Intermittent short somewhere in park/marker lights

[sirrus]

Spent couple hours hunting new electrical gremlin today, thought I may use some help or tips

So couple days ago when I was driving I noticed that my dash lights are off - after quick look around found a blown fuse, 20amp, park/tail lights.

Knowing that fuses don't just blow by themselves I thought that I probably got a short somewhere. But decided to try that 0.1% chance that it was bad old fuse or something, I replaced it and turned parking lights - and they worked! Happy that I fixed the issue in 30 seconds, drove to post office and on my way back, after about 30 minutes of driving around town and couple dozen bumps, it blew again.

So no luck, I definitely got a chafed wire somewhere that short to the body. Where it is - no idea and the whole concept of checking wiring for all parking lights, side markers and license plate lights doesn't look that exciting

What I came up today was to try to isolate the short to one of 4 sections: rear lights, license plate light, front lights or interior (light switch, fuse panel or somewhere in between). After dancing around Jeep with multimeter, taking out all bulbs and disconnecting license plate and whole rear lights (8-pin connector under hood near bulkhead), I've isolated the issue to front or interior. Took out turn light assemblies - they are full of some brownish sludge, but no short.

My next step would be to disconnect bulkhead connector and check again (I'm testing resistance in front side marker bulb socket). However I couldn't get that connector apart for the life of me and it was getting dark anyway.

So my first question is - how do you disconnect that bulkhead connector? And the other one - are there any common places where lights wiring tends to get chafed or damaged in any way? Something that I can easily check first before diving into it any further?

[sirrus]

So a little progress today - I finally took bulkhead apart and now I know that short is somewhere between park/tailfuse and bulkhead connector. Still a few possibilities (chafed wire touching body somewhere, bad headlight switch, something loose in fuse block, etc) - any hints where to look first?

And when I unplugged bulkhead connector it looked like this:


Not pretty at all. Initially I thought that it melted a little, but it turned out to be some sort of grease (maybe 30+ year old dielectric grease?). I'll probably have to clean or replace, but not now

[tgreese]

If the short did not go away with the bulkhead connector removed, then the problem is between the fuse panel and the bulkhead connector - assuming the wire path goes through the bulkhead connector. This is what you wrote, but is this what you mean?

You should be able to detect this short with your multimeter. I would next unplug the headlight switch, if you are actually searching under the dash as you wrote.

That brown grease is typical. Not a problem really - you can clean it if you want to. Put it back with a liberal coating of new dielectric grease, to prevent corrosion and make the connector water resistant.

[sirrus]

If the short did not go away with the bulkhead connector removed, then the problem is between the fuse panel and the bulkhead connector - assuming the wire path goes through the bulkhead connector. This is what you wrote, but is this what you mean?

Yep, that's what I meant.

You should be able to detect this short with your multimeter. I would next unplug the headlight switch, if you are actually searching under the dash as you wrote.

Headlight switch is my next suspect, so I'll start with that. Unfortunately I'll have to remove all dash light bulbs that are controlled by headlight switch as they trigger continuity test on my multimeter. What a PITA

That brown grease is typical. Not a problem really - you can clean it if you want to. Put it back with a liberal coating of new dielectric grease, to prevent corrosion and make the connector water resistant.

Got it, thanks. For now I just put it back, I'll come back to cleaning that dried brown grease sometime later probably

[sirrus]

Unfortunately I'll have to remove all dash light bulbs that are controlled by headlight switch as they trigger continuity test on my multimeter. What a PITA

Correction - it looks like I can pull ACC LPS fuse (3A) instead of pulling all the bulbs out.

After looking more at wiring diagram my three main suspects are headlight switch, panel lights dimmer and radio illumination relay, as other interior things powered from park/tail are not present or disconnected now (radio itself, chime module, etc). Will try to get to all of them today and hopefully fix it

[sirrus]

Well I took out radio illumination relay, panel dimmer and then headlight switch, checking for short after removing each. And of course it is still there - I was hoping that illumination relay went bad, but no luck

Then I have one candidate left - chime module, but I took it out months ago. Just out of curiosity I decided to check it’s connector as one pin should be connected directly to park/tail fuse, even before headlight switch. No short to ground and no continuity to fuse.

All that makes me think that wire going to chime module connector broke of and sits on the body sheet metal somewhere. I’ll try to pull fuse panel to confirm.

Question - how does one pull it? I know there are two torx screws holding it, but it doesn’t really come off when I undo them? Is there something I’m missing?


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[sirrus]

Update - I’ve pulled fuse box successfully, turns out last time I tried it I had bulkhead connected and that prevented fuse panel from coming out.

Now that I have fuse panel hanging, I found that the wire that shorts to the ground is not the one after the fuse, but the one BEFORE - thick red wire that is supposedly coming directly from battery/alternator. Zero ohms to body

Doesn’t make any sense to me to be honest. Any ideas?

Another update - I pulled dome and haz/stop fuses and now red wire is not showing short to ground. Must have been interior light bulbs or something

But now I don’t see any shorts in the park/tail light circuit with my multimeter, so back to square one now

[tgreese]

The only way you could blow a fuse downstream of the current source is if the source is acting like a sink and some downstream source is back-feeding.

I don't know what the '88 wiring looks like specifically? Do you have an ammeter? I suspect that you do not, and that the feed to the fuse panel (powering the dash and lights) comes from a fusible link connected to the starter solenoid. If you short out the feed to the panel, the fusible link will go open with some drama. Don't have time to study the diagram right now, but later I may. You can look at the diagram now and see where the wire in question comes from.

[sirrus]

The only way you could blow a fuse downstream of the current source is if the source is acting like a sink and some downstream source is back-feeding.

I don't know what the '88 wiring looks like specifically? Do you have an ammeter? I suspect that you do not, and that the feed to the fuse panel (powering the dash and lights) comes from a fusible link connected to the starter solenoid. If you short out the feed to the panel, the fusible link will go open with some drama. Don't have time to study the diagram right now, but later I may. You can look at the diagram now and see where the wire in question comes from.

88 does not have ammeter, just voltmeter in the dash.

I thought about fusible links - they all look ok. One of them was replaced with a regular wire by PO I guess, I’ll revert that back to stock later.

I’ve updated my post above, not sure if you’ve seen it - short on the thick red wire was false alarm. After I pulled dome lights fuse and haz/stop fuse it went away, so it was just low resistance of dome bulbs, not a short

What do you mean by downstream source back-feeding? Not sure that I understand that


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[tgreese]

Does not matter if the red wire was not the problem. The fuse protects the circuit downstream from where it is placed. What you were describing is backwards from that, requiring that current flow be reversed. Upstream is a current source, downstream is a current sink.

[sirrus]

Ok, I think I got what you said now. Doesn’t that mean that I should have 12 volts backfeeding into normally downstream side of fuse and short on red wire side (upstream or source for the fuse)? Or is there any other combination that I’m missing?


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[sirrus]

Found the issue and now I’m feeling like a complete moron Culprit turned out to be license plate light - power feed wire got unplugged and was bouncing around, occasionally touching tailgate. I thought I’ve eliminated that when I started diagnosing the issue, but apparently I did not. Now it’s time to put everything I disassembled back together


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[Chubbinius]

Found the issue and now I’m feeling like a complete moron Culprit turned out to be license plate light - power feed wire got unplugged and was bouncing around, occasionally touching tailgate. I thought I’ve eliminated that when I started diagnosing the issue, but apparently I did not. Now it’s time to put everything I disassembled back together.

That's awesome news!
Even though it must have been frustrating for you to keep chasing the different connections, it will hopefully save anyone else with the same problem some time and frustration.

[sirrus]

Yeah, lesson learned (again) - don’t hurry, double check all easy options first and then start digging in

Otherwise you’ll have this


When in fact I just had to undo 2 screws in the tailgate and plug 1 wire back

Anyway, it’s working now and that’s the important thing!


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[67GMC]

Don't feel bad-I had a similar issue and my Jeep looked like your with all the spaghetti hanging out. Turned on parking lights and fuse blew. Re-did a lot of grounds, wiring to headlamps, horn etc. Couldn't figure it out. Had all the relays, seat belt light delay, ashtray and glove box lights out. Nothing made a difference. Then I found that the only light that I hadn't checked was the illuminated defroster switch lamp. That was one of those bulbs with the pins on the outside and it had shorted to ground!!! Man! What a great deal of trouble for a simple issue. I feel better that I did so many repairs along the way but I wanted those hours and days of my life back!.

Glad you found the trouble on yours.

[sirrus]

Wow, that sneaky little bulb in the switch!

Not feeling bad as I also managed to do extra repair while having it all out - pulled parking brake switch, cleaned it and now I have warning light functional again!

I just wish I could have avoided doing all that single handed - I still have a splint on my right hand


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[67GMC]

Must have been hard with one hand. I see you still have a carpet. I pulled mine out thinking it was some wire running underneath. Had to be changed out eventually so now I ride on painted metal.

[Dr.Bob]

Glad you found the problem! And that it was an easy fix.

I had completely non-functional turn-signals; for that, the diagnosis is much easier when it's all or nothing. I swapped the flasher-can with the one for the emergency flashers, and bingo! turn-signals were working again.

Unfortunately, when I had that flasher-can on the emergency-flasher circuit, only the rear lights worked off of it. At least that should be easier to track down, since the lights themselves work, and the circuit from the switch to the flasher is working if the rear lights are flashing, so I shouldn't have to dig into the column (I hope!).

[sirrus]

Easy fix after some tiresome diagnosis I've also put a dab of clear silicone RTV to keep those contacts securely plugged into license plate light. And a valuable lesson for me to be more patient and thorough

I agree that it's easier to diagnose "all or nothing" situation, or at least when you have a way to reproduce the issue 100% - intermittent stuff is always harder to pin down.

One theory about your situation - rear light are wired through relay box that's mounted somewhere under rear side of the body on the frame. I'm not sure whether all of them were this way or it was part of towing package (I have the towing package with aux cooler for tranny). If your rig is wired same way - it would make sense that flasher can provides barely enough voltage/current to trigger relays in the back (so rears do flash), but it's too weak to flash front lights. Just a theory though, so take it with a grain of salt

[Dr.Bob]

One theory about your situation - rear light are wired through relay box that's mounted somewhere under rear side of the body on the frame. I'm not sure whether all of them were this way or it was part of towing package (I have the towing package with aux cooler for tranny). If your rig is wired same way - it would make sense that flasher can provides barely enough voltage/current to trigger relays in the back (so rears do flash), but it's too weak to flash front lights. Just a theory though, so take it with a grain of salt

That would be a good starting point, if my J-20 had that relay box.

Unfortunately, it doesn't.

My GW had that box, though, so I know what you're talking about.

I'll have to check whether my dash-lights illuminate with the flasher, as those run off the front-light circuit coming out of the multi-function switch. Of course, I have to replace a bulb in the cluster for the right turn-signal indicator. But my high-beams also are not working, so I'll get in there with the multimeter and the wiring-diagram and check everything out.