aeb28

Looking at a 2004 SL600 trying to troubleshoot a "Visit Workshop" red indicator with battery symbol. Nearest dealership is 4 hours away, trying to do what I can before I would have to go that way.

Both starter battery (under hood) and accessory battery (trunk) are not old, hold charge (@12.8v), and have been fully charged. Don't seem to be the issue. Terminals have been cleaned and inspected. There is a parasitic draw that I have yet to diagnose that can drain the battery after a couple weeks if I am not careful and car sits. In the mean time I just run/charge it from time to time.

When running, alternator puts out steady 14.1v.

I put an OBDII reader on it, got 2 hits. First, a (seemingly) unrelated P0140 pertaining to the smog pump that I have been aware of for a couple months.

The second is "unsupported" so I can't read it, I do not have a STAR diagnostic. Pretty sure it is whatever code is causing the dash light.

Any suggestions?

kbob999

I've had similar electrical issues, so I'll take a stab at where to look.

When the car is running (14V to the rear battery), do you have similar voltage at the front battery as the rear? If not, voltage is not being supplied to the front battery, and you may have the Battery Control Module not feeding to the front, or the fuse under the passenger floorboard (under the Bose subwoofer) potentially blown.

As for checking the BCM, I'm not even sure how to do that w/o a Star. You can check the fuses (larger one feeds the rear battery from the alternator; the other feeds the front battery from the rear) with an ohm meter, but it is a pain to get to with the subwoofer in the way.

In my case the alternator had failed, and the car was slowly draining voltage when driven until the red message came up. I had checked the fuses (good), and even changed a relay by the BCM (also good), until the alternator was found to be the culprit (it was only 3 months old!).

The parasitic drain is normal for the R230 line. You need to keep a charger on the rear battery when not driven frequently.

Also - the rear battery must always be disconnected first, then the front. When reconnecting, do the front first, then rear.

Let us know what fixes it for future reference.

Lee Wireless

I've had the same issue on my wife's SL going on a year. Red battery light comes on when vehicle started and stays on until ignition shutoff. Doesn't occur again for a few weeks. Both batteries check good, Alternator checks good and no other issues/codes. Had dealership scan and found same. Said only remedy is to change Battery Control Module for $1500. Not happening. I suspect this is a common issue and changing the BCM is a waste until something actually fails. Good luck.

aeb28

Thank you for the replies.

As far as the front starter battery, yes it is getting a good 14v from the alternator. Correct me if I am wrong, but that rules out fuses and alternator itself?

By elimination that would leave the BCM which I couldn't check without star. As you mentioned Lee, it seems like everything is as it should be except the warning light. I really hope there is a way to correct it or that will drive us crazy. So far it has not shut off at all, it is just on all the time with the car running.

kbob999

14V at starter rules out the fuse and alternator. Sounds like another culprit, like the BCM.

If it drives you crazy get a used one off eBay and swap it. There may also be something else causing this, like a relay, so you may want to try something cheaper first, or find a local with a Star (SDS). Last resort, see an Indy, or a dealer. Be prepared to shell out dollars for that one.

Check out bayhas.com and his media server for more details on the electronics and systems of this car. He is active on benzworld R230 forums, and knows this car well.