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

I believe that procedure is designed to avoid a spark in the vicinity of potentially explosive battery gases. If power is only applied after attaching the portable jumper, there is a low liklihood of a spark.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

This is what I have been told concerning using a ground other than the negative battery post. Batteries can give off gases which, if ignited, can explode. When the last cable is connected, it can give off sparks which could ignite gases, thus causing explosion. I have seen the sparks many times. Some people I know even look for sparks to see if they have a good connection. This is what I have always been told.

As far as I know it is meant to prevent either battery mishaps or damaging electrical systems.

Yeah it works both ways but written instructions need to consider LAW SUITS.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

my uncle lived half his life with a huge burn on his right cheek from doing it that way. had his face been any closer, he might have suffered damage to his eyes. batteries DO explode, just not as often as you would think. be safe and use the ground....not the negative post.

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1980 300D

Best place to ground the neg cable?

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'82 240D, 215,000 +
Classic White, Regenbogen

On new cars they usually hide the battery and put ports that are labeled to where you can attach a jumper cable.

Usually just attach the positive to the battery and the negative to any huge metal part.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

Batteries can emit Hydrogen. Making the last connection (where a spark is likely to occur) away from the battery prevents this:

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

On an related battery safety issue. . . I was working on a car in the mid 60's with a friend who was wearing a watch with an expandable metal band. He inadvertently touched the metal band to the positive post of the battery and the car body. The band then became a conductor, welding itself at both ends and getting red hot. My friend now has a permanent expandable-watchband-shaped scar on his left wrist.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

A place where I formerly worked had a shelf for charging batteries. The shelf acted as an enclosure. One day, as I unhooked a battery from the charger--without unplugging the charger first, I got an arc, and ignition of the gases. It was quite a shock, and quite a noise.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags