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Old 06-13-2010, 04:04 PM
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Ground wire on fire...

Hi there! This is the situation: `72 Pontiac LeMans, 454 BB Chevy, big battery in the trunk. All running normal until today. After a short trip of 20 min. at 50 or so MPH the thick negative battery wire went on fire mostly in the last foot where it meets the bat. After disconnecting the bat and let all cool down I tried to reconnect the wire and a lot of sparks were the result. I started checking the wiring and I found that a part of the negative wire just over the differential was a little melted and shows evidence of contact against one exhaust pype. The pype have traces of sparks and some copper melted but the wire is negative! If it where positive I could understand, but is negative.
Next thing I found was the alternator grounded at the charging post. If I disconnect that post the car cranks ok but it doesn`t start even having energy at the distributor and after a couple seconds the blue wire that feeds the distributor starts smoking. What the hell is going on here!?!?!??!?

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Old 06-13-2010, 11:38 PM
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sounds like engine block has loose or undersized ground. where does your neg. bat cable ground to and how is it tied in to your block ground ? high amp draw from your starter with a restricted ground path back to battery will do some like things(i.e. the arcing on your exhaust pipe)
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Old 06-14-2010, 06:55 AM
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Yes sir. The ground wire goes from the engine block directly to the battery and no wires between engine, body or chassis. Will ground it propperly.
What about the distributor cable? Just a dead HEI due to the ground problem?
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Old 06-14-2010, 02:50 PM
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You have a full frame chassis. You can connect the ground wire from the neg. terminal of battery to the frame, From the frame up front connect a ground wire from the frame to the block.
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Old 06-14-2010, 02:55 PM
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You need good grounds to the engine and the chassis for all your systems to work properly.
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Old 06-14-2010, 03:15 PM
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The same Battery gauge wire (#0-#2 gauge) From battery to frame and from frame to engine block.
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Old 06-14-2010, 03:24 PM
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Sounds like the alt grounding at the bat post turned your car into a positive ground vehicle. Your battery, alternator and other components could have suffered as well as what you've found so far.
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Old 06-14-2010, 08:09 PM
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Redundant grounds are king.

I've seen this many time before. The more horsepower or suspension flex the more its needed.

That being said how about a pic of that rig..sounds nice!
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Old 06-15-2010, 12:41 PM
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[QUOTE=south65ss]
The pype have traces of sparks and some copper melted but the wire is negative! If it where positive I could understand, but is negative.

No mystery here, conventional current does flow in the negative wire or path from the load back to the battery negative terminal. A circuit requires a closed loop to function.

BT.
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Old 06-15-2010, 03:04 PM
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"No mystery here, conventional current does flow n the negative wire or path from the load back to in the negative wire or path from the load back to the battery negative terminal."

I believe you meant to say current(electrons) flows from negative to the positive.


"Sounds like the alt grounding at the bat post turned your car into a positive ground vehicle."

This statement is also mostly correct. If you had a dead short at the alt lug to engine ground it is very likely that your cable would smoke and burn. The reason why is because that is the same as connecting the positive and negative battery terminals. It allows the battery to discharge at an extremely high rate. The heat from the friction of the electrons moving through the copper at your single restrictive ground is what caused this to happen.

Why did it occur at this cable?

"Yes sir. The ground wire goes from the engine block directly to the battery and no wires between engine, body or chassis. Will ground it propperly. What about the distributor cable? Just a dead HEI due to the ground problem?"

It sure is a problem that single cable has to caryy all the current/load of the entire vehicle. You should ground the body to the battery, the frame to the battery, and the frame to the engine, and the body to the engine. And throw in a few extra just in case!

How long is your battery cable? Is it over 10 feet?
If so a 0-4 cable is only able to handle 35-50A(maximum current flow;continuous;ie spikes here and there will do no damage and since this is the only connection to the negative side of the battery this means that the entire car should consume less than this(not counting alt charge and starter draw). In addition to that fact you have another restrictive leg running from the main power harness to the battery. You do have a long positive cable running from the main power harness to the battery right?

I guess what I am getting at is that only 1 ground is too restricrive. Therefore, the wire burnt up where the resistance/friction was(not the academic way to state this).

If you ground the frame and body near the battery you could use that heavy cable from the main power harness to positive terminal(at starter?) And then only have to worry about cable being large enough for starter draw and alternator current charge(100 amp alt cranking into a 50 amp bucket is gonna smoke stuff too!)

I would recommend #00 cable for a street car with all the bells and whistles and proper multi-point grounds;never hurts to go larger. Fires happen very quickly!

Sorry for the crude post, typing very quickly while at work when no one is looking.
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Old 06-16-2010, 08:18 AM
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Problem solved. Whas just the positive cable welding itself to the body in several parts. And it wasn`t the negative, it was the positive. After the fire the owner disconnected both post from the battery and some idiot (me) connected them wrong. The cable posts had no colors or identification. Anyway, I`m a lucky guy, nothing electrical suffered any damage. I think I will spend some time changing smoked wires and will be ready to go soon. Thanks for the help guys!!!
By the way, here`s the car.

http://farm5.static.flickr.com/4012/...ff6126520a.jpg
http://farm5.static.flickr.com/4029/...a8372b7aaf.jpg
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