I've been having a problem with the battery in my 1985 380SE going dead. I had the alternator replaced in October. In mid-November, dead battery. Checked the alternator (which was rebuilt) and it's fine. Battery charged back up and held for a few weeks. Tech said it was a short, possibly in the interior lights. Well, now the battery is dead if the car sits more than 24 hours. I use a booster box and she starts right up, but if the car isn't run every day, the charge runs out. I disconnected the wiring harness from the main interior light to see if that made any difference, but it didn't. If I start the car with the booster, it charges right back up and will restart all day long. Once I park it overnight - dead.

Any ideas?

Divide and concur – install and amp meter between the battery & car (with all consumers turned off) – assuming you have something drawing power that shouldn’t you should get a reading of a couple amps.
Start removing fuses (from all panels) and see if the draw decreases; if you find a significant drop your problem is likely on that circuit.

When you connect up the ammeter, you are looking for a value of less than 100 ma. Hopefully, closer to 50ma. If it is more than this amount (200-300ma) then you must start pulling fuses to determine which circuit has the leak.

However, many times these situations can be traced back to the alternator. Disconnect all wires to the back of the alternator and see if the ammeter drops down to acceptable levels.

Have you had the battery load tested?

Can the alternator keep drawing power after the ignition is shut off? The alternator was replaced about a month before all this started.

Well, "drawing power" is a bit of a strong statement. I prefer to use the statement "leaking power."

Yes, a bad alternator can pass voltage to ground when the vehicle is not running.

Any chance you've replaced your trunk light bulb lately? I have the exact same car, and had the exact same problem a couple of years ago. I forget the details, but it's real easy to put the bulb in wrong so it's on all the time. Worth a peek...

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1986 560SL
2002 Toyota Camry
1993 Lexus

my trunk light switch went bad on my 126 and killed the battery after a day or two just like you. Just push the plunger on the switch and see if the light goes out.

Battery has load tested and takes a full charge. I strongly suspect the alternator, but the cars under chest-high snow right. Thanks for all the input.

Finally just replaced the alternator after lots of tinkering. Two days later - dead battery. Any other ideas.

What did your "tinkering" determine? What were the ammeter-test results?

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1986 560SL
2002 Toyota Camry
1993 Lexus

Had The Same Problem. Mine Turned Out To Be A Radio Amplifier That Stayed On Even Though The Radio Was Turned Off. My Amplifiers Were In Trunk So I Just Felt Them And Sure Enough One Stayed Warm. The Other Possibility Is The Antenna Motor Switch Stays Open. Remove Fuses To Antenna Then The Radio And See If The Battery Stays Up.

The alternator is not drawing any power when the car is off. They are baffled. Could it be that pesky old MB alarm under the passenger carpet. It's never worked in the years I've owned the car, but maybe it's sucking the battery dry.

Did you ever figure out the problem on your car?

As a "new" W126 owner, I have been doing searches on these cars to learn more about them for if/when I may have similar issues.

1984 300d Turbo 135k
2001 E320 43k

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