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Discussion Starter #1
Hi Guys,

In need of a bit of advice here. I have just bought a cheap MGF 1.8i VVC.
When my young Son and I went to pick it up, the previous owner said the battery is dead and it had been sorn and unused for 18 months. So I had a spare battery charged and took it with me. The motor turned over, however, there was not enough power it the battery. So I had some jump leads and I was in the car and my son connected the jump leads to the battery terminals.
Oh dear,? the leads were connected incorrectly and now there is nothing. I have heard that the battery would be no good now and that the alternator inline fuse could be blown. Is there anything else that could be fried? I know a silly school boy mistake but it happens? thanks guys.

Dave
 

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'06 MG ZR +120 (HQM) '04 MG ZR 105 (IAB)
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The main thing likely to have been fried is the engine ECU, but if the main fuse has blown, you may have got away without any damage.

When jump starting, connect positive pole to positive pole on both batteries, but ALWAYS connect the negative pole on the slave battery to a good earth on the engine/body of the car that you are trying to start as connecting it direct to the negative pole of that car's battery is also likely to blow a main fuse or damage the ECU.
 

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Discussion Starter #3
Oops, ok, I do have an ECU for the VVC and an MPI, if the ECU is fried would it be the MEM3 with the OBD1 connector just above the fuse box?
 

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mg_tf
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The main thing likely to have been fried is the engine ECU, but if the main fuse has blown, you may have got away without any damage.

When jump starting, connect positive pole to positive pole on both batteries, but ALWAYS connect the negative pole on the slave battery to a good earth on the engine/body of the car that you are trying to start as connecting it direct to the negative pole of that car's battery is also likely to blow a main fuse or damage the ECU.
Hi Man In the Car. Interested in your reply about connecting battery's to the car. I've always connected positive to positive and negative straight to the battery. Can't honestly see what difference it would make wether you connect to the body of the car or the battery as they are both effectively the same connection. I'm always willing to learn, could you explain why you say connect to the body please. Phil
 

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mg_tf
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Hello Phil,
I too agree with MitC’s advice not to connect the last-to-be-connected (usually) negative lead direct to the battery terminal.
My reasoning is based on the possibility of hydrogen gas fumes around the battery (older types anyway) and connecting directly onto the battery terminal increases the risk of explosion via. possible spark. Not sure about remote earth connecting minimising the risk of damage to ECUs etc. since it’s simply connecting to earth at a different point, but I am also willing to learn as well.
Kind regards,
Austin.
 

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The explosion risk is a big part of the reason for connecting the negative to an earth point away from the battery. It also has the effect of softening the charge transferred from the good battery - although not always likely to occur, a direct connection to the earth pole of the flat battery can give rise to spikes and surges which can damage the battery, and have been known to damage electronic circuits which are directly connected.

It is often when the jump leads are disconnected that the spikes happen - the alternator of the 'flat' car reacts to suddenly going from a system being served with normal voltage (from the slave car/battery) to having a battery with very little voltage, and you get a surge which can overload sensitive electronics. When the negative jump cable is connected to an earth point rather than direct to the flat battery, the sudden change when it is disconnected is less dramatic.

It is a relatively frequent occurrence, but some makes/models of modern cars seem to be more susceptible to damage than others.
 

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mg_tf
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Many thanks MitC for the additional explanation. As said, I can well understand the elements of minimising the risk of explosion by not connecting direct to the battery terminal.

However, I am still a little confused regarding the ‘softening of charge....’ explanation since all major metallic parts of the engine etc. will be at exactly the same potential difference as the battery terminal itself - via. the importance of ensuring the battery earth terminal to engine and bodywork cables are sound and the connections good.
Kind regards,
Austin.
 

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Well, it certainly seems to be a 'thing' - there are numerous reported instances of electrical issues related to connecting the negative direct. The PD will be the same, but there will be greater electrical resistance via a roundabout route.
 

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Discussion Starter #10
Yes, I am aware of the hydrogen gas discharge ?but hey, it’s all sorted now, just changed the battery, checked all the fuses and it started first time ?? Lovely little motor and it goes well. What a bargain ?.
 

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Yes, I am aware of the hydrogen gas discharge ?but hey, it’s all sorted now, just changed the battery, checked all the fuses and it started first time ?? Lovely little motor and it goes well. What a bargain ?.
So, you didnt find anyhting had been fried or burnt out? Just replacing the battery and the car started for you?
 

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That is correct Pat, I read somewhere that if the voltage is less than 11.2 volts on the battery, no lights or ignition will light on the dash board. Effectively shutting down, so no damage is done.

So think I was lucky that nothing was affected with the wrong polarity for jump starting.
 

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That is correct Pat, I read somewhere that if the voltage is less than 11.2 volts on the battery, no lights or ignition will light on the dash board. Effectively shutting down, so no damage is done.

So think I was lucky that nothing was affected with the wrong polarity for jump starting.
I had exactly the same problem caused by the same action, like you I changed battery and all seemed to work until I went out at night with lights on, new battery went flat after 10 mins driving in dark. After checking in detail I found fuse blown in engine compartment, changed fuse and never had problem since
 
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