Guys, Im at my wits end. This car has a brand new high capacity interstate battery. Alternator is good. Runs and drives great. Most of the time is will start just fine. Then, every few days, the car will be DEAD. Not even enough charge in the battery to light the dome light. This problem is intermittent. We fully charged it 2 days ago. Went to start it this morning, nothing!

It was at my independants for a week, and he couldnt find anything. It didnt die while it was out there. It is weird because this problem is so intermittent. Whatever is drawing on it, is pretty severe to drain this battery so fast. Its gotten to the point now where I dont trust taking this car anywhere.

Where do I start? Please help.

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K

Unfortunately, I don't know of an easy way to diagnose this. You really need a couple of good meters.... first, a standard VOM with the capability to measure 10A current via the probes... and second, a clamp-type DC ammeter that you can use on the battery cables to do a quick check for a major current draw. Then you have to check it all the time. When you measure a large current draw at the battery via the clamp-type meter, then you have to start isolating one circuit at a time (pull one fuse at a time, etc - don't forget the external ones for the HVAC blower fan, heated seats, fuel pump, etc) until you can find the circuit which is causing the drain. If it's none of the fused circuits, you have to start checking the unfused items (alternator, starter, possibly the CAN modules, etc).

I have a Fluke 36 AC/DC meter, you can find them on eBay for $50-$100 or so, for example this one went pretty cheap. Just clamp it around either main battery cable and you can see if there is a major current draw or not (normal should be less than 1 amp, anything over 1-2 amps will kill the battery overnight).

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Did you have this problem before you changed the battery? If not, I would suggest that the battery itself is bad (although new), if so you will have to check all the circuits beginning with the diode in the alternator. Good luck.

Yes we had this problem before we replaced the battery (twice). It would sit for a while and it would die (we wouldnt know), and this ruined a battery or two (dead cell). Could the alternator be draining the battery? How do you test it?

Thanks!

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K

I did notice that sometimes at night, after I lock it, the interior backlights (dash, window switches, etc) will flicker or pulse a bit. What controls these?

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K

I had this in a 400E for a time. Take readings with a simple digital voltmeter from Radio Shack. Pull fused circuits on at a time . See if there is battery draw hr by hr from the battery. Especially touch your hand to the to the radio amplifliers in the trunk to see if they are warm (battery draw). Disconnect the power to radio antenna as they stick on instead of shutting off. You test these things and you will find the power draw. I bet it is the radio amplifier in the trunk. Good luck. The digital voltmeter shows readings in tenth and hunredths of volt which is what you want. Should cost under $20 and be battery powered.