NICO's Autocross Truck Needs Your Help!

[PEZi]

As some of you know, the truck has had terrible starting issues as of late that got progressively worse. Through diagnosing, we narrowed it down to a single wire getting excessive load, drawing current away from the starter enough to not allow it to crank fully.

So, with your help, I'm hoping we can figure out why there is excessive load on this wire. Here's a little pictorial to get you all up to speed. Feel free to add any insight as to what may be happening!

Ok, so. First pic, I already checked everything in the engine bay. We're clear there. The yellow line is the one burning, and literally burning the jacket right off of the wire, going to the rear of the vehicle. I have not yet gone towards the steering column.



Second pic, the wire burning up splits into three other wires. None of the other three are burnt, although, it is possible they are getting hot, just not enough to melt the jacket.



So, the third pic... the yellow line is the wire that's burning up... It is ONLY burning up where the yellow is. Not forward from there, not backwards from there. Only where the yellow is.



This kind of tells me that it would have to be a problem in this localized yellow area, but the problem is I just put in all fresh wire there, its 100% healthy in that location. Which means, it has to be one of the other directions, but then why are those areas not getting hot enough to melt s***?

[Razi]

What's the yellow wire supposed to be powering?

[PEZi]

read second pic :P

[Razi]

Oh right.. :P

[Amays U G37S]

we drawin' pics in here or wut

[Encryptshun]

This is a completely uneducated and most likely incorrect guess, but maybe there's a dead short in the windings of your fuel pump.

How is this not popping fuses?

[AppleBonker]

How is this not popping fuses?

This x 10579023485672345. If a wire is burning up the fuse should be blowing. Otherwise that is a wicked fire hazard...

[sx moneypit]

we drawin' pics in here or wut

moron

[Dattebayo]

#1.) Okay, so the yellow wire is spliced into three different wires?
#2.) Or is it actually three different wires wrapped up inside a section of shrink-wrap?

The reason the other wires are not burning up is because the load of all three is on the yellow section if #1 is correct... electricity follows the path of least resistance always, it will never back feed into another wire because of the flow of current.
What you need to do is get a multimeter, disconnect each one of those wires in turn and place one multimeter terminal on the yellow wire, the other on each wire in turn with the meter on current (A) mode. This will ID the one that is drawing too much current and then signify whether you must hunt down a partial short in the loom down the line or replace something at the end of the circuit.

If #2 is correct, balls.

[Rev_D21]

I'll check the FSM but I can honestly say I have not seen Nissan split 3 wires off of one. I've seen them do 2 but not 3. Give me a min to grab my 85 FSM.

[SteveTheTech]

You guys are awesome...the same questions I would have already been asked.

My guess would be an incomplete ground for the fuel pump is not completing the circuit. If the load (FP) does not use all of the current in the circuit the circuit will hunt for a new load. What you are doing is basically making a lightbulb out of your wiring harness.

Why I am thinking fuel pump...what else gets power while cranking?

Do a voltage drop test on the fuel pump while cranking the truck.
Check that against a voltage reading from the battery while cranking.

[Rev_D21]

I colored what I think are the wires in question.




Full Size:
http://img69.imageshack.us/img69/5673/pezihelp.jpg

[moso]

ok here is my .02

if the wire is burning up and you just replaced it my first question is, is it the correct gauge? if it were too small it would fry under the load. if it were correctly gauged the fuse should blow. if you are certain it is the correct size and fuzed properly then my next step would be to eliminate loads from the wire at a time until the wire stopped heating up.

[PEZi]

Rev, took your pic and re-edited it to show the wire I am burning up. Sorry for the smaller size, apparently my PB settings must be jacked... I'm sure you can get it tho.



To everyone else, not sure why it hasn't blown a fuse... but I've checked them all.

[Razi]

It must be touching a positive wire somewhere, can causing it to heat up like a lightbulb like Steve said.

[Rev_D21]

You are burning up a ground wire? Have you cut open that 1wire-3wire splice to see what's going on under that tape? According to the FSM that splice is near, or should be, connector 132M which is the chassis harness. This harness runs the lights on the bed and the fuel sending unit. It is located under the carpet on the passenger side floor board. Looks like it could be a hack right there that needs to be sorted out. What do you think?

[PEZi]

I'm actually right there with ya Rev. Because the severeness of the melting greatly decreases away from that connector. The problem is that I am completely by-passing that connector right now and its still doing it. Do you think I should try by-passing all of the wires going through there?

[Rev_D21]

I guess my next question would be is the connector intact or has been been hacked? I assume you are looking at an FSM? That ground wire goes all the way back from to the taillamps on that harness(connector 1C to 3C which connect to tail harness connector 1T grounding to connector 3T). There must be something rubbed through along that chassis/tail harness causing high amperage on the ground side. If you don't have an FSM I'll scan the pages otherwise it's page EL-112 in my 85 FSM.

Here's an idea, try unplugging the tail harness at connector 1T, see if that changes anything. Then simulate the same conditions that make that ground wire heat up.

[PEZi]

I'm looking at a haynes. Its gotten me close enough to the one you've shown to know where I'm at. I actually have the entire harness pulled right now, and I inspected all the wires for rub through and everything. The ONLY thing that is even remotely suspicious at this point is that body ground (i think it says 82M) bottom right corner of the pic, may not have been attached. Not sure if that would have constituted such a ridiculous amount of heat through the wire, but I guess its possible.

[Rev_D21]

It's possible, how likely in this case I don't know. 82M is a ground lug behind the glovebox? Looking things over I see there are only 3 grounds for the main harness. 82M which you checked. It also uses the batteries main ground, it piggybacks off that ground. The only other ground is 3T which is on the tail harness. Should be easy to check for grounds. If you can you should try ohming out those grounds to make sure they aren't creating resistance which will heat up a wire.

[PEZi]

No, actually. I just re-looked at the pic. Connector 117M is the one under the carpet (or where there used to be carpet) and 132M is actually sitting right next to the fuel tank. Connector 117M is right under where the passenger seat would have been, and then all of the stuff going 'upwards' from there in the pic (aka backwards in the truck) is all under the truck and connected to the frame rail. The ground in question is right next to the fuel pump.

[Rev_D21]

Haynes is not doing something right. Those numbers don't jive with my factory fsm.

117M is behind the dash just left of the steering wheel listed as an instrument harness connector color white.

132M is under the carpet .

Want me to scan pics?

[Dattebayo]

Maybe the difference is he is looking at a 720 manual and you're looking at a modern D21 manual?

[PEZi]

Crap... Nope, you're right. As I said earlier in this thread, though, I haven't torn into the wiring going towards the steering column yet.

SO, here is the issue now. There is a discrepancy between the FSM and my actual wiring (the problem is my wiring is stock). The FSM is showing, as you said, only one splice... going to the fuel tank gauge unit and then the 1T connector in the rear. My wiring has a second splice that goes to the fuel pump as well. I'm thinking this may be the problem....... what's weird... is its 100% stock

[Rev_D21]

Nope, i am looking at an official 1985 720 FSM.

[PEZi]

Maybe the difference is he is looking at a 720 manual and you're looking at a modern D21 manual?

Nah, I'm just a tard... Rev has the right stuff up.

[Rev_D21]

Full size dash harness pic:
http://img585.imageshack.us/img585/6579/pezihelp2.jpg

[Rev_D21]

Full size main harness pic:
http://img708.imageshack.us/img708/9424/pezihelp3.jpg

[Dattebayo]

WTF is a 30,000 mile switch? Does it trigger the balloons and confetti shooter?

[PEZi]

Yep, the haynes actually shows both of those perfectly fine. I'm just tired and out of it. What's really just got me lost is the fact that the FSM isn't showing a splice for the fuel pump. It most certainly is there, and is most certainly stock.

This triple splice, by the way, is the one shown in the second pic in the OP