2003 SL500
 

This car will drain the battery enough over a period of 1.5 days. Both battery
test good.The charging system tests good at the battery. I am not sure where to start diagnosing this problem. Any suggestions?

That car is chock full of different modules that all talk to each other on a couple of data bus lines. They don't all shut down when the ignition is turned off. After a short bit though they are all supposed to "sleep" once their "after run" functions are done. However when there is a problem sometimes a module will fail in such a way that it puts noise on the data bus lines. This noise will keep other modules on the data bus "awake" draining the battery the same as if you had left the key on in a conventional car.

This is a very difficult scenario to diagnose without detailed information and specialized tools.

duxthe1 is correct!

Somewhat difficult to diagnosis, the most common issue with that car is the rear SAM staying awake & allowing the interior network to not go asleep.

You could disconnect the power wire to that SAM & see if the battery is still being drained.

What all is on the interior network? Where is that specific SAM? Is a new one plug and play ?

Where is exactly is that SAM? I have noticed if I don't lock the car all is well in the morning......thanks for your response.

Hmmm,

So if you lock the car, the battery dies quickly.

If you do NOT lock the car, the battery is fine and does not die quickly.

Maybe the anti-theft system or whatever module that is connected to is dead?

Did you post this in the SL forum? You may get some more people to chime in.