300CE battery drain

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4 replies [Last post]
monarchd's picture


Hi everybody,
A couple of weeks ago, the 2 year old battery in my 93 300CE started going down but would still start the car. Sunday it was deader than a doornail (whaterver that is). Had the battery checked out and is OK.

Tested the draw with a test light and multi-meter between the negative battery cable and terminal.Key was out of ignition.

The meter pulses .01-03 mA. The test light glows brightly at first and then dims to a very faint glow. Removing the C and 9 fuse kills the light. I think I have tracked it down to a couple of things. You can hear and feel the antenna motor "clicking" when the neg cable is touched to the battery, but if I unhook it, the test light still glows.

The clock is on this circut. Is that enough to trigger the test light?

Also, The relay in the "F" position on the relay panel clicks when the battery cable is touched on and off. One book says it is a two pole relay that controls the auxiullary water pump. I can't find it in any wiring diagram I have. This relay is white and six pole.

Also on this circut is the seat back locking system. This hasn't worked since I have owned the car. I would like to fix this too. Related?

This thing is like working on the Space Shuttle, but I love it!

Any ideas?

Thanks in advance.


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Anonymous's picture

I'm not at all familiar with your car, but it seems to me that 0.03mA is not nearly enough to discharge a car battery even if it sat for a month. Do you mean 3mA? (That's still only 3 amps over 1,000 hours.) The only possibility that comes to mind is a dirty battery, which can self-discharge within a few days.

Take the battery out of the car and wash it down with a solution of baking soda and water. A heaping tablespoonful to a gallon of water should do it. apply it to the battery and if it bubbles, let it. When that stops scrub down the entire battery case to remove any dirt. Once it's clean put it back in the car and see if you still have the problem. BTW do it in the street, not in your driveway. Hope this helps.

Anonymous's picture

I might be wrong with the meter reading. Its an automatic scale that keeps flipping back anf forth. It seemed low to me too, but it does light the trouble light. the battery looks like brand new. no corrosion on the posts.

I also ran a voltage test and it's charging 14 volts with the lights and blower on.

Anonymous's picture

When you touch a battery terminal to the post, there is an initial current surge which is higher than the steady state reading will be. Here's a way around the surge.

Remove the battery terminal from the post. Hook a jumper wire between the terminal and post for circuit continuity. Hook a second set of jumpers with the ammeter between them to the post and terminal. Then remove the first jumper. Read the ammeter. If the meter flips back and forth, put an analog meter in the circuit so you can determine the current draw accurately.

If you are on DC amps and the scale flips back and forth, it could indicate that you are dealing with an AC signal.

Some meters have a method of cancelling the autoranging feature. Check your instructions.

I found a Stansi DC ammeter on ebay with a 1.5 amp scale at a reasonable price. Works good for applications like this.

Former Member
TWEED21's picture
Joined: March 26th, 2007


2002 ML320
300CE Battery Drain


I know time has passed - but did you find the problem..?

I have a 92 300CE that had the same problem which I found out after purchasing it. Replaced the battery and it too went dead after a couple of days. Kept charging it but it kept going flat. Same symtems..F rely clicking when attaching -ve batt cable etc...

I finally pulled all the fuses and put them back one by one over a few days and was unable to locate the offending cuircuit. Finally I discovered a single 25 amp (blue) fuse in terminal X22 which is a little black covered fuse box located just next to the blower fuse and in front of the main fuse box in the left side of the engine bay. That fuse holder had a place for two fuses and according to the electrical diagram one fuse is for heated seats and the other is for a cell phone. The circuit is also defined as a 30 (always hot) circuit. And here was my clue...!

When I pulled that single fuse and put it back I heard the F relay clicking. I discovered that the fuse was in the cell phone (right holder) socket. FYI - I don't have a cell phone or heated seats in the car. So, I relocted the fuse to the left (heated seats) position and the F relay no longer clicked. more dead battery...!! Everything else seems to work fine. That was a year ago. It is possible somebody before me was fiddling and put the fuse back into the cell phone position causing some back feed.

Hope this helps


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