Voltage Regulator replacement
 

Red Battery warning light came on. Fan belt still in one piece. After reading a bit, found that 8 times out of 10 the Voltage Regulator is the problem. Brushes usually worn down. I found the same issue. Brushes gone. Easy repair. Remove negative battery terminal. Just jack up the front-end, place two jack stands and remove the plastic liner. Remove the fan belt, remove top Alternator bolt and just loosen the bottom bolt. Remove three small screws from the back of the alternator and then remove positive and negative leads. Rotate the alternator clockwise as far as it will go (it's not much) and remove the plastic cover from the back of the alternator (if you have the compact alternator.) I used a 90 degree phillips screwdriver to remove the 2 screws holding the voltage regulator in. Just reverse the procedure to install. I also replaced the fan belt while I was at it since it was 4 years since I last replaced it. Took about an easy hour to complete the tasks. No more red battery light on the instrument cluster.

Replace regulator on Alt.
 

Ok I have a 94 sl500 i was passing a truck and was going pretty fast when I left off the gas some lights came on and car stalled.I preceded to start car again it started right up but I noticed red battery light was on and alt not charging battery..
I purchased a regulator so do you think this is my prob? I checked all fuses under hood and all good as I can tell.

Just bought a 2003 SL500 with 23,000 miles on it. (Actually first post after reading MANY and setting up as User). I have seen the red battery light but fiddling with several attempts at clearing from this forum I have somehow caused it to go away. I have read that it is likely the voltage regulator on back of alt, or Battery Control Module. I have ordered both. Also see where the only way to really delete the error is by using a Star Diagnosis tool. Just ordered it from China. The Red Battery code according to a document downloaded from this forum says this code is from the Starter Battery. The ALL SERVICES OFF code (or something like this) is the error from the rear Consumer Battery. (Have seen that one once). Rear battery does seem to drain 1.5V over night. Only had it for 4 or 5 days now, so... I have trickle chargers on both batteries now. Also saw that chargers need to be OFF when connecting them. Haven't driven it yet other than into garage. It is going to be a birthday present for the wife in a couple weeks. She has always wanted a red SL. (Sounds like it'll be a project for me!
ALSO have a SOS telephone error - PLEASE SUBSCRIBE error but it doesn't seem to be anything but maybe subscription or need a switch from analog to digital.