The weak point in the jumper cable connection is usually the clamps. You need to be sure they are really making a solid contact and on as much metal surface as possible. 190D22 is right, ALWAYS connect the last clamp on the body or engine of the dead car away from the battery. When they blow, and they can really blow, they throw battery acid everywhere. I usually do the positive cable first on the respective battery posts and wiggle them to be sure there's a good bite. Next the good battery neg terminal and last the dead battery side car body...I usually look for a bolt down on the fender or front side of the engine. Avoid the exhaust because between the gaskets and the rust you may not be getting the ground you think you are.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold