Jump Starting

[Guest Cattwg]

Having just read the postings on jump starting I became very concerned. I 'jumped' my son's car only yesterday using the traditional connection method. May I ask two questions? I assume that connecting the 'good -ive' to the flat vehicle's engine block is to help protect the electronics of both vehicles. So does this mean that the engine management electronic earth is separate to that of the other vehicle electrics (e.g. starter motor)? I had always understood that all vehicle earths were common. I have always started the donor vehicle before making any connection to the other vehicle - on the premise that this reduced the risk of discharging the good battery. What is the reason for making the connections before starting the donor vehicle? Please understand I am not doubting the knowledge of other correspondents - just wishing to understand. Regards Cattwg

[Guest Dave Newell]

Hi Cattwg (how do you pronounce that?). Firstly does your son's car run alright with no warning lights showing? If so then there is no need to worry as it's obviously not done any damage. The reason for connecting the negative jump lead to the recipients engine block is to reduce the earth path for the current from the donor vehicle to the starter circuit and thereby reduce the possibility of voltage drop through cables and connections. The reason for connecting up first is to reduce the likelihood of voltage surges. A running engine's alternator, even at tickover, can produce around 14 volts. If you connect that to a motor with a battery voltage of say 10 volts it could cause a sudden loading on the donor alternator and result in spikes/surges that could potentially cause damage to electronics in either vehicle. This is as I was taught by my father and confirmed by my tutors in college during my apprenticeship as a motor mechanic and auto electrician.

[Guest Cattwg]

Hi Dave, thanks for replying. Yes, I understand - I think I was expecting some obscure electronic based reasons. I'll follow your guide lines in future. Interestingly the AA 'jumped' a friend's car last year - straight across the battery. You'd think they would know better! My son's car (and our 'van) are OK. Cattwg is pronounced cat-oo-g (g as in god). Regards Cattwg

[Guest Dave Newell]

Cattwg (I still can't pronounce that), the guidelines as I stated them are to minimise the possibility of damage occurring, which doesn't necessarily mean it will cause damage if the guidelines aren't followed. I've always stuck to them and haven't killed a car yet. (famous last words???). Dave.

[Guest David Powell]

Don't worry about it Cattwg, the Auto electrician at a wild guess reckons my old Vauxhall Senator killed its self, as the management system on it was of the early design and just could not stand the sudden surge from the second alternater putting power into the system on top of that from the doner truck. The following year after my model, Senators were fitted with a more modern and much better computer system. As it was the old one that died was always giving trouble by disagreeing with the automatic gear box computer. I expect your sons is a modern one that can stand the odd surge/spike with no ill affects. I have since been told that if I had switched on all my lights and the rear window heater, that may [only may, not would] have absorbed the extra power surge and saved the car. But that is all in the past now, modern auto electrics are all different, and much better.