It could be the battery or maybe the alternator.

If the battery is new, it may be an alternator problem. We had a similar problem. Signal light, hazard lights, overhead lights, nothing electrical worked. It turned out being the alternator. I replaced the old alternator—which, incidentally, had not been grounded by the person who had installed it—and the car ran beautifully.

These symptoms point me to the battery connections, specifically the ground connection. Try this: go to a place like Radio Shack and get a halfway-decent multi-meter. Use it to check the DC voltage of the battery. It should read somewhere around 11 - 13 volts. if it's low, then you know it's either alternator or battery (but I'll wager it'll be okay). If the car runs at all after being jumped and disconnected from the jumper battery, do the same check again with the engine running. If it reads more than it did before, like 13 - 14 volts, your alternator is almost certainly good, too. So, what you'll want to do (and you might think about just doing this, anyway, as it's a good thing to do) is remove the battery terminals from the battery posts. Take a good wire brush battery terminal and post cleaner tool (avalable at any auto parts place for about two bucks) and ream the terminals out so they're nice and bright. Clean the battery posts, too. While that's still disassembled, trace those wires to their other ends. The negative terminal wire should bolt to the bodywork somewhere (dunno where on a 900 series). Unbolt the wire and clean the heck out of the end of the wire, the contact point on the body and the bolt itself and any washers. Apply some dielectric grease (also cheap and available at auto parts stores, this excludes water and fights corrosion) and reassemble snugly. Then, and before actually reconnecting the negative wire to the battery, do the same with the positive terminal wire. You'll find that the big wire from the plus terminal goes to the starter. Do the same cleanup there, and don't forget the dielectric grease. You should also find a few accessory wires. One will go to the electric radiator fan (I know this because I put a '92 940 radiator fan on my 240). Another will go to a junction box that will power all of the other electrical goodies in the car. By the way, the reason I suggest that it is the ground (negative) connection, and not the hot (positive), is because of this multiplicity of plus wires. It is less likely that three or more plus wires will have gotten all grungy, causing you to lose startability AND accessories, then it is that the one ground for all of it will have gone south. Twelve years old is definitely old enough for it to have this problem, so happy cleaning, and good luck. -EdM. '90 Volvo 240DL Wagon "Lola" '72 Volvo 1800ES "Galadriel"