Burnt Wiring

Fortunately, it did not happen on any of my Allis Tractors, but it happened on my GMC General semi with a Cummins 855 motor (not original obviously).  Great running truck but it doesn't run now.  I wonder if it could happen on my tractors though.  Regardless, I really want to understand how replacing a battery caused all of this.  I also need to get a game plan on fixing it.  I was hoping the electrical gurus on here could help me understand what went wrong and suggest an approach on splicing the wires.

First, this truck has had a lot of rig wiring jobs done on it.  The wiring harness picture shows the melting of the wires.  It melted the wires at the battery terminals too in the other picture.

The big fat portion is where all of the smoke came from.  Why did it melt in the center and not the ends?

I am supposed to have 3 batteries and there are three large capacity wires that go from positive to positive to positive and negative to negative to negative.  I put a new battery in place and before I could remove the 2nd old battery, my wiring started to sizzle and melt.  I never got the 3rd one in place.  I tried to disconnect the two batteries from each other but before I could do that the cables melted on the terminals and made it very difficult to get cables off.  By the time I did, my wiring harness melted in the center.  I checked the voltage right after installing the 2nd battery and it showed 10 Volts so it was lower than it should have been.  It is almost like the old battery was dragging it down but also causing it to heat up?  Can anyone shed some light on this?  I don't want to do it again.

My starter on this truck is a 12 volt starter.  I assume I have to replace all of the wires and not just the burnt portions.  I don't know how many I have in that casing yet.  I am not an expert electrician but I think I can splice wires with the right gauge.  Do I have to consider other things?  Anybody had to do this before?

Thanks in advance for any insight.

wire size...

1st problem in the 1st picture is that homemade battery cable' ! The end is NOT properly crimped onto the cable. That alone could have easily burnt the truck down to the ground.

2nd problem, sounds like you have mismatched batteries. It's a BAD idea to just replace one of 4 or 1 of 3 in your case. The 'weak' battery WILL 'pull down' the others to it's lousy level. Smart truckers replace ALL and make sure ALL are same brand and model. best it to get from same batch if possible. When you replace the batteries you ALSO replace ,at the very least the MAIN fat power cable going to the starter.

OK, so it costs a few bucks BUT you now KNOW you've got LOTS of amps AND the wires to HANDLE that power.

Bodging or being frugal WILL leave you at the side of the road !

It's really tough to call what happened there without actually standing there looking over the situation and how you have things wired as well as the size cables for the application. People could come up with a hundred different possibilities of why such a thing could've happened and 98 outta' a hundred could be good explanations, but it's really hard to do looking at a couple pictures. Those types of situations need to be torn into and a lot of diagnosing is in order there. There is a lot of amperage when multiple Batteries are involved. I wouldn't even start guessing or suggesting what happened there. The only thing I would suggest is to start at the power source first and inspect and draw test each Battery individually to eliminate those as a possibility. Even if you did find a internal shorted Battery, you still need to thoughly inspect everything else there. Even the wiring that didn't get BBQ'd. Cable size and condition are important...

Steve@B&B  

I think I might know what has happened.  The fix may not be as bad as I thought.

I pulled open the split wire plastic loom and found only two wires were melted.  Turns out that those two wires plus the wire I showed in the picture were all attached to the alternator.  It appears that the burning started at the alternator which was on the other side of the truck.  Basically, after I connected the new battery it probably started burning then and was working it's way to the other side where I was at.  By the time I saw it, it had already burnt the other side.  The large gauge wire did not melt through like the smaller gauge wires but was on it's way.  Here is a picture of what I think I have.  This fix may not be too bad.  Not sure if it fried the alternator though, but that is easy to figure out.  I missed showing the negative cable going to the truck frame in this picture.

The one oddity is the cable going to the alternator.  It was attached to the negative ground on the battery.  That doesn't seem right.  Normally, the alternator gets it's ground from the engine and there isn't a special cable, I thought?  So, I am going to have to look closer at the alternator.  I have a new suspicion that maybe someone might have made a mistake when they put the battery into the top spot.  I think that cable goes to the positive terminal on the alternator and not the ground.  The reason why it didn't previously cause a problem is because the top battery was not connected to the bottom two batteries.  As I started to connect all 3, that is when all hell broke loose.  

If that negative cable  going to the alternator was really supposed to be the positive cable and was criss-crossed incorrectly, would that have caused the alternator connections to burn wiring?  Possibly.  It's really about the only explanation.  It really looks like the burning started on the other side where the alternator was.  I am going to have to look at that Alternator and try to figure that large cable out.

Here is more of what happened.  This is the alternator on the other side of the engine.  There are two small wires and the large wire, all are burnt.

If you look closely, those two wires going to alternator stopped melting, right here.  So that is going to be my splice point for those two.

while the loom is melted, these wires appear to be fine.  The loom melted but not the wires.  There are a lot of wires in there so that is good.  I will pull the loom off just to be sure though.

you can see I have the big gauge battery cables.  They are all fried so they have to be replaced.  They look expensive.  You might be wondering what that cutout switch is for?  Me too.  I think it may be cutoff switch to the rear tail lights.  They have a tendency to stay on but I didn't realize this cutoff switch turned them off.  I traced it to the back of the semi but lost it without a light.

Post the alternator model, or get a photo of the two small wire conections and LUG on the back... Does not look like a DELCO and it has a tack drive cable ?

the two small wires are normally a POWER wire from the key switch, and a signal wire that might go thru the idiot light on the dash or a small resistor.. the small differential between the two wires tells the alternator to CHARGE.

yes, the alternator charge has nothing to do with the engine START.. Just leave the 3 wires off (taped seperate) and you can start up the engine on 1-2-3 batteries for a test... With the two small wires connected, the BIG LUG should start putting out power with the voltmeter two leads touching the terminal and ground.

Does this picture give you a better idea of the model/make of the alternator?  

I like your idea of taking it off and having it tested. 

Thanks for the link to making the cable whatever size I need.

I have two follow up questions.  

One, I tested the voltage on the one good battery and it showing 12.47.  Is it safe to use that battery?  The terminals got hot but they didn't melt like the cable ends.  

Second, those two burnt small wires, I would like to find their location coming out of the wiring harness to the wiring block.  Is it possible to excite one end and test for current to the wiring block under the hood?  There is this big long wiring block on the engine firewall where I think these wires terminate.  Then they go from that wiring block back under the dash.  Due to the gauge of the wire, I have about 4 possible choices as to which wires these are.  I also don't think the idiot light was really working on the semi.  It should have been on a long time ago.  I also don't think the one wire that should be going to the ignition is making it there.  The ignition key switch is a total rats nest but I could be wrong.

Thanks as always for our help!  Learned a lot the hard way.

YES, you can test the WIRES by using an OHM meter and touching one lead to each end of the wires and check for continuity.... Basically that is a meter that runs a 6v signal thru the wire and see which wire on the terminal block is connected to it.

YES... if a battery tests at 12.4 volts it SHOULD be acceptable for testing and possibly starting the engine.. You will know when you try to crank it, if the battery has enough power / charge to spin the engine.

The last time I took a big tractor starter in for rebuild it cost me about $250 so a new one from the link you sent is about the same price.  I think this alternator is between 130 and 160 AMPs but they have 3 different versions of that.  I will wait and see what he says.

I changed my 190xt to be a 3 wire system.  I think the 3 wire charges at lower RPMs, if I am not mistaken.  Is this one I have for the truck a 3 wire system?

That's a good point about the two smaller wires.  As long as the batteries gets charged, I don't have to worry about them.  Although, I would like for my alternator light to actually work.

The big L/N's are found on Fire Trucks, over the road Trucks, Bucket trucks, Busses etc. Personally, I wouldn't move to a Delco unless for some reason that particular model L/N is gonna be phased out. You would first need to see what model L/N that is and see if there is an equivilent Delco that would be a direct replacement. This way you eliminate the need to fab or alter any bracketry. Who want's to make more work?  Some of the 22 "SI" series/150 amp version Delco's can replace some of the L/N's. Like I said, before you start changing things, have that unit load tested to see if it's still okay yet. 

Depending on the model of that unit, a lot of those L/N's work the same way as the smaller Alternators. Usually, one wire runs to the ignition switch, and the other voltage sense wire runs to a junction block or circuit breaker near the Batteries. There should be a circuit breaker in that system possibly on the frame rail under the cab. Depending on how the truck is wired, in some cases there is a main junction block on the firewall the voltage sense wire runs too. IH & Freightliners are wired that way. When it comes to heavy trucks, a lot of them are wired differently depending on the mfr. of the truck and the engine it's equipped with. Getting back to the circuit breaker, when you find it, it  would be wise to change that out as well because apparently it didn't kick out fast enough and caused the mess that happened..... HTH

Steve@B&B

I was just rolling back up and the circuit breaker I'm referring to is right in your picture! I didn't see it the first time. My old eyes aren't catchin' things to fast anymore.. LOL!

I cannot add the 3rd battery yet due to waiting on cables but I think it was previously starting on one good battery so I don't think I need it.

Just would like to see it turnover and then start. I will deal with the alternator later.