i have never done it and i'm not terribly mechanical - do you leave the positive lead on the battery and just hold the earth lead to it for ten seconds?
won't that cause a load of sparks?

 

Here in the colonies we use a ground lead, for want of an earth lead. But I'll get serious: what you described is correct.

Rule of thumb: it is when there is a path established between the battery terminals that things can get exciting. Connecting both leads to either terminal, with the other terminal disconnected, is safe.

 

i did it to see if a gearbox fault msg and clear gearbox problem would clear - i charged the battery 1st too. - i'll write a more comprehensive msg in a few minutes. - thanks again.

 

Shouldn't be any sparks because there will be no current flow from the battery pos. term, through the car's electricals and back to neg. terminal.
Holding the connections together discharges the decoupling capacitors in the modules, they can 'keep alive' the memory for a short period of time.
Personally I err on the side of caution and give it 5 to 10 minutes (some give it even longer :-).

 

Thanks for replying.

are you saying 'hold' the earth against the actual positive connection rather than just the top of the 'pole' ?
for at least ten minutes?
might seem a daft question but it's not clear -people usually just say touch the positive for ten seconds ...
do you leave the positive lead connected to the battery pole all the time too?

 

No problem - not daft question:-) Just disconnect the car's ground neg cable from the battery and touch it against the pos. terminal or the pole, all the same thing.....
10 seconds may do it if in a hurry but for peace of mind a longer period, because:-

Shorting the + and gnd. cables puts a short across the capacitors within the modules.
BUT the wiring loom in the car constitutes a large amount of copper wire, this has it's own (albeit low) resistance. The higher the resistance, the longer it takes the capacitors to discharge. Because that loom resistance is a guesstimate it's best to leave the discharge time a bit longer.
Basically if the codes don't clear with a short discharge time, try it again with 5 - 10 minutes. Let me know how you get on.

 

i take it by codes don't clear - you mean that should clear codes of a code reader? will it clear a gearbox fault msg if it's battery condition related?

 

Yes, once they are deprived of power the DTCs in the modules' memory will be cleared.
Upon reconnecting the battery the only code you should see is P1000 (recently cleared codes/battery disconnected).
Only when you start the car and drive will any DTCs be rewritten. Personally I don't know if gearbox codes can be triggered by lowish battery, I know ABS modules certainly are.
Just a thought:- If you're happy that the battery charged OK from an external charger
it might worth checking the car's own charging.
Here's some typical 'good' values.... 12.5 to 13v static, no engine running.
14 to 15v with the engine running at idle. (Voltages measured at battery terminals).

 

thanks for taking the time to help - i'll get the indie guy to check that too.