OK, I've read everything I can find, and no answers are forthcoming, so I'm throwing myself on your mercy.

I have 1985 300D with 290+K miles on her, and we were working on an AC problem about a month ago. That took a backseat when an idiot decided to destroy his POS late model mid-sized sedan by driving it in front of a 300D (mine!) traveling at about 30mph. Damage to the MB was trivial and cosmetic (I've scrubbed off most of the rubber he left down the side of my baby, so all that's left are three or four dings not much worse than other cars get in parking lots), the other guy was likely totaled -- at least neither of his passenger doors will open again without the Jaws of Life. Fighting w/his insurance co. has taken the focus off the AC.

3 weeks and 2,000+ miles after the accident, I was driving at night and the alternator light began to ghost -- glowing dimly, almost going out, brightening a little, and dimming down to nothing again. I was 30 miles from nowhere, so I kept going and prayed. I lost almost all electrical power -- dim headlights, dashlights down to almost nothing, the blower motor on the AC slo-o-o-owed to nothing (the blower motor was never a problem with the AC issue), the stereo died, etc. I turned off all non-essentials, and drove on. It would fade down for a while, then suddenly brighten and be fine for a couple of miles, then start to die again. This went on for about 18 miles. Then the lights came up again, and never went down for 13 miles to my destination.

The next morning, I went to the forum's DIY pages and looked at what to do about gremlins. I grabbed the meter and checked -- the alternator was fine. The battery seemed to be a little low, but OK. I drove it -- with no problem -- to the parts store and bought all new fuses. (The alt light might have been ghosting, but if it was, it was too dim to see in bright sunlight). I removed all the old fuses, found a few metal bits off an old blown fuse floating around in the box that I removed (thought I'd solved my problem right there -- NOT!), used a bit of steel wool to clean all the contacts (used a tack cloth to make sure I didn't leave any), and installed all the new fuses. I did this one color at a time and cross-checked to the fuse diagram, so I didn't screw up what fuse went where. FUse #7 and 8 were a little weird -- I got a tiny arc when installing #7 (controls the clock, interior lights, etc.) when I accidentally touched it and #8 (the cigar lighter) at the same time, but the fuses held. I figured it was because #7 would have to be hot all the time, so I wasn't really concerned. When I was finished, I went to start the car and had a nearly dead battery! We jumped it and got it out of the way. A few hours later, the battery wouldn't even try -- it was dead, not even the clock was moving.

I put it on a trickle charge. It took over 18 hours to fully charge. That said the battery was at least weak, so I bought a new battery. When they connected the hot side (very clean cables and terminal connectors), I saw a small arc between the connector and the terminal. Not a good sign. In my limited experience, that means I have a direct short somewhere.

I killed that new battery's charge in about 3 1/2 hours of driving, so I've still got a serious problem. Thank goodness for old-fashioned German engineering that would let me get home with nearly no battery.

Any ideas how I can track this elusive thing down? I've got the Haynes manual wiring diagrams, but narrowing the field would be hugely helpful. Right now, if I keep her on life support (battery charger) when she's sitting, she behaves just fine, and I don't see an alternator light until I've run the battery down quite a bit. It's running about 12.8V, occasionally up to 13, which I understand is too low. Further, it's draining when she's just sitting.

Any ideas? Can you help??

__________________
Anthracite 1980 300D -- 64k original miles with a new engine, on the road again!
Silver 300D -- second owner, Sunny's old baby, Ilse, 210 miles,
Having to thin the herd….
Silver 1983 300SD -- second owner, 325k miles
Gold 1981 300D -- well-traveled, solid little car
Beige 1984 300D -- 292k miles, grease machine, parting out
Seafoam green 1981 300SD -- 250k, windshield frame damage
too many assorted w123 & w126 cars, parts cars, and extras

I would suggest removing all of the fuses and then connecting the battery. If you still get a spark when you connect the battery, the short is before the fuse box. Check the various hot leads from the battery, make sure that there are no places where the insulation is worn through so a wire can touch the metal body or another wire.

Get ahold of an ammeter (auto parts store, not expensive, maybe $20). You don't need a fancy one, even the ammeter section of a digital voltmeter will do, as long as you don't try to start the car! That will blow the fuse in the digital voltmeter (unless it doesn't have one, in which case the meter will fry). You want to be able to measure small currents of a few hundred milliamps or an amp or two -- the hidden load that is draining your battery. Connect the ammeter between the negative post of the battery and ground (really doesn't matter which but I think the ground lead is easier to work with).

If there is no current when you hook up the battery, gradually add the fuses back, in any order you want. Watch for sparks and/or the meter to jump, indicating current is being drawn. Make sure the key switch is off, doors closed, all accessories off, etc. If you add a fuse and get a drain from the battery, somewhere in the circuits controlled by that fuse is your short. Use your Haynes manual to see what fuse controls what stuff. The fuse panel cover should have a chart on the inside, too. If yours has gotten lost, someone here can get you a copy or a text version (I'll be off line for the next week or I would volunteer myself).

Once you have it isolated to a single fuse, it should be fairly easy to track down, and at least you'll be able to prevent a dead battery by pulling that fuse.

Good luck and let us know how it goes.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

How do you know the alternator is OK? Have you had it tested, what is the voltage across the (charged) battery when the engine is running? I also get a small arc between the battery post and the clamp, I don't believe that indicates a "dead short" because the arc is much too small and my battery does not drain down significantly. It sounds to me like the alternator is not providing sufficient current to keep the battery charged. I would go to Autozone, etc. and have the alternator tested. The last time mine did something like this I ended up just needing a voltage regulator.

If you really do have a drain on the battery, you should be able to find it by measuring the current across each fuse (with the fuse removed). But, I suspect you are barking up the wrong tree.

Thank you both. I had though to check the drain in the morning by removing the fuses one at a time to see when the drain stopped, so I was sort of headed your way, although your plan is better since I'd not thought -- as I should have -- about it possibly being before the fusebox. My in-box diagram is intact, so I'm good on that, anyway.

I did have the alternator tested and it was charging -- heck, the first thing I checked was for a loose belt (wishful thinking). Still, it may bear doing again if nothing else works, since I wasn't all that impressed with the tech who did it, and it was done where I couldn't observe.

Thank you again. I'll keep you posted on what I find.

Sunny

__________________
Anthracite 1980 300D -- 64k original miles with a new engine, on the road again!
Silver 300D -- second owner, Sunny's old baby, Ilse, 210 miles,
Having to thin the herd….
Silver 1983 300SD -- second owner, 325k miles
Gold 1981 300D -- well-traveled, solid little car
Beige 1984 300D -- 292k miles, grease machine, parting out
Seafoam green 1981 300SD -- 250k, windshield frame damage
too many assorted w123 & w126 cars, parts cars, and extras

I'd have the alternator checks, or, if you have a spare, swap them. Could be a bad voltage regulator, or diode in alternator, loose conection, somewhere. Even if you have something draining the battery, with the car running I'd think the alt would keep up with it or pop a fuse.

Pull all of the fuses, connect an ampmeter and put them in one at a time. If that doesn't find it, swap alternators.

Chuck.

sunny,

U do not have a direct short. If U had a direct short (0 resistance) you would blow a fuse or if the short was in a non fused wire, the wire would get so hot it would smoke the insulation.

Someplace you have a switch that is not opening or a leak in an electronic circuit. U have to disconnect circuits one at a time to find the bad one. Remember some curcuits will always draw a few milliamps even when everything is turned off. So its best to measure the current when testing and disconnecting the circuits.

Have U checked the light in the glove box or trunk?

P E H

I've been in the auto electric business for over 30 years. The best way to locate a battery draw is by connecting a test light in series between either battery terminal & the battery cable. Don't use a Snap On light, the bulb has too much resistance. After connecting the light momentarily touch the cable end to the batt terminal with test light still connected, then remove. This will allow any components that have capacitors such as radio, a/c amp, clock winding mechanism, etc to recharge. You may see a slight arc when you do this...it's normal. If you use an ammeter you'll be chasing phantoms, as it will show you values you need not wory about. A test light will allow a small amount of current to flow keeping caps charged without glowing. If it's not enough to light the light , it's not enough to drain a battery under normal service intervals. If your test light glows, you have a battery draw. Remove the plug from the alternator & check test light. I've seen many alternators bench test ok for output but still have a draw when not running. From your description you may have an intermittent charging problem. Check all connections from battery to alternator including grounds, especially the ground from the alternator to the bracket. You might want to remove the regulator from the alt to check brush lenght.
Be sure you fully charge your battery before testing alt output, a weak battery will give you low readings, and stress your alt...it's made to maintain a good battery, not charge a weak one. Ideally you want around 14.2V , but most MB's seem to run around 13.5V. Below 13.2V is unacceptable.
If all checks ok, start pulling fuses until your light goes out.

Good luck

John,

Lots of good information in your post. What is the watt rating of the bulb U use in your test light?

P E H

More good info, which I'm sure will help others. as well. Thank you so much -- it appears I have a lot of quality time to spend with Ingrid! (Time to get that carport built -- more motivation!) And there's a new addition to our stable that wants attention, too, but I'll run her in another thread.

Thanks, guys! I'll let you know what I find.

Sunny

__________________
Anthracite 1980 300D -- 64k original miles with a new engine, on the road again!
Silver 300D -- second owner, Sunny's old baby, Ilse, 210 miles,
Having to thin the herd….
Silver 1983 300SD -- second owner, 325k miles
Gold 1981 300D -- well-traveled, solid little car
Beige 1984 300D -- 292k miles, grease machine, parting out
Seafoam green 1981 300SD -- 250k, windshield frame damage
too many assorted w123 & w126 cars, parts cars, and extras

Not sure what the wattage is. It's a #1816 12V bulb rated 3cp. Any light that takes a # 57, 1895, 1816 bulb will work fine. The only ones I've found that will not work right are the Snap Ons. Too much resistance in their bulb, letting caps crash, causing light to come back on, making you think there's a short when there's not. I've made a lot of money tracing shorts other mechanics swore were there because of their Snap On lights!
I have nothing against Snap On tools (own a few myself), don't want to start an argument over their merit, it's just that their test light is not suited for this type of test.

You normally will see a very small arc when attaching the battery cable. Due to a recent battery failure, I found that this was not a very determining factor in finding an electrical problem. I found from an ammeter connected in series, that my car draws a few (100-300) milliamps of current from the clock and seat memory.

I would suspect an alternator failure since you just changed out the battery. Double check the grounding of the alternator as this may be causing your failure. What are the voltage readings across the battery pre/post starting?

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

PEH and Sam were right (no, I'm not at all surprised!), I didn't have a direct short. I had an intermttently dying alternator, one that would bench-test fine, but put it back on the car (what a PITA that is taking it off and putting it back on!) and it would charge the battery occasionally, but not often enough or long enough.

So. A new battery and a new alternator, and she's back to behaving like she's always done. Which means I need to do that monovalve kit thing right quick -- as soon as I find that angled driver with the flat tip screwdriver tip.....

Thanks for all the help, guys -- it's always a comfort to be able to ask and see such well-thought-out and thorough answers. We'd never manage if it weren't for you guys.

__________________
Anthracite 1980 300D -- 64k original miles with a new engine, on the road again!
Silver 300D -- second owner, Sunny's old baby, Ilse, 210 miles,
Having to thin the herd….
Silver 1983 300SD -- second owner, 325k miles
Gold 1981 300D -- well-traveled, solid little car
Beige 1984 300D -- 292k miles, grease machine, parting out
Seafoam green 1981 300SD -- 250k, windshield frame damage
too many assorted w123 & w126 cars, parts cars, and extras