Ignition Wires Getting HOT

[Jtb327]

Hey everybody! I've been lurking on here for quite some time, and the forum has helped me fix a good handful of problems on my 99' Qx4. I finally found something that's kinda got me stumped, so I figured I'd put it out there and see if anyone has any advice. 

 

In short, my problem is that the wires leading from the ignition switch are getting hot. Like, hot enough to melt the harness plug and completely fry the switch. Originally, I thought the heat was just coming from the bad connections of the worn out switch. Naturally, it's not that simple. I've got a new switch coming in the mail tomorrow, but for the time being, I had hardwired the various wires in the harness so I could still drive this week. First, I connected everything directly to the constant power in the harness. I hooked it all up to a switch (The starter is on its own switch) which got ridiculously hot to the point that the plastic was smelling burnt and the electrical tape started to smoke. I figured this was due overloading the wires/switch, so found the hottest wire and ran another switch to direct power from the battery. Now every wire stays cold except for the one leading to the wire labeled ign1. I cleaned my terminals, cleaned and checked my grounds, and tightened everything up on the battery end in case that was the problem. All the fuses and relays seem to be fine, so I've ruled that out too. I experimented with various combinations of switches and directly connecting wires, and each time the source of the heat seems to move to the "weakest" point of the system. Everything I've used is 16 gauge wire, so I don't think that's the problem. I followed the wire as far as I could from the plug to under the dash and couldn't find anywhere there would be a short in the system. So my question is A. Where all does this wire go? B. Is there something I'm missing that would cause this much heat in the wires? 

 

Thanks in advance for whatever advice you can give! 

[Jtb327]

Thought I should add that the car and all accessories can be turned on indefinitely without a problem. It is within seconds of actually starting the car that the wires/switch get hot.

[Slartibartfast]

Yeah, that's not good. I'm not familiar enough with the R50 wiring to know where ignition 1 goes. You can check the EL section of the service manual to try and figure out where it goes (by color if nothing else). (I don't know where to get a full diagram for the R50, unfortunately, so you'll have to work with the partial diagrams Nissan apparently thought their techs would need.) Something on that circuit is clearly drawing way too much power, but is somehow not blowing a fuse.

 

What I'd do is try and figure out what all is on that circuit, start unplugging things (or pulling fuses or fuse links, though, again, I'd expect the fuse to melt long before the ignition switch did) until you can turn the power on without that wire heating up. Then investigate the last thing you unplugged. Also, if you had some part of the truck apart recently (particularly dashboard or engine bay), check your work there first in case something got nicked or pinched where you were working.

Good luck! Electrical issues can be a real PITA.

[Jtb327]

Thanks for that link to the service manuals. Definitely a ton of good info there. I did some more experimenting, and it seems like giving my terminals a thorough cleaning helped with most of the heat. I've installed the new switch for now, and while it gets a little warm, it is nowhere near the temps it was reaching before. Certainly not melting the plug now. The harness runs through the fender and then to both fuse/relay boxes under the hood, so should be pretty easy to keep an eye on things downstream. So for now, I'm hoping the problem was just old wires and bad connections. Thanks again for the input! If the truck goes up in smoke, I'll be sure to post pictures. 

[Slartibartfast]

Hopefully that's all it was! I can't say I've noticed any warmth in my ignition wiring, but then I can't say I've felt the wires while I'm driving, either. I'd be inclined to cut off the old plug and splice in a new one if you can find one (wreckers or see if it's available new) just to be sure you're done with this.

 

Ford made a run of alternators that had a similar problem. The rocket surgeons decided to use two thin wires instead of a charge cable, and spade terminals in a plug rather than the usual stud and nut arrangement. Cost cutting, I guess. The spades corrode, the plug melts, and the whole mess catches fire.