Fuse Box Electrical Fire ( almost)

[chasten]

So i have about 6k miles on this engine and its been running great. i drive it mostly in the tow tune with my slide-in camper on the truck and no issues. its been sitting for 3 weeks and i took it to run a few errands, made 4 stops and then home. park it and head inside, all normal. i happen to come outside to grab a kids toy and hear the lift pump running, WTH? I cycled the ignition to wake the computer up and hopefully fix itself. doesn't work so i tried stating it and nothing, dash lights come on and look normal but when turned to crank its just crickets. thinking maybe batt. is dead i grab the trickle charger and pop the hood, and the burning electrical smell hits me. lift pump is still singing so i pull the fuse on power wire from batt. to pump and it stops. neither of the fuses in the add-a-fuse are blown, in-fact i cant find any blown fuses. i pulled the fuse panel off to reveal the melted connectors underneath. after all the excitement i noticed the wipers were up like i shut it off mid wipe, but i hadn't used the wipers. aside from that everything else seems normal.
so aside from tracking down new parts and replacing some wires i want to track down the cause. the lift pump trigger wire with the add-a-fuse has a 3amp fuse and seems unlikely to cause this. could a bug or something crawl up inside the board and cause something like this? if something was shorted or backflowing seems fuses would blow? any help with locating parts or cause would be great. thanks

 

[chasten]

some damage pics

 

[2004LB7]

rodent piss

 

[dndj]

Could be a +12v to GND short to cause this kind of meltdown, or a connector terminal getting resistive over time. I took a very close look at your pictures. The 4th one showing the underside of the fusebox clearly shows pin N5 is the culprit since the pin is pretty much vaporized and you can see the melting radiates out around this location. Schematics show that this is a GND pin. I clipped the relevant schematic for this (I think). Basically this circuit provides grounds to a bunch of relay coils (low current), but also a direct ground for the wiper motor as well. The most likely culprit is the high current wiper motor. If rodent piss caused the connector underneath the box to get resistive, this can lead to a cascading failure like this. Resistance = heat = more resistance ... etc. Have you been using your wipers recently?

Maybe this would help provide a clue as to the cause, because you don't want to replace parts and have it burn again.

 

[chasten]

ya i thought the N5 was the cause also but could find that right schematic for it ( thank you) i did use the wipers the time i drove it before this, about a hour into the mountains to go camping and back. this last time before the meltdown was just a errand run and no wipers.

 

[dndj]

So the wiper motor has a 25A fuse on the +12v side so under normal circumstances the pin N5 ground in the UBEC would be rated for at least that much. If there was some kind of direct short in the wiper motor, that 25A fuse should blow before your fuse box melted down, unless N5 was resistive. Also, since you hadn't used your wipers on the last trip, it doesn't seem likely that would be the cause of the spontaneous melt down. To create that kind of damage, an unfused +12v source (or something much more than 25A) must have contacted something on that circuit above.

 

[2004LB7]

if that connection had corrosion or higher resistance then normal then all it would take if the wiper motor was drawing 20 amps is a 1 volt drop to have 20 watts of heat concentrated im that little pin. just remember that many turn signal or break lights are about 20 watts and they get hot enough to glow white hot at the filament. it doesn't take much resistance to create a lot of heat on a higher amp circuit

 

[dndj]

if that connection had corrosion or higher resistance then normal then all it would take if the wiper motor was drawing 20 amps is a 1 volt drop to have 20 watts of heat concentrated im that little pin. just remember that many turn signal or break lights are about 20 watts and they get hot enough to glow white hot at the filament. it doesn't take much resistance to create a lot of heat on a higher amp circuit

Totally agree with this. OP said he didn't use the wipers on the last trip before it started actively burning in the garage so something got physically shorted to ground at that point, possibly due to previous melting / compromised integrity of the fusebox.

 

[2004LB7]

Totally agree with this. OP said he didn't use the wipers on the last trip before it started actively burning in the garage so something got physically shorted to ground at that point, possibly due to previous melting / compromised integrity of the fusebox.

yeah, it's going to be really difficult to tell exactly what shorted out or drew excessive current but you can definitely see that the buss bar got hot enough to discolor and the traces are burned. that would take a huge amount of current. maybe 50 amps or more. one thing about most plastic is once it start to char it leaves a higher amount of carbon behind which can conduct and cause further heating and burning. sort of a run away effect if the conditions are right. first one could have been a smaller pin that got some corrosion and burned then it set the path for the buss bar to conduct to some other ground or power pins causing even more burning

 

[dndj]

I looked at the pics again, I think you nailed it. I had missed the blue discoloration on the +12v bus bar, plus clear evidence of arcing about half way down, along the edge. Sequence of events: I think resistive terminal N5 ground melted during wiper usage. Then on the last drive (w/o wipers) the melted terminal tipped sideways and contacted the bus bar. Direct short to ground and fun fireworks.

 

[Chevy1925]

ive heard these fuse blocks have had this issue with a little corrosion or loose connection. have not personally seen it though

 

[chasten]

that make a lota sense. this truck sat engine less for a year and the fuse block was in a box of parts. lots of opportunity for contamination/ dirt to get in there. i never really thought twice about it. just hit it with the air compressor and dielectric'd everything on assembly. the wire that attaches to the N5 pin is melted but only back a couple inches from the plug. other wires i've seen that are overdrawn melt across the whole thing, which makes sense if the issue was in the box not at a component.
found a guy with a totaled identical truck who was will to hack the harness for the plugs. probably over paying, but way cheaper than buying a harness. couldn't find any at the local wrecking yards.

 

[chasten]

is there a special tool i can buy to remove the connectors wires from one plug and transfer to a new one so i don’t have to splice the entire harness?

 

[dndj]

I checked through the GM upfitter manual (link) but couldn't find a spec on the UBEC (underhood fuse box). However, looking at the left instrument panel junction block on the floor down by the driver's left foot, says the pins are GM p/n 15304711 and the release tool is GM p/n 15315247 but you can find generic sets like this on amazon (link). If the pins look the same to you, those are probably the right ones.

 

[chasten]

so this is interesting, i bought used UBEC with all the plugs. identical truck with 80k miles. i pulled the plug to swap pins and wouldn’t you know it the exact same pin is melted!!! this has got to be a factory issue. the question is how do i improve the wiring to never have this problem again.

 

[2004LB7]

wow, good find. now I need to inspect mine

if it was mine, and since I'm a cheapskate but still like to overdo it, I'd probably cut out that burned connection and surrounding plastic. I'd then soldier the female connector right to the male pin. since its on the side you should still be able to remove the connector block

 

[dndj]

so this is interesting, i bought used UBEC with all the plugs. identical truck with 80k miles. i pulled the plug to swap pins and wouldn’t you know it the exact same pin is melted!!! this has got to be a factory issue. the question is how do i improve the wiring to never have this problem again.

Holy crap! I guess my first inclination would be to return it to the seller. If the price was right, and they're hard to find, I guess I would salvage it. If the male pin on on the bottom of the circuit board looked good, and there was no burnt looking stuff on the circuit board, i would replace the female receptacle in the connector, then ground the wiper motor elsewhere by intercepting the black wire coming in on G1 and connecting it to a solid ground elsewhere on the chassis or battery. The rest of the ground loads on that pin look to be very low current (relay coils). It is kind of odd that GM ran the ground of the wiper motor through the UBEC in the first place, likely because the wiper relays on embedded on the UBEC circuit board and they wanted to keep the grounds close together?