off and ignition switch on the battery supplies 12V to one side of the bulb and the other side grounds through the alternator. So if the alternator fails in a way that disrupts the ground path the battery light won't come on. I'm no electrical engineer, but that strikes me as a pretty silly design that lets the alternator fail without giving the driver any indication. I guess it was pretty common in that era though, although I don't remember it being the case on American cars.

Glad you solved the problem. Good luck with your CE. I love mine.

http://www.peachparts.com/shopforum/tech-help/21416-300ce-e320-coupe-owners-roll-call.html