hi everyone. question on my car... 82 300 sd. the first day i had it, it ran fine. the next day it wouldnt start up- i found out that it had a dead battery, one of the cells had dried up. my dads friend helped me put another working battery in the car. after we put it in, it started up and ran fine for a couple of days. i drove it around the block and parked it-thjen tried to start it up and it wouldnt go. my friend and i jump started it, and it started right up. i drove it home and parked it and it wont start now. the battery that we put in it was also from an old mercedes, and it was an old battery...but still worked, and it ran fine for a few days. do you think it was just a bad battery, or could it be an alternator problem? how could i tell? input would be appreciated. thanks!
zach

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1982 300sd turbo. First car and the start of a new collection...

While it is running, pull the ground off the battery. Turn on the headlights. If the headlights turn on, your alternator is at least putting out current.

On a gas car with a bad alternator, as soon as you remove the ground, it will die, but the diesel needs no electricity to run.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

If the alternator isn't dead yet, that trick will probably kill it. Your car doesn't have battery/alternator warning light? Just get yourself a voltmeter and make sure the battery voltage is more with the engine running than when it's off.

Well, there's no way to know for sure unless you test them out. If I were you I would take the batteries to a parts store that will test them for free to see what kind of life they might have left in them (whether or not they can be recharged). Being as old as they are sounds like they will probably be shot. If you want you an also pull the alternator and have them bench test it while you are there - that way you will know the status of your alternator and it won't take running a new battery dead to figure it out. Aside from a bad battery or bad alternator, the other alternative is that you have some parisitic drain somewhere that you will have to identify - but don't worry about that until you have determined the status of the battery and alternator.

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman

If you have a Harbor Freight Store somewhere near you a Digital Volt/Ohm (Multimeter) can cost as low as $3 when on sale. I have at least 5 of them and try to keep one in each of my cars.
The autopart stores sell the same meter for 5 times as much.

Turn the Ignition Key on your car but not far enough to start it. You should see the Battery charging light glowing Red. If not the Light Bulb could be burned out.
The problem with a burned out light bulb is that your Alternator needs that little bit of current draw/flow that the Light Bulb causes and your Alternator will not charge without it. No light no charge.
Ounce the Engine is going and the Alternator starts chargeing the Red Ligh is supposed to go out.

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84 300D, 82 Volvo 244Gl Diesel

Start the engine and with a DC Voltmeter, simply measure across the battery with the engine running. If the alternator is working, you will read about 13.5 to 14.0 volts. If the alternator is not working, you will only read about 12 volts or less.

If you can turn the alternator by the cooling fins with your finger. The belt is too loose.

If the above tests come out negative (bad alternator) replace the voltage regulator & retest before you replace the alternator.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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When a voltage regulator goes bad, can it let too much current through and boil the electrolyte out of a cell?

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85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
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98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

Yes it can. The only problem with your suposition is it tends to boil all the cells not just one..You can find a shorted regulator again by measuring the battery voltage with the engine running. Usually well above 13.5 volts.

The old battery with its bad cell could have possibly damaged the alternator. That senario would have made the alternator charge constantly.