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
********************
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 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

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

The good news is a dead battery is less acidic than a charged battery.

I was watching two people jump a car once and I heard a loud BOOM. I looked over and the battery had exploded. They were lucky none of them were near it.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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I do not agree with a battery being less acidic. That thing is full of sulfuric acid. Water dilutes it, while a base like baking soda neutralizes it.

The explosions are the result of the hydrogen gas being ignited by a spark.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Agree or not, it's a fact that you can easily measure. The ions are fused to the plates during the discharge, resulting in a much lower concentration of acid.

If you connect the neg cable last, and disconnect it first, it shouldn't spark, should it? I know electrons flow from neg to pos but the current flows from the pos and I've never seen the neg cable spark, say when reattaching it to the battery with the pos cable already attached.

I could rationalize it if it was the pos cable, because I've seen it spark many times, but never the neg.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo

I have seen the negative spark as well, because of bad connection or by going slow to connect.

As far as the battery acid being weaker, do a litmus test on it and look at the ph level. The sulfuric acid is about 12m when added, then diluted a little with water. Either way, the acid will burn you.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

I never said a discharged battery has no acid in it or it's safe for a discharged battery to blow up in your face.
All I said was a discharged battery is less acidic than charged battery. I don't know what you tried with your litmus paper but you can't argue with the laws of chemistry. When a battery discharges the hydrogen atoms fuse to the lead plates causing a less acidic solution. That is a fact whether you choose to believe it or not.

Anything else you get out of that your making up.

__________________
1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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