Dash and Running Lights Out, Trailer Harness Off - What am I Missing? - Toyota Nation Forum : Toyota Car and Truck Forums
T-100 Forum Discussion forum for the short-lived but extremely popular Toyota T-100 model.

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post #1 of 17 Old 09-28-2014, 05:07 PM Thread Starter
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Dash and Running Lights Out, Trailer Harness Off - What am I Missing?

Well, I think I finally caught the dreaded trailer harness adapter electrical gremlin. Dang it.

I thought my dash lights weren't working the other day, but it was bright and didn't check it later. But I went to drive my T100 this weekend on a trip, and I got down the road and noticed the dash lights were definitely not working. The dimmer had no effect.

Thanks to this wonderful forum, I remembered that trailer harnesses can cause all sorts of issues. So on a hunch at the gas station, I checked to see if my taillights were working.

They weren't. So I ran in a bought some new fuses for the taillights and dash lights to rule them out. New fuses had no effect. So I checked my adapter and the plug had a lot of water in it, and knowing that can cause issues, I got under the truck at the gas station and followed the trailer harness wires hoping that I could unplug it and that would fix the issue. Well, unfortunately there was no plug, and the PO had the harness wired in using Scotchlok connectors, all wrapped in tape. Well, I pulled all the harness wires out, and my my lights still didn't work. So back home we went, since it was getting dark, and we had to switch cars.

Everything was fine not long ago, and I towed my parent's trailer recently and everything went well...the trailer lights all worked correctly. So everything was fine recently.

So the harness is unplugged, but I haven't pulled all the Scotchlok connectors off yet. That may help, which I will do in a bit.

Is there anything else I could be missing, or any other suggestions? The only lights that don't work are my dash lights, license plate lights, and my running/driving lights.

The headlights, brake lights, and blinkers all work fine.

'97 Extra Cab 3.4 4x4 5sp 113K and counting...
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Last edited by SC T100; 09-28-2014 at 05:18 PM.
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post #2 of 17 Old 09-28-2014, 05:16 PM Thread Starter
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Update: Pulled the Scotchlock connectors off. Lights still aren't working. I know I could have some corrosion in my main harness, but the so far the wires appear to be in good shape, and the connectors didn't have any rust in them that I could see.

What am I missing? I know I also need to tape/seal up the wires that were used for the harness, but the lights should still work once the trailer harness is disconnected if that is the issue.

'97 Extra Cab 3.4 4x4 5sp 113K and counting...
Aisin hubs, Marlin shifter bushings, a few fellow TN member mods, and lots of elbow grease.

Last edited by SC T100; 09-28-2014 at 05:21 PM.
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post #3 of 17 Old 09-28-2014, 05:34 PM
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Probably a blown fuse in the box under the dash on the drivers side, located on the outside wall just above the foot rest, and caused by a short in the trailer wiring or the ground.

I learned the hard way.

Leon
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post #4 of 17 Old 09-28-2014, 05:38 PM
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Dimmer switch unplugged or bad?

Check fer voltage at the connector prior to where the Scotchlok connectors were.

1998 T-100 SR5 2WD auto, Roadmaster Active suspension, dual oil catch jars, AC mod, aero cap, lower aero package, 67% grill block, aero single wheel trailer
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post #5 of 17 Old 09-28-2014, 08:09 PM
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After my electrical ordeal, I'm a big fan of checking for a faulty ground. There are two on the driver's side and one on the passenger side in the cab. Unbolt them and hit the washer with a file/sandpaper to get any buildup or corrosion off.
Good luck on this issue. I absolutely HATE electrical problems!!

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post #6 of 17 Old 09-29-2014, 10:18 AM Thread Starter
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Quote:
Originally Posted by Ole Joe Clark View Post
Probably a blown fuse in the box under the dash on the drivers side, located on the outside wall just above the foot rest, and caused by a short in the trailer wiring or the ground.

I learned the hard way.

Leon
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Dimmer switch unplugged or bad?

Check fer voltage at the connector prior to where the Scotchlok connectors were.
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After my electrical ordeal, I'm a big fan of checking for a faulty ground. There are two on the driver's side and one on the passenger side in the cab. Unbolt them and hit the washer with a file/sandpaper to get any buildup or corrosion off.
Good luck on this issue. I absolutely HATE electrical problems!!

I checked and replaced the fuses in the kick panel for the dash and tail lights, but not all of them. I will give them a once over again, though.


Dimmer was working just fine until all the dash lights went out. I will check to make sure it isn't unplugged, but that doesn't mean the dimmer didn't go bad all of a sudden. Voltage check both at the wires and before sounds like my next step.


I thought about the grounds, too. I will definitely check and clean to ensure that bad grounds are ruled out.


Also, does anyone know off-hand if there may be a relay that got tripped, and would unplugging the battery for a bit do anything to possibly reset it if there?

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post #7 of 17 Old 09-29-2014, 12:50 PM
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Looking over my EWD...guess what I find that powers instrument cluster lights as well as the license plate lights? The Tail Light Relay!

It should be located behind the driver's side kick panel right off the fuse panel. It looks like it should be the round relay that located above the 15A Tail light fuse. It will have 3 lugs on the relay compared to the 4 lugs on the other relays.

I don't think unplugging the battery will do anything to reset it. It might be worth yer time to unplug the relay, push it back in to see it will fix it or not.
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post #8 of 17 Old 09-29-2014, 12:54 PM
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Here's the part number...
85916N RELAY, TAIL LAMP
90987-01003 1 $52.90


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post #9 of 17 Old 09-29-2014, 01:17 PM Thread Starter
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Awesome! Bam you are the man. Thanks so much for the info...it gives me something else to check. Maybe I just knocked it loose (hopefully).


Thanks again!
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post #10 of 17 Old 09-30-2014, 07:15 AM Thread Starter
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FIXED:


Well, I have to eat a little crow here. In my haste at the gas station the other night, I actually replaced the stoplight fuse, not the taillight fuse. I was in such a hurry to get back on the road and get to the beach for the weekend with the kayaks, I made a dumb mistake and mis-read the fuse panel labels.


So as I looked at the relay Bam noted last night, I decided to give the fuses a once over again. Well, guess which fuse was blown...yup, the taillight fuse.


Replaced it, and all was working well again. Dash lights, taillights, etc.


So just FYI - if you're in a hurry, take a deep breath and slowly go over what you're working on to make sure you're actually doing it correctly. Unlike me.


So to recap, the taillight fuse in the kick panel of a 1997 T100 4WD manual trans actually controls the taillights, the dash lights, the license plate lights, and the lights-on buzzer. It's a 15A fuse.


Thankfully it was a dumb mistake. All I have to do now is seal up the factory wire harness where the trailer harness had been spliced in. Then I'll need a new trailer harness.


Can anyone recommend a good one that plugs into something as opposed to needing to be spliced in?

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Aisin hubs, Marlin shifter bushings, a few fellow TN member mods, and lots of elbow grease.
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post #11 of 17 Old 09-30-2014, 06:18 PM
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I had a Hoppy branded one on my truck until the controller gave out. So I cut out the original controller and wired in a U-haul one. It still plugs into the wiring harness.
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post #12 of 17 Old 10-01-2014, 06:07 PM
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post #13 of 17 Old 10-01-2014, 09:28 PM
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Wire it in via solder and heat shrink. WHen not is use fab a mount so its upside down and OPEN (not with a cover on it like many have). Just leave it upside down and maybe dab a little electrical grease in there to fend corrosion.

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post #14 of 17 Old 10-06-2014, 02:23 PM Thread Starter
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I had a Hoppy branded one on my truck until the controller gave out. So I cut out the original controller and wired in a U-haul one. It still plugs into the wiring harness.

Thanks Bam, I'll take a look.

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Haha, yep.


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Originally Posted by noahrexion View Post
Wire it in via solder and heat shrink. WHen not is use fab a mount so its upside down and OPEN (not with a cover on it like many have). Just leave it upside down and maybe dab a little electrical grease in there to fend corrosion.

Good idea if I can't find a plug-n-play unit to use.


Thanks!

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post #15 of 17 Old 10-13-2014, 10:03 AM
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Quote:
Originally Posted by SC T100 View Post
FIXED:


Well, I have to eat a little crow here. In my haste at the gas station the other night, I actually replaced the stoplight fuse, not the taillight fuse. I was in such a hurry to get back on the road and get to the beach for the weekend with the kayaks, I made a dumb mistake and mis-read the fuse panel labels.


So as I looked at the relay Bam noted last night, I decided to give the fuses a once over again. Well, guess which fuse was blown...yup, the taillight fuse.


Replaced it, and all was working well again. Dash lights, taillights, etc.


So just FYI - if you're in a hurry, take a deep breath and slowly go over what you're working on to make sure you're actually doing it correctly. Unlike me.


So to recap, the taillight fuse in the kick panel of a 1997 T100 4WD manual trans actually controls the taillights, the dash lights, the license plate lights, and the lights-on buzzer. It's a 15A fuse.


Thankfully it was a dumb mistake. All I have to do now is seal up the factory wire harness where the trailer harness had been spliced in. Then I'll need a new trailer harness.


Can anyone recommend a good one that plugs into something as opposed to needing to be spliced in?
Nice work SC! I too have come across this Thanks for posting up the results and how you fixed it. Electrical problems call always be fun...

1995 T-100 3400 2WD 87k miles
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