On three occasions over the last three weeks, my battery was dead without warning....typical scenario: start car at work (no problems), drive home 40 miles, stop car, pick the kids up, start the car (no problems), go eat pizza, get back in car and find that battery is dead...not enough juice to crank the starter.

On one occasion, I checked the battery voltage after the car would not start, and it was about 9.3 V (dead). I started the car easily with booster pack, drove 40 miles to work, then checked voltage both at idle and with engine off: 13.6 at idle, 12.3 with engine off. Seems ok. I have checked battery & charging voltage several times since most recent dead battery, and the battery appears to take a charge and hold it, and alternator puts out 13.6 at idle.

I just came from Auto Zone where I had them use their machine to check battery and alternator, and both tested OK.

My conclusion is that there must be something drawing a lot of current over a short time when car is off (glow plug relay stuck?, or something similar). However, it seems to be intermittant since I have used the car without any problem for up to a week or so between episodes, then unexpectedly, the battery is dead, after just having used the car. So the culprit appears to be something that works fine most of the time, then unexpectedly kills the battery.

Relevant background info:

1) Battery is 1 year old, nice white MBZ battery. It was drained once in July when starter solenoid was stuck and starter would not go off until I pulled battery cable. Repaired starter, no battery problems since.

2) Alternator is 4 years old, 100,000 miles, Bosch rebuild...seems to be ok

3) The only recent problem with the car which coincides with the dead battery problem was a bad climate control push button unit...just replaced it today. Footwell flaps were not working in economy mode....all vacuum elements tested ok (no leaks) .... I then checked the 5 electric vacuum valves (behind the pushbutton unit)....all worked ok when 12 V applied, but on various push button settings, and heating and cooling settings, the proper vacuum valves were not being activated in certain push button modes. I changed push button unit with a spare, and all is well.

4) Glow plug relay is recent (2 or 3 years old) .... I don't think this is the problem (because its relatively new), but this type of failure would explain the apparent fast battery drain.

5) I dont have any high current accessories in the car...pretty much stock except for a small amp for the radio.

6) Recently, the weather has turned cold...somewhat coincides with dead battery problem, but car operates fine most of the time.


Questions:

Could there be a failure mode in the climate control push button unit that could draw current when the ignition is off??? If this is possible, the unexpected nature of the dead battery could be explained (different push button settings, different ambient temperature, different temp settings, heating vs. cooling modes, etc.)

Could the voltage regulator fail intermitently? (I probably should check electrical connection at the alternator/volt. regulator, and check condition of brushes), but alternator seems to be performing OK.

I'm thinking of wiring up a volt meter from the battery to monitor battery voltage and charging voltage on a continuous basis.

Is there any way to temporarily hook up an ameter to continuously monitor current draw, or do I have to disconnect the positive battery cable to periodically check current draw.

Any other suggestions??


Thanks,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)