In the 91 350sdl, it has a new bosch alternator and a fresh battery. Both correctly installed. These are the givens.

As long I drive the car everyday, it is fine. However, when i let it sit for 4 days, the battery gets too weak to start the car.

My hypothesis is that there is drain on the battery somehow. Kinda like a leaking artery that is sucking the life out of the battery.

Can you suggest EASY ways to find the problem? All suggestions are welcome.

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Jennifer
90 350sdl

I think your hypothesis is right on the money. Do you own a Digital Multimeter?

It sounds expensive. What is it?

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Jennifer
90 350sdl

It used to check volts,amps, ohms, etc.... can be expensive, but you do not need a Fluke meter, just a cheap one.... under $20.00. I had the same problem once, pulled the radio fuse and no more drain on battery, replaced radio, this was on my Fiat. Good luck.

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1984 300D Turbo Sedan (Hilda) 272,115 miles.....
Anthracite Gray/Palamino Leather, I am 3rd owner

2001 Dodge Stratus (Silver) (wifes) 55814 miles...

1982 280TE Wagon Astral Silver Metallic/
Anthracite Velour
260,512 miles (Eva)

1969 230 Sedan Olive Green/Black MBTex
4 Speed Manual
84,213 miles ????
Haus Frau


2004 Boreem Ninja Pocket Bike
Highly Modified 49cc (Ling Ling)

Einigkeit und Recht und Freiheit

Buy the meter and set it on high current setting (buy one that can measure up to about 10 amps. it usually has a seperate plug for that function) Disconnect a battery terminal (battery should have some charge for this) and see if current is flowing. If it shows more than about 200mA leave it there (connected), if not, switch to lower setting. You should see something, because the clock draws a little bit, maybe 30 mA? anyway, if you have maybe 100mA draw or more you have a problem. Revove 1 fuse or relay at a time. This may very well tell you which circuit is leaking, as the current flow should drop as soon as the offending circuit is disconnected.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

FWIW, you can get a decent DMM (digital multi meter) from Sears for $50-75. The model I own (I'll get you the number when I get home) can measure DC current up to 10A for brief periods of time.

You can use the amp meter to determine if there is a constant current draw (10A or less) on the battery.

You might also just disconnect the power antenna...it may be frozen up and not shutting off it's little motor.

edit-I've never seen <$20 meter that would measure any current other than milliamps.

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Never a dull moment at Berry Hill Farm.

Likely culprits are power antenna, electric vacuum pump for the door locks, and light on in the trunk. Yeah, get a multimeter and search here for "current draw" to find out how to track that sucker down.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

funny because the power antenna is indeed broken. i'll be happy to disconnect it if you tell me how. also, assuming i have a good enough DMM, what is the procedure for checking for constant current draw.

you guys are the best. do you realize that if i took this car to the dealer, i might be tempted to never come back for it after they tell me how much it's gonna cost.

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Jennifer
90 350sdl

I haven't torn into the trunk of my SDL but on the 123, you remove the plastic trunk liner on the left-hand side to expose the antenna mechanisim. You should be able to find the wiring connection for it in there somethere. Unplug it and you'll be golden.

I doubt if it's the alarm because, even if the audible warning was somehow disconnected and the alarm was triggered, your headlights would probably be flashing.

Be sure and check all the usual culprits: glove box light, trunk light...et al, ad nausaeum.

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Never a dull moment at Berry Hill Farm.

Probably around $25. Any auto parts store should have one as well as wally world etc. You can get an analog one for less than $10, it won't be nearly as useful but probably fine for this problem.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k

Follow exactly what Pete was saying above, with the following modifications/additions:

1) For safety, disconnect the negative battery terminal and put one probe of the ammeter on the battery terminal and one on the cable end. Make sure everything in the vehicle is shut off and the doors are closed.

2) If pulling one fuse at a time does not find your culprit, disconnect the alternator wires from the back of the alternator and see if the meter drops below 50 mA. If so, then you have found your problem.

When I go looking for these problems, I don't use the big expensive meter that can measure 10A. The little Radio Shack meter that is the size of a pack of cigarettes is much more convenient. It will measure up to 1A. Your current leak is certainly less than this amount (probably 100 to 500 mA).

Harbor Freight has a digital meter that will actually read up to 10 amps for 3.99 on sale. The 10 amp side is not fused so you may have to buy another one if you switch on the glow plugs... It is not too accurate but close enough for this kind of tracing. What you are looking for is a drop in current draw as you remove each fuse. Minor drops i.e. milliamps are probably not your problem. .5 amps or higher is probably more likely for a four day drain. I'm certain that someone here could do the calcs of exactly what the drain would have to be to reduce power over X period on a certain size battery.... (see head swimming) oh never mind...

I'd add stereo and alarm system (if installed) to the list of likely culprits.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique