QuiGonJon

1991 4runner, 3.0 Auto

So I have a short somewhere in my brake wiring. Keep popping the STOP 15A fuse, killing my brake lights and the shift lock mechanism. As reference, the main wire powering brake lights is green-white (G-W) and is the shorted wire.

I removed both rear light harnesses, tested for continuity, confirmed no shorts in the housing harness. Testing the harness coming from the body, I have continuity from the white ground wire and the remaining 3 wires. Left and right tested the same. I'm not sure why I have continuity with all 3, that makes me think I'm missing something


I tracked down the brake pedal switch and disconnected the harness there. No ground to the G-R wire coming from the fuse. G-W has continuity to ground. So the short is not in the brake pedal switch.


The G-W then splits, going to the brake lights and the shift lock control. I found the G-W in the shift assembly and tested it there. Cont to other the G-W to confirm its the same wire. It has cont to ground here with the harness disconnected so I'm pretty sure the short is NOT in the shift lock assembly. I retested the rear light harnesses and am still getting cont to ground.


I guess from here I'll keep following the wires until I find the problem. It has to be somewhere in the harness between the pedal switch and the lights. Is there any known trouble spots I should check first?

I wanted to outline my diagnosis so far so someone can tell me if I made a mistake, missed something, not understanding correctly, etc.

The hunt continues

QuiGonJon

This is at the rear driver side door, right before the cargo area. Harness leading the the rear lights, all tested good. No cont. Other end had cont at G-W, G-B and G, which are the other 2 wires at the rear most harness. G-B and G had strong cont, G-W cont to ground I think was weak? My meter beeped but did not stay beeping like when cont is strong. Im not positive what that means

QuiGonJon

As a note, the taillights work fine! I'm confused as to why I'm getting cont to ground with all 3 wires if only the brake light is shorting. Makes me think I'm doing something wrong

QuiGonJon

So I've narrowed it to the front end. Checked the harnesses underneath both front seats, not cont heading to the back, cont to ground heading to the front. Gotta be somewhere around the dash

scope103

First of all, your meter is set to "Diode Test." I think you want "Continuity" (or "Resistance"). https://www.instrumart.com/assets/fl...117_manual.pdf (I don't own a Fluke 117, but in general a diode test puts a fixed small (~10ma) current between the probes, and measures the voltage drop. Which for the average silicon diode is around 1.6vdc. I'm not sure what reading you'd get with a short or open, and I can't interpret the odd readings in your photos.)

For completeness, my ('94) EWD shows ground is W-B (not white), brake G-W, tail G, Turn G-B and G-Y, and Reverse R-B. Since the ground for all 4 bulbs is W-B, in the rear light housing you should get "continuity" between all 5 wires (through the bulbs, but they have a low resistance). Remember that the G-W brake wire goes to ground through the brake light bulb. If you disconnect one of the tail light housing harnesses, you should still get continuity to ground from the harness-side G-W, because it's connected to ground through the opposite brake light bulb.

If the Tail lights (and reverse and turn signal) all work, and the Brake fuse is fine with your foot off the brake, and the Brake fuse blows immediately, every time, you step on the brake, then you have a continuous short somewhere in the G-W wire. To find it, I'd disconnect both Tail light housing harnesses and the high-mount stop light. When you step on the brake, does the fuse still blow? (Testing for continuity at this point is troublesome, as the G-W wire also goes to the ECM, the shift-lock, and the Cruise control, and I have no idea what they would show for resistance.)

Gosh, you could run out of fuses if this takes much longer. The "real mechanic" way to do this is splice a high-current bulb (like a headlight bulb, or one of these: https://www.harborfreight.com/12-vol...ght-93904.html ) into one of your dead fuse bodies. The high-current bulb has such low resistance that the brake light should work pretty much the same, but if there is a short the high-current bulb just lights up with its 55watts. This gadget is especially handy when you have an intermittent short. Just wiggle the wiring harness until it lights, and you're close to the short.

QuiGonJon

I read the manual and yea I guess I wasn't using the meter "correctly" but the diode setting has always worked for me to test continuity. I guess it's not the right way though. I used the continuity setting and retested the rear harnesses. Getting the same results, cont from ground to the remaining 3 wires, G-W, G, and G-B. I disconnected both sides so its not getting contact through the bulbs.

The short isn't immediate. If I put in a new fuse it'll last for a couple minutes of driving. I put the shifter in N to start so it never went to or from P, just to make sure again it's not the shift lock control.

Now this morning my turn signal fuse blew. They're right next to each other in the fuse box. I took out what I reasonably could around the fuse box and didn't notice any damage, everything looked pretty untouched behind it. There is an aftermarket Viper alarm/door control system but I'm pretty sure it doesn't touch the brake wires.

QuiGonJon

Ok can anyone tell me what this is? No way it's factory


This is right inside the cargo area, behind the light housings. Green wires tapped into all 5 wires. Is this part of the Viper alarm?

Edit: nope it all goes to the trailer wiring. Not sure what the box is for. This might be the issue

scope103

Oh, Yeah. Even people who are careful can have problems with trailer wiring. Whoever installed yours, doesn't appear to have been that careful.

Let us know what you find.

QuiGonJon

So I did some research and apparently it was dealer added. Not factory, but Toyota dealer. I took it all out, cleaned up the wiring, and the problems gone! Haven't blown any more fuses, everything works.

Thanks for your help. Guess I was testing wrong at first

scope103

If it makes you feel any better, the "dealer installed" trailer wiring on my truck looks even worse than yours.

13Swords

Those red pieces are Crimp-Lock Splicers, used to add a wire to the existing wire, probably for trailer lights, I'd think.

2ToyGuy

I always called those things vampire splices, and, IMHO, they SUCK. They're a quick, cheap, and easy way to add a wire to an existing circuit. IMHO, the key word there is CHEAP.
Much better to use a butt splice. There are splices that are made to take two wires in one side. They accomplish the exact same thing, but if you crimp them down correctly, and use heat shrink, you get a MUCH better, more reliable, and safe, splice into a circuit.

Easier to remove, too, should you desire to. The vampire splices also damage the wires they're added to. The butt splices can be removed by either cutting off the wire you no longer need, or cut the two original wires on either side of the splice, thus removing it, and put them back together with a regular butt splice and some heat shrink.

All just my opinion, and we all know what good those are...
Pat☺

dropzone

iTrader: (6)

miracle how that crap works. not sure how many times i have traced wiring issues on used stuff i have bought and helped friends with issues that were related to those things used by wire hacks.
Glad you got it figured out.
invented by a lazy person who had no frigging clue, sometimes called a t-tap.
my opinion too! damage the wiring ....

QuiGonJon

4 days later, everthings still good. That was the issue for sure. If I ever need to add trailer lights, I can add them again the right way but that was a mess. I would like to say I'm surprised that that hack job was done by a dealer but it is the cheapest and fastest way to do it. Obviously not the right way. I'm just glad it wasn't a problem in the Viper alarm. Under the dash looks like a rats nest with it tapped into everything, I dread the day anything under there goes wrong.

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