Jump starting

Post by **Sidney'61** » Mon May 25, 2009 11:29 pm

Hi all,
Maybe I'm just being stupid here but...
When jump starting a friends car the other day we thought we better check we were doing it right and consulted the manual, it states that you must connect up the positive terminal first, this is a modern car so it would be negative earth.
Logically I'd have thought the safest option was to connect the earth first and not the live one, but apparently not.
Why is this?

**Runs and hides for fear of somebody posting a really obvious answer and me looking stupid**

Post by **d_harris** » Mon May 25, 2009 11:46 pm

The idea is to prevent sparking I believe....

Post by **PSL184** » Tue May 26, 2009 1:01 am

Always remember the earth rule "disconnect first - reconnect last" If you were to directly earth 2 vehicles (via jump leads) then connect one positive battery terminal and accidentally drop the other one onto the car before connecting to the battery it would go ever so slightly "BANG"

Post by **PAULJ** » Tue May 26, 2009 9:22 pm

if you connect earth first then the live, if you were to drop the live or connect it to the wrong part you will have a direct short. Connect live first and it wont matter what the earth touches or hits. Same dissconnecting. Earth first

Post by **Sidney'61** » Tue May 26, 2009 11:03 pm

Thanks guys, I'll remember that in future, when I have to jump start another unreliable eurobox...

Post by **bmcecosse** » Tue May 26, 2009 11:45 pm

The secret in 'jump starting' of course - is to connect the cables - and then let the donor car charge up the flat battery by fast-idling it's engine for 5 minutes before attempting the start!