Burnt wire going to drivers door pin switch, 96 4.0

My son calls me today and says his headlights will not come on, ok it needs a switch, no biggie. I pull the XJ into the garage and while disconnecting the headlight switch I smell wires burning so I jumped out and pulled the battery cable. After some inspection I found that the yellow wire that runs from the door pin connector at the kick panel to the main light wire harness was melted and the door pin had come apart (the pin actually popped out of the switch).

I got lucky and nothing burnt/melted up into the main harness and other than he needs a pin switch for the drivers door and a repair made to the 3" long wire all seems to be ok.

Sorry for the long read but what the heck caused the wire to burn/melt? I was and always am carefull when working under the dash and I have changed many XJ light switches without pulling a battery cable.

Any thoughts?

the wire is live always and grounds when the door opens. I'd say something snagged the wire and pulled the switch apart, leaving a live wire swinging in the breeze. Maybe a misplaced foot during entry/ exit? Maybe during a speaker installation? Maybe dad did it when he was reaching under the dash to get to the headlight switch and the headlight switch really was bad??

It's hard to say, but I've always wondered about the "smarts" that went into that circuit design.

Shorty said:

the wire is live always and grounds when the door opens. I'd say something snagged the wire and pulled the switch apart, leaving a live wire swinging in the breeze. Maybe a misplaced foot during entry/ exit? Maybe during a speaker installation? Maybe dad did it when he was reaching under the dash to get to the headlight switch and the headlight switch really was bad??

It's hard to say, but I've always wondered about the "smarts" that went into that circuit design.

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Could be, but the weird thing is that from the connector to the door pin, the wire's are fine (they are also a much heavier gauge), but from the connector to the main harness, the small yellow wire is burnt. I think there was a break in the insulation on the yellow and black wire and they touched causing the grounded circuit. I don't understand why a fuse didn't blow when it happened. All seems to be well with it and the dome lights work with all of the other doors. I just need to get another door pin switch and finish the repair.

As far as the headlight switch goes, the old one was toast! The stem would just slide in and out of it not pulling any contacts at all, I plan on dissecting it to see what the issue was.

A fuse wouldn't have blown, since it was shorting ground instead of power. Basically all of the cabin lighting is constantly powered with a spiderweb ground (the yellow part), with multiple switches providing access to chassis ground (local black). Anytime one of the jamb switches pop out the yellow spiderweb is connected to ground at that switch and all of the cabin lights come on. So with the yellow wire brushing against the chassis, it is the cabin lighting ground circuit hitting the chassis ground periodically. The damage would be from arcing at that point, but there wouldn't be anything that would make the fuse blow since it's "working" when that happens.

What's curious to me is that the cabin lights should have flickered when it first started to short, unless the cabin lights were already on from something else

Yep they were already on, I had the drivers door open and was under the dash pulling out the headlight switch when it happened. I guess I could have shorted something when I pulled down the switch. I saw smoke starting to roll out from under the dash and made a beeline for the battery cable! LOL!

As others have said those wires just ground out to turn the lights on. For that wire to melt you would have had to short power directly to that wire which is possible from the headlamp switch.

MJR said:

As others have said those wires just ground out to turn the lights on. For that wire to melt you would have had to short power directly to that wire which is possible from the headlamp switch.

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Yep, the switch is what did it. I did some checking with a ohm meter and the switch must be melted inside due to the readings I'm getting.