I have a 1983 300SD TurdoDiesel. I recently let a friend of mine use my it and he accidently left the cd player on and the car died. He jump started it, and now the battery does not seem to hold any power. I charge up the battery, but by mid-day, the car is dead. At times when I'm driving, the electrical systems in the car slowly seize to function, lights, windows, speedometer, everything. If I remove the negative from the battery during the day, the battery seems to stay charged. I'm thinking it's a bad alternator, or could it be something else?

You may be on the right track when you say it stays charged if disconnected.

Will it start the car if disconnected for a longer period, such as over a weekend? If so, the battery capacity is probably OK.

If the battery itself is OK, you must do some detective work to find out what is drawing current when it is shut off. Try searching on other battery-related threads here for some tips on detective work. They involve looking for current use when there isn't suppost to be any, such as a trunk light that doesn't go off even with the deck lid shut.

I wouldn't suspect the alternator, unless it won't put charge back into the battery.

BCingU, Jim

Disconnect one of the battery cable clamps from the battery. Connect an ammeter in series between the battery and the cable clamp. A reading of zero or a very small number is normal. A high number means something is drawing current that shouldn't be. If this is the case, pop/reinstall each fuse from the fuse block until you find the source.

Your battery going dead in only a few hours means which means (a) The battery is discharging internally and must be replaced. (b) There is a high current draw somewhere, or (c)The battery is very cold and you don't have enough cold cranking capacity. If you can rule any of these out, then you may have found the problem.

Be careful applying current to the small portable voltmeters... they are fused with a 10 amp fuse. You might try a test lamp first in case there is a current draw higher than 10 amps. The test lamp should not light at all.

Gary

It can be a bad alternator. I had a similar problem about 6 months ago on my 300E. Turns out that the triode was shorted out, and also had the windings bad. My alternator would actually get pretty warm to the touch merely being hooked up normally and with the key off. It might be something else, but, might be the alternator too.