Finding a voltage drain

[czwbkx]

Although my 400E has behaved perfectly since I bought it, the car let me down Wednesday. The battery was dead and when recharged drains back down in two to three hours. Absolutely no problem when I parked it, now it drains.
First - any clued what to check first?
If not, then how can I monitor battery drain while pulling fuses to find the bad circuit?

Thanks,
R.

[Southern]

I would check the dome lights first (glove box, under hood, etc.) to see if they all turn off when closed.

Next I would check for accessories (heated seats, alarm system, etc.)

Battery drain can be measured by an ammeter in series with the Positive (+) wire to the battery. A clamp-on ammeter would be easier to use. Pull all of the fuses and put back in one at a time and measure the current draw from the battery. A small draw (mA) from the radio circuit is normal.

Don't assume that the battery itself is not the problem. My local Sears store will check it for free if you bring the battery in.

[czwbkx]

I've hooked up a circuit tester and found that there is a large current drain. This drain doesn't go away when the fuses are removed, so the problem is in an unfused circuit somewhere.
Has anyone else had this problem with a 400E or other W124? Any hints would be appreciated.
Thanks,
R.

[240Joe]

I don't have the diagram but here is my guess. I would think the only thing not fused is the starter. However, you could have a relay that has failed (stuck closed) so that current is being drawn from the battery all the time. You might try pulling the relays one at a time to see if that isolates it.

You should be able to do this by watching the current drain with a meter (what are you using to measure it?) while a battery charger is on the battery.

The other way is to feel around to see if any thing is warm to the touch.

How much current are you seeing? It has to be more that 15 or 20 amps because otherwise your battery would stay up for 5 or 6 hours (per the amp-hour rating of the battery).

That's why I wouldn't think it is a dome light or truck light as those don't draw that much current.

Keep us posted...as these are interesting problems.

Joe

[Southern]

My guess would be the rear window defogger, it tends to pull quite a load. I would take Joe's advice and check all of the relays.

[czwbkx]

Running again. I hooked up a voltage tester with an audible signal when a drain of >200ma was present between the negative battery terminal and cable. I then started removing fuses and relays hoping to hear the tone stop, indicating I'd found the culprit circuit. No luck and finally gave up and took the car to a local independent MB shop.
They found the CD player was cycling even when the stereo was off and the key was off! The player is on a seperate circuit with an in-line fuse located in the trunk.
The car is fine now, but I still haven't figured out what to do with the CD changer.
Thought you'd like to here how the mystery was solved.

R.

[240Joe]

Thanks for solving the mystery for us.

I take it that the cd changer is an add on. It probably needs to be wired to something that applies power when the key is on, but only after it is repaired or replaced.

Thanks again.

Joe