Car jump starting

  VNAM75 22:25 29 May 2008
Locked

My brother phoned me and asked me how to jump start his mates car which wouldn't start. I gave him the instructions in my ford manual:

connect both positives ends
connect the negative end of the boost battery (clio dci - diesel) to part of the engine frame of the flat battery car away from the battery (punto - petrol)

I couldn't remember if the red was the positive end so they got alternative advice. They ended up connecting the + and - ends of each battery. He said he was told to rev the boost car to 2000 revs which started the car.

My questions are:

1) Are there any risks by not connecting to the engine frame of the flat car but to the - of the battery. Would there be any damage to the car?

2) I've read that a diesel car shouldn't be used to boost a petrol (12v) car, nor should it receive boost from a petrol car.

Are these just basic precautions which you can get away with as a one off or are there genuine risks if they are not followed each time?

  Strawballs 22:34 29 May 2008

1 No the negative strap from the battery goes to the frame and the engine block so that all of the electrical items don't need to have wires running back to the battery.

2 I can't see why there would be a problem with that one as they are only 12v batteries in both.

  Pamy 22:38 29 May 2008

the + and + should be conected and the - from booster car should be connected to the engine as far from battery as possible. In most cases if you connect + to + and - to - it should be OK , but it is possible that you can dammage diodes in alternator, so i was told by mechanic.

  VNAM75 22:42 29 May 2008

Strawballs, I don't understand 1. Should you connect to the negative battery end or the engine frame of the flat car or is either fine?

  peter99co 22:43 29 May 2008

Some cars have very sensitive electronics and can be written off if they are subjected to a voltage surge.
It is far safer to charge or change the battery. The computer controlled engine management equipment can be very expensive to replace.

  VNAM75 22:45 29 May 2008

Pamy, I think you're right.

The punto was neglected. The battery looked very old and tired. Don't think the terminals were ever cleaned and regreased.

  peter99co 22:58 29 May 2008

Check the started car with a voltage tester on the battery terminals. At 2000 revs+ you should get 14 volts approx. If this is the case the alternator is working.

  VNAM75 23:05 29 May 2008

I've asked him to get the alternator and battery checked at a garage. I've had to replace the alternator on my previous 2 small petrol cars.

Was 14v too much(unnecessary to rev the car?)for the 12v battery?

  peter99co 23:16 29 May 2008

No worry at all, you need 13.5 to charge the battery and 14.2 is the cut off point. A charged battery should show 12.7 volts. (with the engine off) only 12 volts and the battery needs charging up.

A voltage check is safe if you take care to put the red + and the black - leads on the proper terminals of the battery.

  peter99co 23:19 29 May 2008

If you have had alternator damage is it because you have had electric welding carried out on the car? ie failed to disconnect the alternator before welding started?

  Ranger 23:20 29 May 2008

My sister allowed her car to be used to jump start a car, it blew something in the management system which cost over £100 for the part alone, since then niether I or my sister give anyone;s car a jump start

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