Expert: Sean Halliday - 8/26/2008

Question
The battery in my 1996 Mercedes C280 used to go dead after a few days.  Now it goes dead after less than a day.  Once the car is jumped, the battery seems to hold a charge in terms of starting again until the ignition is off for several hours.

I checked an add-on electronic rust arrester module and found that it drew about 1.8 amps, which could be part of the problem, so I disconnected it.  I will rewire it later to be on only when the inition is on.

With the battery removed, I tested the ohmic resistance between the positive and ground battery terminal cables, and found 2 ohms.  That is low, so I guess either a) there is a partial short somewhere or b) the active components (transistors, capacitors) are giving my meter a bad reading, or c) there is something else wired into the circuit that isn't disconnected when the ignition switch is turned off.

I tested the fuses in the trunk, removing them one by one, but it made no difference in the ohm reading.  I tested resistance from the fuses under the hood to ground, and found open circuits in all except two, which were labeled on the fuse chart to handle diagnostics.  The ohm resistance from the fuse test points to ground were around 2 ohms in both cases.  My meter is not super accurate.

If there is a partial short, how do I go about finding it?

I don't think 2 ohms hot to ground is correct for an ignition off condition.  I would expect a much higher reading, because with 2 ohms the ignition off current flow would be 6 amps, which would drain eve a large battery in less than a day.

I had a similar problem in a 1980s vintage Mercedes 109E.  I ended up buying a knife-type throw switch for the battery terminal and just cutting off the power that way whenever the car was parked.  I have bought another knife switch for the current car, and I could live with it, but it is not convenient, particularly since in this car the alarm goes off when the power is reconnected, so I always have to carry the remote with me.  Normally, I just use the key to open and close the doors, because remotes can get lost or have their batteries run out and then I couldn't deactivate the alarm.  Maybe it is the car alarm that is using the extra current.  The problem is that it's an add-on car alarm and does not appear to be wired into the the fuse in the trunk that it is supposed to be.  I can't seem to find a way to deactivate the car alarm completely (maybe the Mercedes dealer could).  Also, there is a tiny red LED on the radio that flashes when the ignition is off.  I don't know if it's a phony alarm signal or what its purpose is.

Any help or advice would be appreciated.


Answer
Normal drain is approx.02 ohms for most cars,radio,computor,clock etc.You will need to remove every fuse one at a time to find the drain.When you pull the fuse thats causing it,then you can go from there.A lot of times its a light in the trunk or under the hood.Have a electrician remove that alarm as they cause more trouble.