Correct Procedure:
Connect positive terminals of both batteries.
Connect negative terminal of booster battery to a good ground of the vehicle to be started. In other words not directly to the neg terminal of the car to be started.
This is what I've been told.
This is what the Haynes manual says.
I think I read it in the Mercedes manual also.
Why? I don't know. Can anybody explain this to me?
Why not to the neg terminal of the car to be started? There is probably only a few milliohms difference between the neg terminal and its ground. Electrically its almost the same point. I have a portable jump starter battery that has a switch to apply power after the cables are connected. I recently used it to give someone in a Toyota a jump start. It was dark, cold and I really couldn't find a good ground in the Toyota other than the neg battery terminal. In addition the cables on the jump starter are short so there really aren't too many options where you can connect to ground. Even with long jumper cables there don't seem to be good grounds available. So it went on the neg terminal. Toyota started without incident. What could have happened? How do you guys jump cars?

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1985 300D Turbo
"Evolution is God's way of giving upgrades" Francis Collins