The battery on my 300 recently started a slow draining process. They battery has been load tested and is fine. No charging codes are showing on dash. When I run the diag. through the HVAC control it show 11.6 volts. I have seen different ranges so not sure if that is too low. Should it be closer to 14?? The next question is would it be more likely a failing alternator or something else?? Thanks for the help guys.

You must do some diagnostics before this can be answered.

Presumably you have a DVM that can measure amps?

Remove the negative battery cable and connect the DVM in series between the battery and the cable. Set the DVM to the lowest possible scale and make sure everything in the vehicle is OFF. Be sure the doors are closed.

Read the meter.

If the meter reads more than 90 milliamps (hopefully less), you have a "leak".

Now start pulling fuses, one at a time, until the meter goes down significantly (it won't go to zero due to the clock and some other functions that might draw a small current).

If the meter never drops, then go and disconnect the alternator. See what the meter does.

Had the same exact reading on my 96E300d, and it turned out to be a bad diode in the alternator. In my case, battery disconnected, had no problem holding a charge. Connect it back up, and it would be almost dead withing 10 hours. With the engine off, and easy test is to disconnect the wires from the alternator, and then using your multi-tester in the continuity mode, touch one lead to the battery connection on the alternator, and the other lead to the alternator case. If you have continuity, you have a leaking or bad diode. Good luck!!

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96 E300d