Georgef has raised a pertinent issue, re. when removing the -ve battery connector, as touching it firmly to the +ve pole will discharge any capacitors wherever they may be incircuit. Now it's a feature of c-mos memories they DO tend to hold parts of their memoy even when you think those data have been dissipated. C-mos is usually a 12volt device, and I've known with volatile memories they can keep some data even if the power supply drops as low as 2volt, which could be the voltage on a semi-discharged electrolytic capacitor.
You don't know what capacitors are involved, they may not even be on the microprocessor boards, being in some cases at various points on the chassis. My Jag. had this type of design.

So I'd advise after disconnecting the -ve connector, touch it to the +ve battery pole for about a minute. This should totally clear any c-mos memories. Of course, it doesn't apply to TTL logic. The usual caution should be applied to working around any lead-acid starter battery, make sure you don't inadvertently short the battery.

Leedsman.

 

Maybe I'm not understanding what you're describing. Are you saying to simply disconnect the battery negative cable and touch it to the positive side of the battery?

 

Er...yes.

Leedsman.

 

Why do I feel like this is NOT something you should do.

 

I would not consider doing that ............. ever. If you want to take long shot old mechanics tale at face value and clear [volatile] codes that are cleanable before the caps have emptied then undo and remove both terminals and clamp them together.

 

As long as a least one of the battery terminal is not connected to anything there is no danger............ever.

This procedure will clear values held in the computer's memory such as Long Term Fuel Trim (LTFT), these values are not reset by clearing the codes with a code reader.


If it seems safer to you, disconnect both terminal, but it is not necessary.

 

So just to be perfectly clear so there's no question.....

I disconnect the negative battery cable, leave the positive cable connected to the battery, then touch the disconnected negative cable to the positive cable and post (which is still connected to the battery). Holding them together for about a minute, this will clear any code memory stored in the OBD system. And to make this even more clear, this will damage NOTHING....., nor will I wipe clean the ECM or PCM system of the information needed for the vehicle to run properly?

Last thing I want to do is see a rash of sparks and find I just destroyed most of the electronic system on my van because I did something which goes against the inner fibers of how car batteries work.

 

Correct.

It will clear the codes but more importantly it will clear the "learned" data in the PCM.


It will not damage anything, the PCM will be reset to factory defualts.

How can there be sparks if one terminal is disconnected?

Like I said above if it makes you feel more comfortable you can disconnect both cables and touch the ends of the cables together.

 

Rule of thumb for electrics (also applies to 230volt house electrics):--

If you think you are going to do something a bit dangerous, like shorting a starter battery, apply your SMALL* 12volt test lamp between the terminals you are about to short together FIRST. There are four possibilities then:--
1) The lamp will not light at all, indicating no current.
2) The lamp will light dimly, indicating a small current, less than the full current draw of the lamp. This is usually safe.
3) The lamp will light brightly, showing it's time to do another check with a bigger lamp. If the second lamp also lights brightly, excercise caution.
4) The lamp will light briefly, a 'pulse' of light. This indicates you have discharged a capacitor somewhere -- i.e. the whole point of this excercise.
Remember electrolytic capacitors can charge themselves up a little even after you think you have well-discharged them! An electrolytic is constructed like a cell/battery inside, hence they are always polarized unless it says on the label "non-pol".

Hope this relieves any anxiety.

Leedsman.
p.s. The old way to check if an electric 230volt terminal in your house was "live" was to have a 15watt lamp on wires, connect one wire end to a firm ground/earth/mass and the other to the terminal you are about to work on. If the lamp lights at all in this situation, sort it out. You can use a neon screwdriver too, but there can be complications with these. (One complication being that the finger on the screwdriver may not be earthed. It may be relying on capacitive effects, esp. if the electrician is wearing well-insulated rubber-soled shoes). Also make sure your ground is good, a copper pipe known to sunk in the ground, the supply ground, etc. are good earth points.
*Small in this case means a flashlamp size.

 

georgef,

It only ever worked, if at all for the old pre-CAN BUS, on modern telematics BUS's I can't see any value only an increased risk, this is a pure physics rather than a car / computer debate. Arguing that no circuit = no current ([equaliser](equalise relative to what) doesn't wash for me. All modern cars are incredibly voltage sensitive and asserting as you do its safe without telling me what is equalising ..... relative to what ......... seems an unnecessary increased risk. The static voltage in any car can be much higher than the value of the negative terminal.

You can as I stated earlier achieve the same outcome without risk - """remove both terminals and clamp them together""".