Why not the positive??

 

I was wondering why I had to do this when I installed my AirRaid tonite. When I went to put it back on I definitely jumped when I touched the clasp to the terminal and the horn honked and the wipers started moving

 

Negative is less likely to arc.
You do it to reset your computer so it learns with the new mods instead of using the old settings.

 

also, as a mechanic i was always told that the computer is directly connected to the positive side and if a spark occurs upstream of the ECM it might travel on to the computer. If you disconnect the negative with is downstream of the ECM and it sparks you have little to no chance of shorting out anything important

 

I'm not a scientist or anything, and someone else may have better info than I, but I think there is at least one good reason:

If you are removing the Negative cable and you happen to ground yourself against the vehicle while in the process of removing or reconnecting it, you are much less likely to get shocked. To be shocked this way, you would have to be both grounded to the vehicle and touch the negative post of the battery without first connecting the ground strap and something would have to be switched on that demanded enough juice to shock you. If you touched the positive terminal while grounding yourself to the vehicle your body completes the circuit and you take the full charge of the battery.

 

from a electrical stand point from which I come from vechicle electrical system are DC It does not matter which side you take off to reset computer eve nfrom a voltage spike it needs to go somewhere to get the spike as far as getting shock from a car battery it wont happen unless you jam a strand of wire into your skin your skin has to much resistance for a 12v dc source to shock you

 

It is hard to get a shock from a 12 volt system but if you ground out a wrench while loosening the positive cable you can kiss your computers goodbye.

 

It doesn't take that much to fry the typical computer!

 

I've taken shocks from 12v, 110v, 7500v & 15000v and I can tell you from personal experience, that I'd rather take 15000v @ 30ma than 12v @ 500a all day long. I used to work with a buddy who owned a neon sign shop installing neon, and the big transformers put out lots of volts at very low amperage. You can get one of those babies to arc 6 - 8 inches but when you get bit by one, it only stings a bit.

I've been jolted working on my boat, (a 12v system) and that thew me for a loop. It left a burn mark on my hand where it came in contact with a Remote positive terminal while my other hand was resting on the intake manifold.

All that aside, the worst shock I ever got was from a 7500v 60ma transformer on a neon burn in rack. It makes my hands shake just thinking of it.

Fortunately DC voltage doesn't cause you to grasp and hold like AC does, or at least that is my experience. Every time I've been hit with a DC charge, I've been repelled by it. I've heard stories of people getting bit by AC, and not being able to let go, but those may be old wives tales.

As always, your mileage may vary... but I have learned to be very cautious around electricity regardless of the voltage (ok, maybe I'm not as careful with AAA batteries as I could be)

 

One Chrysler service manual said to disconnect both and then touch them together for a minute or so to totally re-set the system. This drains any leftover electricity that is stored in capacitors I think...