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Unsolvable TAIL fuse issue

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Old 11-08-2018, 04:31 PM
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TAIL fuse issue

1986 4Runner 22re.. Out of nowhere my Tail fuse blew, and I have torn through the wiring looking for a short, only to find nothing.. I have disconnected everything on that circuit, taillights, marker lights, dimmer switch, and rear deck light .. Also all the rear window relays just for safe measure even though I have it all bypassed.. Without the tail relay its fine, obviously, but as soon as the relay makes contact with the pins, boom it's popped again.. I've also checked the grounds, all good, even cleaned em off real quick for good measure.. Only thing I can think of is that relay, possible corroded inside, is there a way to test it without buying another one? Or bypass it? Also checked the combo switxh, looks good.. No other issues anywhere else, just the Tail circuit..

Last edited by erock13; 11-09-2018 at 11:10 AM.
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Old 11-08-2018, 04:57 PM
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Schematic is below.

Originally Posted by erock13 View Post
...I have disconnected everything on that circuit,...
...is there a way to test it without buying another one
Let's start with this:
1) By "disconnected" you mean disconnected the wires connected to the relay? or removed all the bulbs?
2) How about the license light? Did you inspect the flex cable between body and tailgate as well as the wires inside?
Also, please see signature.

In fact, the reason for testing the relay is so you would not have to buy a new one. We can go there after you answer questions above.


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Old 11-08-2018, 05:06 PM
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Forgot to mention that, the flex cable is completely disconnected and removed from the truck, no license plate lights back there anymore.. Everything except the front markers are disconnected via their plugs, the front markers just have their bulbs taken out..
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Old 11-08-2018, 05:09 PM
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Also no trailer wiring to deal with either
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Old 11-08-2018, 06:51 PM
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Please change title. Yotatech will solve it - LOL!

Tail light relay is upstream of the fuse. Any of its pin shorted to ground would not blow the tail light fuse.

Let's check...
1) Power off
2) Tail light fuse installed
3) Pull out the tail light relay. I believe it's the cylindrical one forward of the blinker relay in pic below ...
4) Measure resistance between bare, shiny metal of body ground and pin 3 of the relay socket. That's the pin with the red wire behind it.
Tell us what you see.


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Old 11-09-2018, 11:01 AM
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Lol, yeah i put unsolvable because ive been goin at this thing for 3 days now.. It's got continuity though, so that would mean the ground to ground is good correct?
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Old 11-09-2018, 01:39 PM
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Originally Posted by erock13 View Post
Lol, yeah i put unsolvable because ive been goin at this thing for 3 days now.. It's got continuity though, so that would mean the ground to ground is good correct?
What's got continuity?
Gotta describe describe symptoms and problems in EXACT terms. Pls see signature. Did you EXACTLY as I suggest above?
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Old 11-09-2018, 01:49 PM
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Also, what exactly is disconnected from the circuit and what is still connected to the circuit?

You may get different readings based on what is connected.

If I am thinking along the same lines as RAD4Runner, you are looking to find a short circuit in the wiring and not knowing what is still connected could give you false readings.

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Old 11-09-2018, 02:25 PM
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Originally Posted by RAD4Runner View Post
What's got continuity?
Gotta describe describe symptoms and problems in EXACT terms. Pls see signature. Did you EXACTLY as I suggest above?
Power was off, new tail fuse installed, disconnected relay, black probe to ground and red to pin 3 on the fuse panel, multimeter buzzed and gave a reading that varied between .03-.05 ohms

Originally Posted by old87yota View Post
Also, what exactly is disconnected from the circuit and what is still connected to the circuit?

You may get different readings based on what is connected.

If I am thinking along the same lines as RAD4Runner, you are looking to find a short circuit in the wiring and not knowing what is still connected could give you false readings.

Coming from the rear first, the tails are disconnected at the plug, the harness that is supposed to go into the tailgate is disconnected at the plug, the cargo light is disconnected at the plug, the relays in the quarter panel are disconnected, the dimmer switch is disconnected at the plug, and the front marker bulbs are removed
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Old 11-09-2018, 05:07 PM
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I just realized that the alarm is also tied into the tail circuit, to flash the lights.. It's an aftermarket basic viper system, it's got two fuses and one of them is blown as well.. Was installed by a shop, so I'm not sure how it got wired up.. I'll try disconnecting it and see what I get
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Old 11-09-2018, 05:17 PM
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Yeah, the alarm and how it is wired definitely complicates things from our end, as we cannot see how components are connected to the alarm system.

Hopefully the shop did a good installation job and not the "twist and tape" method.

Tell us what you find and if disconnecting the alarm changes anything.

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Old 11-09-2018, 05:50 PM
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The alarm led me to the issue.. Theres a split in the loom by the fusebox that carries either the tails or markers, not sure, but the split is for the alarm, and the fuse that was blown is connected to it.. Well that wire is partially hidden, it goes into the opening by the kick panel where the door switch is located, and then is bundled up with the door switch wires, well right out of eyesight was a portion that was kinked and had just the tiniest bit of exposed wire and was grounding out on the body, straightened out and moved away from the body, its back to normal. Thanks for the help guys, cant believe it took several days to find the stupid thing
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Old 11-09-2018, 06:50 PM
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Originally Posted by erock13 View Post
..., cant believe it took several days to find the stupid thing
Me, I'm impressed you found it that quickly!!

Good write up, lessons for us all.
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Old 11-09-2018, 10:25 PM
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Glad you found it. As I always stress problems are often caused by bad wiring (possibly caused by poor workmanship of the shop that installed your alarm) and/or maintenance.

Originally Posted by erock13 View Post
... black probe to ground and red to pin 3 on the fuse panel, multimeter buzzed and gave a reading that varied between .03-.05 ohms...
Yep that^^^ right there indicates short to ground.
When we do things methodically, they become solvable

Originally Posted by erock13 View Post
... straightened out and moved away from the body, its back to normal...
You insulated that wire, of course?

Last edited by RAD4Runner; 11-09-2018 at 10:27 PM.
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Old 11-16-2018, 12:24 PM
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This might sound crazy but I had the same problem /same troubleshooting/same results for 5 days (NO TAIL LIGHTS) but!!!
I noticed that the dash light bulb behind the defroster bezel had gone out. I replaced the light bulb-PROBLEM SOLVED!!!
Very small bulb comes in two wattage's. Use the higher wattage.
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