Started car in gear! Doh! Starter motor now dud?

Stooby2

TrickyWicky wrote: »

Thing is nearly all cars these days are ECU driven and have an immobiliser - part of the ECU. Therefore even if you do it the way you mention above, the ECU is still live when you connect the other vehicles battery to the flat battery. There will still be that initial voltage spike regardless and the ECU will still be powered from it.

Yes ABS lights and all sorts can come on. Typically because the battery was flat in the first place and the ECU has detected that something somewhere isn't working properly and raises it as an error. My car brings up the brakes as a problem when it has a flat battery just like it brings up the injection system when the fuel pressure is too low due to it having a blocked filter.

I was given to understand that attempting to start the broken car whilst connected to the good car produces the voltage spikes, not the connecting disconnecting of the leads.

I've been reading various forums about this and there doesn't seem to be a clear answer - the same mixed bag of opinions we've got here. I will keep doing it the way I've been taught as I'd rather be safe than sorry.

TrickyWicky

Stooby2 wrote: »

I was given to understand that attempting to start the broken car whilst connected to the good car produces the voltage spikes, not the connecting disconnecting of the leads.

You have a flat battery at say... 8V. You suddenly connect a 12V battery via jump leads (and in the process you get a spark from the crocodile clips). When do you think you are going to get the surge? - When connecting or once it's been connected for a few mins?

The intermittent sparking when connecting pretty much happens with all jump starts - you might not see if but if you look very closely when you connect the second lead (positive or negative) you will always see a small spark there. THAT is when you're going to get a sudden voltage surge.