Tail Fuse Repeatedly Blowing (1 Viewer)

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Joined
Nov 30, 2012
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Location
Danvers, MA
Hi gang,

I know there are several posts on here about things connected to the tail fuse that commonly cause it to blow, but I'm having trouble finding them now that I need them. I'll just post a few details.

1996 fzj80, tail fuse repeatedly blows. I have a Slee illuminated switch tapped into the dimmer wiring, as well as a light tapped into one of my front parking lights, but they have been connected thusly for months and this problem has only just come up in the last week. Just turning the lights on does not cause the fuse to blow, but driving the truck almost guarantees that the fuse will blow, which leads me to suspect that some vibration or other is causing something to ground out somewhere and blow it.

So basically I'm just wondering who else has run into this issue (I know many people have). Any particular spots that are the most likely culprit before I start going over the whole damn circuit? So far I've seen trailer wiring harness (my truck has two hanging out the back for some reason...), the rheostat itself (seems unlikely), the stereo connections behind the center dash (mine is aftermarket, but it still seems unlikely for such an all-of-a-sudden problem with no prior stereo issues, plus I don't think my stereo is hooked up to the dimmer anyway), and various wires/harnesses running to tail/side marker bulbs. Any other ideas?

Thanks guys, you've been truly terrific help sorting out my electrical gremlins in the past so I have come to you once again, hat in hand, for a bit of help.

Colin
 
After swapping the horn relay and the tail light relay (identical part numbers), I've found that the horn works, so that rules that out. When I put a voltmeter on the two pegs of the fuse slot with the circuit powered, it fluctuates, jumping from 70v, to 7.9v, to 0, then back again, repeatedly. This also occurs when I put the ground side of the voltmeter directly to a ground point instead of bridging the fuse pins with it. Does this tell me anything specific about the nature of the short/ground problem? I disconnected the factory trailer wiring harness entirely and checked the length of the aftermarket one that was presumably installed to replace it; no breaks, chafed wire, and no readily apparent issues. I also disconnected the rheostat (dimmer) harness and bridged it, then tested voltage across the circuit and still had the same fluctuating voltage, so I am fairly certain it has nothing to do with the dimmer unit itself.

I've looked at the EWD and had some further questions: the circuit begins at battery, goes through main fusible link, comes through the tail relay (which I know to be working) where it splits off to the switch and to the cab fuse block. With the circuit powered (light switch on), a test light on just the positive side of the in-cab tail fuse slot illuminates. Does that guarantee that the issue is somewhere after that in the rest of the circuit (mainly wiring and harnesses for lights/markers and some junctions/splice points), or could it still be in the switch/light retainer relay, the main fuse/tail relay/fuse block, or the wiring between any of those first parts of the circuit up to the fuse?

Also, where is the J1 junction connector? Is it behind the fuse block/rheostat area? Anyone happen to have a picture?
 
Update: the voltage checks I made were inaccurate due to DVM issues, so after resolving those, the voltage across the fuse pins is 12.36 versus 12.41 across the battery terminals, so nothing out of the ordinary there. So I guess it's check every single harness on the circuit. Will report back.
 
Did you check the trailer harness module? You have to pull off the plastic molding that snaps in around the rear passenger corner of the cargo area. (Take off the steel rear strip first) then you can just pop off the plastic. The module is stuck on to the inside of the rear passenger wheel well with double sided tape.

Also you should check the integrity of the unsheilded green wire that runs from the drivers side tail light to passenger side. Take off the shiny metal strip in the back and pull up the carpet a few inches and you'll see it. It will wear down where it goes up and over into the drivers/passenger sides. There is a sharp edge there and will ground and blow out that circuit.
 
I checked those right away once I found the whole fubar'd situation with the not just one, but two separate attempts by the previous owner to install a trailer hitch adapter. Not one, but two green wires running across the rear under the carpet. Not one, but two funny little black boxes with wires/harness sticking out both ends. I pulled the obviously-defunct one entirely and wrapped up the remaining wires into a bundle and taped it up to avoid shorts, and checked over the still-intact one for shorts or signs of wear, which there were none. Still nothing.

I added an aftermarket remote start/alarm system, but I don't recall having tapped into anything along the dimmer/tail circuit, so I don't think that's the culprit. The radio works without a fuse in the tail fuse slot, so it's not that either. I'm thinking it's the glove box light/switch? Is that in the tail circuit?

Appreciate the reply, any ideas are good ideas at this point.
 
Oh, and another new update is that the fuse blows immediately now, no revving of engine required to trigger it.
 
When my tail fuse kept blowing is was the trailer harness wiring that was rubbing on the frame by the spare tire. Have you checked under there yet? I had to lower the spare to actually see the shorting going on. Just a thought.

Additionally I had an issue (not tail light related) where my tach and some interior lights would short out and that was due to the main harness rubbing/melting on the EGR.

Just a few thoughts. Good luck!
 
Thanks for the suggestions guys. I was looking at the diagnostic part of the EWD about how to spot shorts and started disconnecting harnesses along the tail circuit starting with the ID2 harness (the longest/flattest of the 3 just behind DS kick panel relay cluster) which removes everything from the wire leading to the rear lights/side markers and back from the circuit. The circuit worked again, so I moved to the next harness, the BQ1 harness in the DS quarter underneath the bottle jack storage tray. I wound up removing the tray because of hands of ham, but you may be able to pull it apart without doing so.

With ID2 reconnected, I disconnected BQ1 and the circuit was complete through the floor wire 1 section underneath the DS doors. I kept disconnecting all the possible harnesses between the rear DS lamps, license plate light, and PS lamp and side marker. I checked inside the tail light assemblies and removed the bulbs, inspected wires, and I believe the culprit may have been the cracked right side marker lense allowing water to infiltrate the harness and (possibly?) ground out there.

Between disconnecting and reconnecting all the wires in the rear section of the tail circuit, including the ones accessible by lowering the rear storage tire, messing around with that side marker area and taping the lense up for now and cleaning out the bulb harness, that seems to have fixed it. We'll see how long it lasts, but I may want to consider a new lense and probably replace the corroded contacts underneath.

Anyway, thought I would share what became the fastest way to isolate the issue when faced with the seemingly endless number of shorts it could have been. Move from harness to harness and test the circuit. I wound up just doing it with one fuse and waiting to see if it would blow, but I imagine a DVM or test light could work for you. Wasn't all that conclusive on my end. I think with the blown portion I found 11.9V and after disconnecting ID2 it read 12.3V, but I don't know if that's even significant.

Happy hunting your next electrical gremlins! Solved for now.
 

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