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Trailer Light Sorcery . . . I beat the Hex !

Desert Whaler

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Full disclosure . . . I suck at electrical.
Dude on the freeway pulls up next to me, honks, gives me the 'Roll Down Yer Window Fool' sign & yells, "Your drivers side turn indicator and brake light are out on the trailer"!
Cool heads-up.
I order 2 new LED replacements, since these are almost 10 yrs old.
Package arrives . . . I go to back my boat out of the side yard, but use the short bed chevy for easier maneuvering instead of my tundra. . . for shits & giggles I plug in the lights to check the left side lights are indeed not working . . . . they're working fine !
Un-hitch the trailer and back my Tundra up . . . no dice.
I always use a 'Flat-4' to 'Round-7' adapter plug in case I have to pull with some other rig.
So I get the flashlight / tester out and start testing each lead coming out of both trucks receivers . . . both are working 100% . . . running lights, turn signals, brakes, etc.
Then I put the adapter plug in and test the same leads on the 'flat-4' side coming out . . . works 100% on the chevy, and yep . . . you guessed it . . . the left side power coming out of the plug on the Tundra is no bueno !
Weird that the same adapter plug works on the Chevy and not the Tundra where it's always worked fine ! I even used a contact-cleaner soaked rag on the end of a screwdriver and cleaned the terminals . . . then dialectric grease. nada
$6.00 amazon order for a new adapter plug and it's 100% . . . .
But the thing is, and how I got lucky was . . . If I had hitched-up my tundra 1st and tested the lights, it would've failed, and I'd have re-wired a whole new set of lights with the same result !
Someone was looking out after me on that one. 🙏 Now I've got spare lights.
 

Desert Whaler

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The wizardry of wiring.....:) I've Lost a few brain cells troubleshooting
No kidding…. I figure after all these years of being haunted by the trailer light voodoo, I cashed in on a little ‘credit’. Lol.
 

Springfield

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Okay quiz, my left trailers lights work, no brake or turn on either. Would they both go bad at the same time or go to the hex and start for loose wire/disconnect.
 

LazyLavey

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Possibly bulbs bad... or yes.... bad wire connection......

Check for power on the plug at the light...if you got power at the light plug its the bulb (or whole unit)

no power? trace wiring for bad connection, including tow vehicle plug
 

mash on it

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Okay quiz, my left trailers lights work, no brake or turn on either. Would they both go bad at the same time or go to the hex and start for loose wire/disconnect.

So, clearance and marker lights work. (brown wire)

And the left turn and left brake light is not? (yellow wire)

I'd check the yellow wire, starting at the truck connection, working my way back.

And check the ground. (white wire)

Dan'l
 

ka0tyk

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I enjoy electrical... its like math. Theres no "interpretation" of it, it either is one way (correct) or it shocks you.
 

Springfield

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My plan is to start at the plug connections before replacement, thanx guys

Running lites and tail lights fine, just left brake and turn which both just working until they werent
 

ToMorrow44

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My plan is to start at the plug connections before replacement, thanx guys

Running lites and tail lights fine, just left brake and turn which both just working until they werent
Brake and turn are the same circuit, same wire. So if everything else is working fine except brake/turn on one side, then there’s probably something up with that wire somewhere.
 

oldschool

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For me, electrical and wiring are based on the “FM” principal. FUCKING MAGIC.
 

Malibuvride07

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After learning the hard way a couple times now. First thing I check is corrosion on both the connector from trailer and receptacle on truck. For some reason my extreme trailer has always hide slight corrosion on the plug resulting in dim and not working lights. Finally gave in this season and replaced it and works like a champ now!
 

spectra3279

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I put reverse lights on my spectras trailer. I put a seven pin connector on it also. Was thinking later on could make some connectors that I can connect and charge batteries while I'm towing from the power in the 7 pin
 

lbhsbz

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If I have at least one light doing something…I roll with it.

Those 7-4 pin adapters fail a lot…I think I have about 4 of them.
 

Taboma

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I enjoy electrical... its like math. Theres no "interpretation" of it, it either is one way (correct) or it shocks you.
I disagree, certainly there's plenty of " Open, Closed, or shorted " in it's rudimentary nature and that's what the text books are written about.
But it's the mysteries of "How or Why da f*ck it do dat" that had me awake at 2AM and kept the over 50 years I spent in the trade so interesting for me. 😁
 

Ragged Edge

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Keep in mind that on older tow vehicles the pins inside the electrical connector wear out. Short term fix, clean the pins and sockets the best you can, take a small flat blade screw driver and push it between the split in the pins of the connector spreading them out slightly. This will improve the contact point between the pins and the sockets. Buy a new electrical connector for your tow vehicle and replace it as soon as you can. The trailer side plug can also wear out, not the case with the OP as his adapter plug works fine on another vehicle.
 

Springfield

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So, clearance and marker lights work. (brown wire)

And the left turn and left brake light is not? (yellow wire)

I'd check the yellow wire, starting at the truck connection, working my way back.

And check the ground. (white wire)

Dan'l
AHHA..yellow wire broke off connector at light itself.
 

fishing fool

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FYI..... If you have to back up a 5 pin trailer (trailer brakes) and you don't have a 5 pin plug, you can turn on your lights and plug the plug into just the last pin of the 4 pin. The last pin is lights on a 4 pin plug which will give power to the 5th pin trailer brakes and will allow you to back up....
 

Nanu/Nanu

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This is why still to this day it's called "electrical theory". We got a decent idea of how this stuff works but we still ain't quite fer sher!
 

02HoWaRd26

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Okay quiz, my left trailers lights work, no brake or turn on either. Would they both go bad at the same time or go to the hex and start for loose wire/disconnect.
I’d just have the whole trailer refurbed...... it’s the yellow wire, I’d say not the ground if everything else is working, unless the ground just at the two lights on the rear is broken.
 

Springfield

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I’d just have the whole trailer refurbed...... it’s the yellow wire, I’d say not the ground if everything else is working, unless the ground just at the two lights on the rear is broken.
It's the yellow wire broke off at the light. Easy fix till it goes to Adrenaline for the full refurb and new lights. 😊 ;)
 
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