Battery Light stays on.
 

Howdy,last night as I was coming home the battery light came on briefly at low speed but then went off when I went faster.When I got home and shut it off the battery light stayed on.Checked the fuse box to see if anything was weird and found the C fuse had blown,replaced it but the light stayed on.Called the mechanic today and he said alternator,I asked about maybe voltage reg and he said the light would go off if that was it but it stays on if its the alternator.Any suggestion ,hints info on this.I have not checked how much its charging but when you start it up the light goes off.Thanks

Easiest way to check is to turn on the heat/ac/radio and =yank the battery cable off after its started. On gas cars it just dies but on the diesel you'll have to wait until the drain from the accessories does the job. Either way it sounds like youve got a bad alternator or regulator. However aren't they the same component on these cars?

Most parts shops will have a tester that can be used while the alternator is in the car. Stop by and have it tested, if you can.

If you're bored, you can pull the regulator and check the brushes.

My light had come on dimly, and would sometimes go out for 10-20 mins. On the end of the second day, the light stayed on. Had to get a jump to get home. Lucky for me, I had a spare alternator - "borrowed" the brushes from it, and all was well again :)

PS: I think they are flathead screws, and the one will need a stubby to get to it.

An easy test for alternaters....take a screwdriver, and hold it against the center of the back of the alternator...you should feel a slight magnetic pull if it is good.

I had one where the light would come on when I took the key out of the ignition. It turned out to be a shouted diode in the rectifier.

Had same problem last friday
 

Just had the same problem last friday on the 300SD. It was the alternator. Easiest way to check is to use a voltmeter. Check battery voltage with the car not running. A fully charged battery should be around 12.8VDC. Anything less means the battery is not fully charged. Anyway, with the car running the battery voltage should be around 13.1~13.8VDC meaning the alternator and voltage regulator are working fine. If the battery voltage remains the same as when the car was off than the alternator is bad. Not worth the time to try to tell if its the voltage regulator or brushes, just replace the alternator. About $75 at Schucks, unless you have a spare lying around and nothing else to do. Also check for tight and clean connection at the battery posts and all other electrical connections. Not rocket science, just basic electrical troubleshooting. SKI

I agree with Larry, It usually 90% of the time the brushes on the regulator are shot. Just replace the regulator. You don't even have to pull the alt on a 300 or 240.

Be very careful, there are spinning belts in that area and you don't want to get the screwdriver caught in the alternator.

Do not do this. The regulator uses battery voltage regulate the voltage out to the vehicle and you can spike / burnout all the electronics in your car doing this. There is a reason why there is a tag on new alternators that say to have to fully charged battery before instal.