Let's all learn how to debug a GAUGE fuse blown due to a short circuit

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So I was driving my truck a few days ago and the dreaded "gauges, windows, door locks, heater/AC, everything" went dead. This seems to be fairly common based on the multiple previous threads. Here are just a few: 1, 2, 3 Just like the others, my GAUGE fuse was blown. OK, turn the truck off, replace the fuse, turn the key to start and the fuse pops immediately. So what I know from that is that there is a ground fault in one of the many downstream circuits that rely upon the GAUGE fuse for protection.

I've never encountered this problem before and I can't seem to find a step-by-step on how to debug this condition and fix it. I thought this would be a good chance to figure it out with all of your help and document the shortest and easiest path to victory. If you know of such a step-by-step, please reply and give a link. Otherwise, grab some popcorn and come along with me on this one.

BTW, my truck is a 1996 LX450 with no significant modifications to the wiring except for a two pairs of Baja Designs lamps on the front bumper. They are properly wired with their own fuses, relays and push button switches to the left of the steering column. So my truck is stock as far as wiring goes.
 
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The first thing I did was figure out how power was going into and out of the fuse block for the GAUGE fuse.

powersource.jpg


The power to the GAUGE fuse is a black-yellow wire and the power leaving the fuse is a yellow wire. You can see it at the bottom of the above image.

In theory, if I remove the fuse from the slot and I use a multimeter to test for continuity between the yellow terminal for the GAUGE fuse in the fuse block and a good ground point, it should NOT have continuity. That's if everything is working correctly and there isn't a short downstream. In my case there is clean continuity to ground showing that there is a short somewhere. This is my first assumption that I'd like to get confirmation on. Does that sound correct?
 
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Digging deeper into the EWD doc, I started writing down a list of all of the circuits downstream of the GAUGE fuse and which junctions and connectors feed them power. My plan was to disconnect all of these downstream circuits and test them for continuity to ground at the connector. Or, I could reconnect them all, one at a time until there's continuity between the yellow terminal in the fuse block and ground. That would tell me which circuit has the short circuit.

The problem with this plan is that nearly all of the connectors are up in the dash and very difficult to access, if not impossible, without taking the whole dash apart and out.

I did notice that many, if not all, of the downstream circuits pass through junction J1 or junction J4. Here are two examples of circuits that go through one of those two junctions:

j1.jpg
j4.jpg


Because those two junctions are in all of the downstream circuits, I think I can speed things up by pulling one of them apart and testing for continuity to ground at the fuse block again.
 
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So where is J1 and J4? Here actually:

junctions.jpg


J1 is the easiest one to access I think so I tried to go find it and test out my theory.

CB2063D0-7515-4646-9991-410C74B75072.jpeg
3E7B14FE-5FDA-4F91-B330-921CE324D331.jpeg


I remove a few trim pieces and made enough room for me to look way up under the driver side dash, above the steering column to what I *think* is junction J1. The pin out matches what is in the EWD, but I'm not entirely certain because all junctions have the same shape and pin out. I pulled the junction cap out which should break the connection to all of the downstream circuits that go through J1. I tested for continuity to ground and it is still there. That means one of two things: this isn't J1 and it is J2 instead, or it is J1 and the short is downstream of J4.
 
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That's it for tonight. I'm going to dig around up there to see if I can find the other nearby junction to confirm that I'm dealing with junction J1 and J2. Then I can pull the other junction apart and test for continuity on the off chance that I found J2 first. Either way, if there is still continuity to ground then I've got to investigate the circuits downstream of J4. I recently replaced the coolant temperature sensor and both O2 sensors and I might have nicked something and caused a short. I'm not even sure if those circuits are downstream of J4 and GAUGE yet but those are what I changed recently right before this problem developed. Wish me luck tomorrow.
 
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The GAUGE fuse powers a wide variety of items.
AC PRESSURE SWITCH
ABS ACTUATOR
ABS RELAY
AC AMPLIFIER
ABS ECU
AIR INLET CONTROL SERVO
BRAKE FLUID LEVEL WARNING
BLOWER SPEED CONTROL
CDL CONTROL
COMBINATION METER
DASH COOLING FAN
CRUISE CONTROL
DLC 1
DIFF LOCK ECU
DOOR LOCK CONTROL
ECU
ECT
FUEL SENDER
OIL PRESSURE SENDER
OD SWITCH
NSS
PARKING BRAKE SWITCH
REAR HEATER
REAR WINDOW DEFOGGER
REVERSE LIGHTS
SEAT BELT WARNING
L4 POSITION SWITCH
NEUTRAL POSITION SWITCH
>>>>>>>>TRAILER SOCKET<<<<<<<<low hanging fruit
SPEED SENSOR
WATER TEMP SENDER
DEFOG RELAY
HEATER RELAY
POWER RELAY
 

smritte

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At this point, install a fuse and see if it blows again. The reason is, if you have a chaffed wire and you accidentally moved it while accessing something, you could have made the short temporarily disappear. If it didn't, keep searching. If it did, there's a chance the issue is in the area you took apart.
Periodically, retest to insure the issue still exists.
 
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At this point, install a fuse and see if it blows again. The reason is, if you have a chaffed wire and you accidentally moved it while accessing something, you could have made the short temporarily disappear. If it didn't, keep searching. If it did, there's a chance the issue is in the area you took apart.
Periodically, retest to insure the issue still exists.
An easier method would be to use a meter with a continuity beep tone. 1 meter lead in the load side of the fuse block, the other lead to ground. The meter will give an audible beep during continuity. When the beep stops, you have found the issue.
 
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I checked the trailer lights wiring this morning. Near the plug, the wires were exposed and the plug itself was corroded. I thought, "I hope it is this easy". So I opened the rear interior driver side panel and unplugged the wire to the trailer hit from the black box. I tested for ground continuity using my multimeter with the beep tone turned on and it was still beeping. Just to be 100% sure I put a fresh fuse in the GAUGE slot and turned the key to ON and it immediately popped. So at least it wasn't the wire to the hitch. I unplugged the whole Y cable and black box and tested continuity again and same thing. So unfortunately, it is not the trailer lights wiring.
 
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I checked the trailer lights wiring this morning. Near the plug, the wires were exposed and the plug itself was corroded. I thought, "I hope it is this easy". So I opened the rear interior driver side panel and unplugged the wire to the trailer hit from the black box. I tested for ground continuity using my multimeter with the beep tone turned on and it was still beeping. Just to be 100% sure I put a fresh fuse in the GAUGE slot and turned the key to ON and it immediately popped. So at least it wasn't the wire to the hitch. I unplugged the whole Y cable and black box and tested continuity again and same thing. So unfortunately, it is not the trailer lights wiring.
There is factory trailer wiring under the rear cross member. That would be the first place to look.
 

dirtyboots

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Page 7 of the EWD:
*Wire harness sharing the same short terminal grouping have the same color*
On P. 192 of the EWD it shows J4 = G and J5=W-G...check the color of wire at the J unction and EWD (couldn't get sharpie or anything to stick to the junction terminal so I wrapped one of the junction wires with a label for quick reference)
The one you have pictured above looks B-Y and if so that's J3
IMO double check all of the stuff you just installed (O2, temp sensor)..HTH

J3 = L-Y
J4 = G
J5 = W-G (dimmer stuff IIRC)
 
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smritte

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An easier method would be to use a meter with a continuity beep tone. 1 meter lead in the load side of the fuse block, the other lead to ground. The meter will give an audible beep during continuity. When the beep stops, you have found the issue.
I wont disagree with that method considering that's one of the ones I use. As many things that are in parallel in that circuit, I would unplug items before using a continuity tester. The total resistance may be low enough to give a false reading. I know it will on my Fluke. If there were fewer items, that would be diffrent.
 
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I need to check the factory trailer wiring under the rear crossmember. If that doesn’t fix it, I’m going to have to take my whole dash apart. All of the things plugged into this circuit have plugs mostly up under the dash I think.
 
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I wont disagree with that method considering that's one of the ones I use. As many things that are in parallel in that circuit, I would unplug items before using a continuity tester. The total resistance may be low enough to give a false reading. I know it will on my Fluke. If there were fewer items, that would be diffrent.
I know my Fluke meter will still produce the beep with a resistance as high as 1k Ohm. I never use the continuity feature without double checking the actual resistance.
 
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I know my Fluke meter will still produce the beep with a resistance as high as 1k Ohm. I never use the continuity feature without double checking the actual resistance.
Right. And parallel circuits reduce the overall resistance between hot and ground. Makes sense. I bet your fluke can be tuned to only beep at less that 10 ohms.
 
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In my case I’m getting .01 ohms between the yellow terminal of the gauge fuse and ground. There is a very good connection to ground somewhere. But where? This is going to be a serious hunt I think.
 
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Page 7 of the EWD:
*Wire harness sharing the same short terminal grouping have the same color*
On P. 192 of the EWD it shows J4 = G and J5=W-G...check the color of wire at the J unction and EWD (couldn't get sharpie or anything to stick to the junction terminal so I wrapped one of the junction wires with a label for quick reference)
The one you have pictured above looks B-Y and if so that's J3
IMO double check all of the stuff you just installed (O2, temp sensor)..HTH

J3 = B-Y
J4 = G
J5 = W-G (dimmer stuff IIRC)
You're correct about this. Based on the EWD, it looks like the following colors correspond to the relevant junctions (from the 1996 FZJ80 EWD):

JunctionWire ColorConnector Color
J1YellowGray
J2White-BlackBlue
J3Blue-YellowGray
J4GreenGray
J5White-GreenBlue
J6Black-WhiteGray
J7White-BlackBlue
J8White-BlackBlue
J9White-BlackGray
 
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So I was driving my truck a few days ago and the dreaded "gauges, windows, door locks, heater/AC, everything" went dead. This seems to be fairly common based on the multiple previous threads. Here are just a few: 1, 2, 3 Just like the others, my GAUGE fuse was blown. OK, turn the truck off, replace the fuse, turn the key to start and the fuse pops immediately. So what I know from that is that there is a ground fault in one of the many downstream circuits that rely upon the GAUGE fuse for protection.

I've never encountered this problem before and I can't seem to find a step-by-step on how to debug this condition and fix it. I thought this would be a good chance to figure it out with all of your help and document the shortest and easiest path to victory. If you know of such a step-by-step, please reply and give a link. Otherwise, grab some popcorn and come along with me on this one.

BTW, my truck is a 1996 LX450 with no significant modifications to the wiring except for a two pairs of Baja Designs lamps on the front bumper. They are properly wired with their own fuses, relays and push button switches to the left of the steering column. So my truck is stock as far as wiring goes.
What was the last work done on the 80, specifically inside the cabin
 
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What was the last work done on the 80, specifically inside the cabin
I added a center lock switch and a USB charging port. I added USB charge ports to the rear doors. All of those still work after the gauge fuse blows. My door locks still work but the windows don’t.
 

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