I have always been curious about this. Everytime someone recommend do something (replace/repair) in the car, he would say first "remove the negative battery wire". I understand you want to cut power to the electronics so they don't get fried. But why the negative? Isn't removing the positive wire accomplishing the same thing? Just wondering.

The simple rule of thumb is to ALWAYS dis-connect the negative side first. That way, if you accidentally short the positive side when removing it, no damage will be done.

It's too easy to hit the wrench to ground when removing the positive cable. Thus you always remove the grounded cable first, which is almost always the negative cable.

Exactly correct.
If you ground the negative cable by accident, there is NO possible danger/damage/hazard.


If you ground the positive cable by accident, the following thread covers the facts of what can happen in great detail.

Battery Safety
http://www.peachparts.com/shopforum/diesel-discussion/141479-battery-safety.html#post1057827

I remember lending a few tools to my neighbor so he could service the battery. From my window I could see the sparks fly as he was attempting to remove the positive cable first.

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95 E320 Cabriolet, 169K

Negative is where the electrical flow goes...high low gradient...?
Paul

The reason is 100% because of safety.

Say you are removing the positive terminal with a wrench (before removing the negative terminal), and that wrench hits ANY part of the vehicle whatsoever (the engine, A/C compressor, P/S pump, a pulley, the chassis, the hood, the frame, a cable, WHATEVER), it WILL short out because that which it hits is grounded. This is VERY dangerous because, with the amperage a battery puts out, it is likely the wrench become WELDED to the surface which it hit - causing a short circuit in the battery you can't stop in time. 1 of 2 things will happen if you don't get that short rectified (usually within 1-2 seconds):
1) Your ground cable will melt or catch on fire
2) Your battery will explode

Don't risk it.

This brings back a violent memory. Had a tech buddy of mine not had the hood in the fully 90 degree position on a 123 back in 1984 he would have definately blown the hood or hinge off the car. The explosion was as violent as anything I have ever heard. The top of the battery separated from the rest when it blew off. All because a wrench slipped down and grounded between the pos post and the chassis. Happened in what seemed like milliseconds. There were pieces of the plastic bomb and acid everywhere. That guy from that moment on (years) wore safety glasses at all times in the shop. He had perpetcha poop in his pants...

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64 European Coupe Brilliant Silver with Red interior 4 Speed Sunroof
87 TDT 286k
87 SDL 195k
83 SD 202k
83 SD 201k
83 D 286k
02 R1150RT 32K
00 122HP FLSTF
07 KTM 250SX

"Danger takes many forms"