Hi,
I've got a 1989 W124 300E that I only drive once a week or thereabouts. Recently (at least the last 4 or 5 months) the batttery has been fairly flat when I come back - certainly too flat to start it. It's a new battery (about 3 months old) because I had the problem and thought it was a battery issue. Probably not now.
I've put an ammeter across all the fuse positions (with the fuses removed) and the only current of any significance is Fuse 9 which has the interior lights etc. It shows about 0.35A. Apart from the radio and clock, I don't think there should be any drain. Is this too much for them? Should this be enough to flatten the battery in a week - 7 x 24 x .35 = 58 amp hours - sounds a lot less than should trouble the battery to me, but I may be wrong.
What else should I look for?

Have you removed one of the battery cables and put your ammeter in series there? There are other circuits besides the fused circuits through which you could have a drain.

Additionally, set up your meter for DC voltage and put it across the battery terminals at idle and make sure the alternator is making about 13.5 to 14.5 VDC.

Again, not an MB tech, but make sure you leave it hooked up for at least 5 minutes in case there are some "late bloomers" that don't turn on immediately upon re-connection of the battery. Granted, this is usually an issue for later model vehicles, but it won't hurt to be thorough.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

Start out by removing the negative battery terminal, and hook it up via a test light. If the light comes on brightly, there is clearly too much draw.

Now remove fuses one at a time. If one of them turns off the test light, you've found your draw. If all fuses are removed and the light is still on, unplug the OVP relay fuses.

The last one that came to me with this problem had water in the OVP relay and was leaking enough current to make the ABS click occasionally with the key off, a new OVP and all was well.

Good luck.

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Gone to the dark side

- Jeff

Hi all,

Thanks for the suggestions. I'm having no luck with finding the problem via the fuses so I've pulled the OVP relay and will resolder it up during the week (I have to go to my farm to get my soldering iron). The ABS light would flick on and off occasionally - once a week or so - so maybe the OVP relay is the problem.

Does anyone have any comment on whether the 0.35A draw should be enough to flatten the battery in a week?

Regards

John

It is too high, not sure it should flatten the battery though.

__________________

Gone to the dark side

- Jeff

Check the door switch on the drivers side are good, and not shorted out. One of them on the driver's side controls power to the convenience features, such as power seat. Keeping that relay closed costs 250mA all day long. Enough to kill a decent battery in a week. Ask me how I know. The previous owner purposely wired those two terminals together so he could adjust the power seat with door closed before starting the engine. Since he drove daily not a big issue, recharge battery good to go. Park at an airport for a holiday come back dead, different story.

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Cheers!
Scott McPhee

1987 300D

Assuming a 65Ah battery, it would be fully discharged in 7.8 days with a steady draw of .35A.

Also, unless you have a deep cycle battery (like an Optima), every time you "flatten" the battery, you reduce the level to which the battery can be recharged, thus reducing the Ah rating and the CCA rating.

If I were you, I'd go get a small battery maintainer and leave it hooked up to the car when not in use. They're usually in the 2-3 amp range and can be had for around $25. Keeps your battery nice and fresh; as a matter of fact, my 380SEL sat for 2 years with a maintainer on it. Once the necessary repairs were made, it started right up (and I must have cranked it for a total of 10 minutes during the repair process).

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

My guess here would be to try wiggling your door switches a little or even just push and let them go a few times with your hand. Otherwise, if your car has one, turn the rear light switch in the middle of the dash board on and off a few times. My final thing to check is make sure that none of your power seat switches are stuck. I had that issue when the passenger power seat switch was somehow stuck in the middle and it caused the car not to start. The moment I got it unstuck, saw the interior lights go on and the car was able to start.

On my 1987 W124 the trunk light was staying lit and drawing the battery down. It is easy to remove bulb and do your checks again.

Aha! That's like wondering if the fridge light goes off when you shut the door. Everybody assumes it does, but do you every really know? So, did you actually have a helper shut you in the trunk to see... and trust them enough to open it again?

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Cheers!
Scott McPhee

1987 300D

Actually if dark outside on my car you can see a wee bit of light by the 3rd brake light. I had my 3rd brake light off recently and must not have slid it back together properly. Good question tho. !