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Disconnecting Negative Battery Cable When Car Is Running


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#1 andy_1984

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Posted 21 September 2011 - 01:43 PM

seen a tip here somewhere regarding removing the negative cable from the battery while the engine is running to check if alternator is doing its job correctly.

i know from searching google to that this isnt exactly safe for modern cars but i tried it anyway since im going to get it checked by a mechanic.

when i remove the negative cable the car runs but if i turn the heating on the traction control and abs lights come on for a split second then the car dies.

does this sound like normal behavior to anyone ?

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#2 artscot79

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Posted 21 September 2011 - 02:18 PM

seen a tip here somewhere regarding removing the negative cable from the battery while the engine is running to check if alternator is doing its job correctly.

i know from searching google to that this isnt exactly safe for modern cars but i tried it anyway since im going to get it checked by a mechanic.

when i remove the negative cable the car runs but if i turn the heating on the traction control and abs lights come on for a split second then the car dies.

does this sound like normal behavior to anyone ?


not sure if its normal but its certainly a big big no i have seen fried alternator voltage regulators because of people doing this there are cheap testers and garages will test the battery and alternator mostly for free you cant test the alternator corrcectly doing it this way ide have taken it to a garage and asked if they do a battery and alt test youre attempting this i believe To see if its the altornator not charging, disconnect the negative to the battery, if the vehicle dies, then the altornator is bad.

It doesn't work on any car thats been made in the last 30 years though. You need a multimeter (at least) to test the voltage of the battery while the car is running, and test the amps when the car is shut off. Depending on the car, many have a GEM module now that stay energized for up to an hour before shutting themselves off, and will show a higher-than-normal draw until then. So it takes a bit of time to test for a draw/

Typically a bad alternator shows up as less voltage running than when the engine is off. Look for 12-13v off and 13-14.4v running. Anything less and you might have an alternator issue but remember smart charge can increase or decrease this voltage automatically depending on many variables

#3 jeebowhite

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Posted 21 September 2011 - 02:29 PM

Sounds about right, because your pulling a lot more juice and its got nowhere to ground, so that puts your entire car at risk, the battery will probably detect an excessive battery and just cut out, causing it all to fall over.

Bad idea disconnecting the negative - purely on the basis you touch any metal part and you could have shocked yourself, a 12v shock isnt bad but isnt nice - its the amps that it kicks out though...

what seems to be the problem with the alternator?

#4 andy_1984

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Posted 21 September 2011 - 02:46 PM

jeebowhite, still got the car running rough at idle and despite artscots best efforts in previous posts and my vain attempts at diagnostics i still havnt resolved it. got a big pay cheque coming up shortly so going to get my garage to give it a proper going over and actually solve the damn thing once and for all.

i think im imune to some 12v shicks. im always tinkering with electrics in some form or other :unsure: its the coil pack on the ford that gives me a kick :lol:

#5 Lenny

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Posted 21 September 2011 - 04:47 PM

sorry guys, that was me who wrote the post on disconnecting the battery, I was told by a mechanic before that if i was buying a car it’s a way of checking it out, as well as checking the inside of the oil cap for white traces etc.

ill edit the post as I now know that this is unsafe practice.
However I must stress that I didn’t specify to remove either positive or negative terminal from the battery i had assumed it would have been self explanatory to remove the positive from the battery ONLY, as this feeds the power in to the battery,
removing the negative while the engine is running is a whole different ball game.

and since the person carrying out this method would have a concept of power and electricity I didn’t think it would be a high safety risk factor to the individual as the reason for removing the positive link from their battery would be to see if the alternator is circulating power,
power in terms of electricity

thus having some understanding of an alternator, electricity and its risks basically don’t try and taste the cable because it looks like a strawberry :blink:

in my personal opinion; if anyone touches the terminals of the battery or the bare end of the cable's during this practice needs more help than just a guide for checking the working of there car.

I apologise for any incontinence caused and have also learned a lesson today myself reading this thread, thanks guys :)





#6 johnH

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Posted 21 September 2011 - 04:53 PM

just a bit of info, the alternator can push a min off 105amps!!! 1 amp can kill

with your car off the battery should be pushing 12.5V

with engine running you should have around 14v across the battery, this shows the alternator and smart charge is working.

if the engine is running and your getting between 11-13v your alternator is still working but the smart charge isnt

typically alternator faults are shown by a battery light on the dash.

and oh ive heard disconnecting the neg from the battery is a big no no when the engine is in use. basically you have just removed a safety earth! and turned your car into a massive capacitor and they can hold charge for years! and from personal experience of being an earth for a capacitor its not nice and it hurts like crazy...life of a RAF techy lol

#7 Will Clark IOM

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Posted 21 September 2011 - 05:30 PM

Getting an electric shock off household mains (a.c) is nothing compared to (d.c) the only way to get off an electric shock from a battery is someone hitting you off it with a wooden plank due to you being the batteries discharge.

#8 Lenny

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Posted 21 September 2011 - 05:42 PM

Getting an electric shock off household mains (a.c) is nothing compared to (d.c) the only way to get off an electric shock from a battery is someone hitting you off it with a wooden plank due to you being the batteries discharge.


What a Guide that would be Will :lol:

Step 1:start car
Step 2: excreta .......


.............
step 6: ensure your mate is on standby with a rail way sleeper in his hands wearing a pair of wooden sandels ( just in case) ;)




#9 GRAHAM46

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Posted 21 September 2011 - 07:34 PM

A mate of mine is an auto elctrician by trade and i've seen him test hundreds of batteries and alternators over the years. Takes seconds and normally done for free.

Graham.

#10 stef123

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Posted 21 September 2011 - 07:55 PM

What a Guide that would be Will :lol:

Step 1:start car
Step 2: excreta .......


.............
step 6: ensure your mate is on standby with a rail way sleeper in his hands wearing a pair of wooden sandels ( just in case) ;)


:lol:

a brush handle makes a good weapon, i mean useful tool to remove someone from the live part

#11 artscot79

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Posted 21 September 2011 - 07:59 PM

:lol:

a brush handle makes a good weapon, i mean useful tool to remove someone from the live part


since all has failed it may be that theres a diode gone in the ecu which can cause a rough idle and slight misfire thats the last thing i can think of that requires the ecu to be unplugged sent off for testing and repair i believe its around 250 quid

#12 andy_1984

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Posted 21 September 2011 - 08:43 PM

was well aware of the risk. im just at my wits end with this car. if it turns out to be a major problem i might just end getting rid of the thing and getting a bicycle.

#13 artscot79

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Posted 21 September 2011 - 08:54 PM

was well aware of the risk. im just at my wits end with this car. if it turns out to be a major problem i might just end getting rid of the thing and getting a bicycle.


the diode was the only other thing i know of that can cause the issue rough idle and misfiring at idle

#14 Lenny

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Posted 21 September 2011 - 09:14 PM

was well aware of the risk. im just at my wits end with this car. if it turns out to be a major problem i might just end getting rid of the thing and getting a bicycle.


Careful mate even a bicycle has a dynamo :lol:



#15 jeebowhite

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Posted 22 September 2011 - 02:19 PM

LMFAO... brilliant!

This really is turning into a good thread!

The moral of the story is, dont disconnect negative and start your car, you may end up redisigning your hair, and turning your car into a giant electric shock bomb...

On the plus side, nobody was injured and we all seem to have had a laugh at the idea, and have learned how to save your friends !Removed! if they do end up doing this.

Anyway, hopefully you got the car sorted, and it doesnt cost a bleeding fortune! with any luck it wont cost you an arm and a leg and then you find out its a broken cable... like johnH found...

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