Think about this logically. If you have a bad ground, jumper cables won't help. Well, I should ask first, where are you putting the negative jumper cable, on the battery or on the engine? Current has to go through that ground, (and the rest of the circuit), regardless of whether it's coming from your battery or a different car and jumper cables. The only thing that's being bypassed with jumper cables is the connections on your battery. The story I'm typing up about voltage drop tests isn't going to help because GM insists on using those side post batteries with connections you can't access to take measurements. What I would suggest is taking both battery cables off, cleaning the connections, then making sure they're tight. You may want to also remove the negative cable from the engine block and shine up that terminal with sandpaper.
Some other things to consider are if the battery is getting old it can still measure fully charged but it will lose its capacity as the lead flakes off the plates, the battery could be too small, or the worst one by far is people who poke add-on wires under the battery cables. Those wires lift the terminals so they don't make full contact with the battery.
You should also measure the battery voltage when the engine is running. It must be between 13.75 and 14.75 volts. If it's low, suspect the generator. Since they started using the current design in 1987, it is real common to go through four to six generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time. The old battery might work in an older car, but as they age, they lose their ability to dampen and absorb the huge voltage spikes these generators produce due to their design.
Friday, October 14th, 2011 AT 8:34 PM