84 318i

[jackno2]

I have a 1984 318i with 105,000 mi.

It just died when I was driving it.

I got it home and jump started it and kept it hooked up to charge. But the minute I took the jumper cables off, it died again.

Alternator, battery perhaps?

Or could it be something else.

I thought I would ask before I attempted anything.

[winfred]

could be, sorta hard to diagnose without getting dirty, charge the battery and see what it's voltage is, under 12.3 or so after it's rested a little while after charging and it's bad, with the car idling with a good battery it should put out over 13 volts close to 14 would be good, 12.5 or less and somethings bad, try the brush pack on the alternator first it's on the back held with two screws, the two brushes should be atleast 1/8" or longer and the faces should be shiney from riding on the armature, a short and dull looking brush is a dead give away that it's worn out, a new brush pack should cost $20-50 and shouldn't be hard to find

[Bravo10000]

I have a 1984 318i with 105,000 mi.

It just died when I was driving it.

I got it home and jump started it and kept it hooked up to charge. But the minute I took the jumper cables off, it died again.

Alternator, battery perhaps?

Or could it be something else.

I thought I would ask before I attempted anything.

The "95%" answer is that this is an alternator issue...and old trick with cars from back in the day was to pull he battery cables with the engine running, and if the engine died, you knew that the alternator was the issue.

You shouldn't try that anymore...my mechanic tells me that this can damage electronic components, I don't understand why...but I would all but eliminate the battery as the initial cause. That's not to say that the draw on the battery (after the alternator malfuntioned) didn't ruin it - it very well may have.

When I have issues like this, I prefer to use a multimeter, and attach it to the battery terminals with the engine running to see if I am getting the right voltage and amperes from my charging system (I generally see a voltage output of 14 and change). Still, you'd need to know the rating of your alternator in amps. My wife's Mercedes has it listed right on the alternator, but I don't know that my BMW does the same...then I shut the car off to see if the battery has voltage (and if it drops rapidly, a la shorted cell).

I guess you can't really do this, since you can't keep your engine running without a charger or "jump" on it, and that would throw off your readings of course. Just about any parts store can test the battery for you to let you know if it has a shorted cell. If you have a charger, you could try getting a full charge on your battery - if it takes a charge, you could run the test to see the voltage/amp output of the alternator, since your engine will run for a short while on the battery's power.

[winfred]

unregulated dc power surge that may or may not contain a surge of ac current that could cook the computer or fry the alternator, the battery acts like a buffer providing something for the alternator to push against and let it regulate itself by

You shouldn't try that anymore...my mechanic tells me that this can damage electronic components, I don't understand why.