I have read that when you "reset" the computer in your vehicle....clear codes etc.......that you disconnect the negative cable.

My question is will the computer reset it self by disconnectinf the positive cable only?
Richard

 

Yes it will.

 

Yes, but you can file that under Very Bad Ideas.

Basically, the negative side of the battery is connected to the vehicle ground. There are some exceptions, but about 99% of the cars out there are negative ground*. So every time you touch the door, the hood, the frame, etc. you are indirectly touching the negative side of the battery. If you touch the positive side of the battery, and lean on the fender, you can get a very nasty shock. It's quite unsafe.

Always disconnect the ground (negative) side of the battery first.

* my friend had a 97 Taurus SHO, and I was very surprised to see it was a positive ground.

 

Disconnect the negative cable, and touch it to the positive cable, and the PCM will reset immediately.

 

might want to disconnect both cable before you do that,or there will be a lot of other things reset immediatly also. thought I would clear that up in case the original poster was confused

 

There is no need to disconnect the positive cable. But, you do bring up a good point. Just for clarification you DO NOT short the battery posts together!! You disconnect the negative battery CABLE, and touch it to the positive cable (which is still on the positive battery post).

 

The reason I bring this up is because my g/f's son bought a 97 Land Rover Discovery back a couple months ago that he allowed to run hot so it has a replacement 4.0 engine but the other day, his alternator apparently went 'south'. He drove the vehicle here to my house and I checked the voltage from the alternator and it showed to be 12.6 volts approx.

He removed the positive cable from the battery and the alternator and I installed a remanufactured unit from Advance the next day. I got the same reading out of the replacement......12.6 volts while running.

We replaced that alternator and when I picked up the newer unit, I had them check both pieces at Advance and they were both found to be 'good'.

Anyway, I put the newer unit on an got 13.7 volts out of the unit. I also add that the voltage fluctuated from 12.8 to 13.8 with newer unit. I was told that I needed to reset the computer by disconnecting the negative cable and not the positive....like he did.

Mu g/f's brother-in-law told me that he resets computers buy turning the ignition switch to "on" and then removing the positive cable from the battery while the switch is on. I told him I just remove the negative cable while the switch is off.

 

To disconnect the positive cable with the key on is a very bad idea, unless you like replacing the ECM. Voltage spikes will ruin computers quite well, and while removing the positive cable, there will be plenty of them. That is why the negative is the one to remove, the arc goes to ground, rather than through the computer. That also is why when you hook up jumper cables, the ground is always the last to be connected. The key does not need to be on, the memory will be erased all the same, as it is only there while power is applied, disconnect the power, and the memory will be erased. The computer is on a continuous feed, like the memory circuit on the radio. It will be erased in the same way as the memory circuit on the radio is.

 

I disagee, a voltage spike from Di/Dt will be created whether you disconnect the positive, or negative battery cables. You should not disconnect either cable when there is current flowing (charging, or discharging).

 

I won't argue there, that is also why I stated there is no need for the key to be on...