johnmh

I have a low mileage 2004 E320 which I was driving it home last night. I saw the error message briefly as I was parking, this morning it would not start. No problem, battery.

The main battery tested ok under load and voltage. I replaced it with a new one, it still would not start. I put the original battery back (it tested ok and is pretty new) and the car started no problem about an hour later. Seems to be running ok now. Weird.

My tech found no error codes except one concerning the ignition lock out. He cleared that error code and it did not come back after a drive. I doubt the lock out was the issue, it wanted to turn over, but lacked the current / voltage (felt like a dead battery).

Other than a possible bad starter motor (or solenoid) am I missing something? I am not used to cars with this level of electrical complexity.

Greetings to all from a new joiner.

winter211

To me this sounds like a connection issue somewhere. I think the car is losing connection to the battery and was running off the alternator for a moment - not good for any modern car let alone a Merc. Make sure the insides of the terminal connections are clean. Next time your car won't start, check voltage at the starter with a multimeter. Should be 9V while cranking minimum preferably more.
If voltage is low, check voltage at the battery as well while cranking. I would be concerned with any more than 0.3V drop from batt to starter.
if voltage is good, likely a starter issue. Whack it with a stick see if it spins. However I really don't think the starter is the problem here because of the dashboard message. You could do a voltage drop / resistance test on the battery wires, preferably with a load on it as a wire with only a couple strands functioning can show no resistance with low load but won't support real current. In fact I would put your car to ignition on and get as much current going as you can - high beams, heated seats, radio, defroster - if your car shuts off then that's definitely a connection issue because a new or good condition battery would support that for at least 10 minutes (don't do it for 10 minutes).
I also don't think this is a battery issue since two batteries had the same problem. This is wiring, hopefully, or computers.
Did your car have previous owners? Look for shoddy jobs where power wires are, like amp wiring or remote start hackery. I had a car that wouldn't start sometimes because the PO just twisted together a couple ignition wires and duct taped them.

CaliBenzDriver

Couple pointers based on your power distribution issue:

Ask your tech to measure engine to chassis drop voltage under loaded condition.
Is it less than .30V?

Measure charging voltage under load at battery posts.... above 13.7V?


I am guessing your engine GND strap now needs to get serviced.

Also consider adding a 2nd GND strap in addition to existing but in a higher, more dry location near engine top/side.

Getting a reliable GND will help improve power distribution with low drop voltage right at the source: most loads reference chassis not engine GND. Hence a good strap is essential.

cetialpha5

You can also check the voltage of the battery on the dash and see what happens when you crank it. Switch to the temperature display and hit the reset button 3 times with the car not running and it will display the voltage on the dash. You should also check the status of the aux battery and in addition to the alternator, the older pre-face lifted cars also have a battery control module which might also be bad.

CaliBenzDriver

How can you find out for sure the BCM is faulty on these W211 ?

konigstiger

Here's one way.