My WTF rear-hatch electrical/wiring problem

[KelOfTheRockies]

First, my current truck is a 2000 Ltd.

Previously I had a '98 SR5 which got a broken wire in the harness at the hinge of the rear hatch. Back window went down overnight in -5 degree winter temps and wouldn't go up.

I figured that one out, and fixed pretty easily

A few nights ago, my Ltd, started with a very strong burning smell which I was scared was going to burst into flames. Stopped at a well-lit store and got under the dash and hood but found nothing even warm, let alone burning.

Limped home and got it into the driveway, then disconnected the battery and hit the rack for the night.

Woke up early with the problem figured out in my head - broken harness at the rear hatch. Found it in the morning but here's my question;

WTF didn't the 20A fuse—or ANY fuse—blow before the problem turned into this:

Thoughts?

—Kel

 

[brillo_76]

Do you have led bulbs in your rear stop brake lights and reverse lights?

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[KelOfTheRockies]

Do you have led bulbs in your rear stop brake lights and reverse lights?

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No light upgrades, all incandescent.

But still, I don't see how that could make a wire nearly melt without blowing a fuse. The one with the burnt copper is the 16ga power feed.

—K

 

[brillo_76]

No light upgrades, all incandescent.



But still, I don't see how that could make a wire nearly melt without blowing a fuse. The one with the burnt copper is the 16ga power feed.



—K

Interesting. I have seen circuits melt down when led bulbs are in use. I can't say I saw what happened to your hatch before. Extremely odd, as you are correct, the fuse should have burned open. Unless the melt was just below the fuse threshold.

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[KelOfTheRockies]

Interesting. I have seen circuits melt down when led bulbs are in use. I can't say I saw what happened to your hatch before. Extremely odd, as you are correct, the fuse should have burned open. Unless the melt was just below the fuse threshold.

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I can't imagine a scenario in which a 16ga wire can burn completely through, taking the insulation on several other wires with it, and pull under 20A. Would kinda negate the 20A fuse purpose if so, it would seem.

 

[brillo_76]

I can't imagine a scenario in which a 16ga wire can burn completely through, taking the insulation on several other wires with it, and pull under 20A. Would kinda negate the 20A fuse purpose if so, it would seem.

Oxidation on the copper as the insulation cracks and falls off.. Oxidation causes resistance. Resistance creates a pile of heat thus the melt down of the wires.. As that is a flex point and the wires insulation will crack off in time. Fuses work when a circuit is dead shorted to ground or higher amperage then then the fuse can handle and burns open.

I suspect the wire got very oxidized at that point and caused the melt down which is why your fuse didn't burn open as no short to ground or no higher then the fuse amperage occurred.

 

[phattyduck]

No light upgrades, all incandescent.

But still, I don't see how that could make a wire nearly melt without blowing a fuse. The one with the burnt copper is the 16ga power feed.

—K

20A = >250W available before blowing a fuse. Think how hot a 250W bulb would get in that location and you have an idea how it can happen.

This is a very unlucky occurrence - it sounds like it had just the right resistance to build lots of heat without blowing a fuse. A dead short would blow a fuse quickly. You'll need to replace that whole length of wire there with quality wire of the correct size.

Good luck!

-Charlie

 

[KelOfTheRockies]

Thanks, Charlie.

So I went to salvage and pulled two harnesses for the hatch door. One had been repaired - fairly well - and the other was near pristine with no sign of impending doom.

Problem is the white connector which goes behind the headliner is completely different on my truck.

Mine: 4 wires

Salvaged (both ones): six wires, different colors and bigger connector.

The connectors inside the hatch door are a match.

Guess I will just have to repair the original one. Shouldn't be hard now that it's out and I can just work it on the bench instead of standing on my head in the back of the truck.

Disappointed I can't use the one in perfect condition with no splices or cracks.

—Kel

 

[08ShadowRunner]

20A = >250W available before blowing a fuse. Think how hot a 250W bulb would get in that location and you have an idea how it can happen.

This is a very unlucky occurrence - it sounds like it had just the right resistance to build lots of heat without blowing a fuse. A dead short would blow a fuse quickly. You'll need to replace that whole length of wire there with quality wire of the correct size.

Good luck!

-Charlie

I think the distance from the fuse also played a part combined with the cold temps. Since it's the heat generated from the short that blows the fuse, the long distance combined with the cold wires didn't allow the required heat to blow it. It could have also been an intermittent short due to vibrations from driving. :shrug:

 

[phattyduck]

Thanks, Charlie.

So I went to salvage and pulled two harnesses for the hatch door. One had been repaired - fairly well - and the other was near pristine with no sign of impending doom.

Problem is the white connector which goes behind the headliner is completely different on my truck.

Mine: 4 wires

Salvaged (both ones): six wires, different colors and bigger connector.

The connectors inside the hatch door are a match.

Guess I will just have to repair the original one. Shouldn't be hard now that it's out and I can just work it on the bench instead of standing on my head in the back of the truck.

Disappointed I can't use the one in perfect condition with no splices or cracks.

—Kel

You need '99+ wiring to match your hatch. '96-'98 have a completely different rear hatch wiring setup. Looks like the pins will match, so you can also custom make something there to repair what is bad.

-Charlie

 

[brillo_76]

I just splice out the wires that are bad. Yes its a shame but its only a matter of time before the wires go bad again. I always use a non flex area of the wire for the replacement. This way you can match the colors of the wires (if they match)

 

[brillo_76]

I think the distance from the fuse also played a part combined with the cold temps. Since it's the heat generated from the short that blows the fuse, the long distance combined with the cold wires didn't allow the required heat to blow it. It could have also been an intermittent short due to vibrations from driving. :shrug:

Possible. Very good thought. The distance shouldn't be a major factor though. Granted colder temps does increase resistance. We have several circuits coming through that flex point. You can see that the higher gauge wire was bare then broke. The vibration of driving did exactly what you said. short no short.. Thus creating the resistance to melt the insulation off the wires as it melted into other wires and circuits.

I am glad the OP caught it before it got any worse. Its rare, but can happen..:-(

 

[KelOfTheRockies]

Thank you, everyone! I really appreciate the information and clarifications.

Good to know that the '99 and later cable will work.

I learned how pull the pins out of the connectors, so I am disassembling the good '98 cable and will splice in lengths out to the pin, so that all the splices will be staggered, but well inside the rubber sheath.

Hope to be done with repairs over the weekend. (would have been done by now if we hadn't gotten two feet of snow - working in the driveway)

—Kel

 

[brillo_76]

The rule of them I go by.. Always match the harness to the chassis.. so 1996 to 1996 that way you never have to worry about issues. Things can change from year to year in rigs.. Granted the 3rd gens have 3 sub gens but if you follow this and verify the wiring harness you never have issues.



Sometimes trim and feature models change harness as well.. Just for future reference.. Example: E locker harness and not in the vehicles not all 1996 to 2000 have them.

 

[greembow]

I had the same problem in my 99 BMW M3, I rebuilt the entire wiring harness for the trunk with silicone jacketed wires- that kind of insulation doesn't crack or break, and is very resistant to burning. You could look at replacing the wires with silicone jacketed if you're worried about reoccurring problems.