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tail light wiring.

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1
Hey, can someone help me undrstand tail lighting. The right rear light blinks but is very dull and no park or brake. The bulb is fine. All other lights work.
Im not understanding how to fix it. There are two wires to the light. But it has 3 functions, right. I can start stripping tape to find a problem or run new wires. Or could it be the relay? Are those wires connected to other lights or do they run seperate. As you can see I dont know squat. Can I tie into the left rear with new wires for the park and brake? Argggg.
 

smokey

Hitech hillbilly
Staff member
Sounds from the description of what you are saying is that you have a Ground issue.
Check all the grounding points on the lighting circuit. Pull the connections apart and clean the ones in the harness where it unhooks at the bumper.
Most lighting issues are from poor connections.
 
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Ok, so, where is it grounded? There is two wires to each light and I dont think one is a ground. Wouldnt the light be grounded by the frame or is the another place a wire will go to be grounded?
 

smokey

Hitech hillbilly
Staff member
If this is the flat bed it is hard to tell where they grounded this. On a normal bed in a 70 series truck there is a small wire that is attached to the bed by each tail light.
Do you know if it still has the original harness on it?
If so I can get you a wiring diagram with color codes.
Try testing the lights with a test light along the a harness to see where the problem starts and work back from that point to find the bad spot.
 
The two wires supply power to two different filaments in the bulb.
one wire is for running lights (you called it park)
The other supplies power when brakes are applied or turn signal is turned on, If the brakes are applied while the turn signal is on it interrupts the power turning it on and off only brighter.
Each socket has to have a good ground (wire)to complete the circuit






Smilys aren't working, maybe They have a bad ground
 
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I cant tell if its original. Its a mess of taped up wires. Im going to pull my hear out and start peeling tape off.


smily smily smily
 

smokey

Hitech hillbilly
Staff member
If it is taped up I would bet it is a bad connection in where they joined the wires.
When repairing the connections Solder and shrink tube the connections otherwise you will have headaches down the road.
 
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Thats what Im thinking is its original wiring up to the split at the rear. That would be exposed to the elements more. I hope I can find the problem, I just hate wires. Cant I just wave my arm out the window?
 
To start with ..Everything runs through the Turn signal switch

The wiring Ford uses allows for the brake light filament and TS filament to work like they normally due.

This T-Shooting guide may help you.

The ground strap for tailights is located behind the taillight attached to the box on a styleside.



78wiringdiagram-1.jpg
 
I hate wiring. Let me get that out of the way :)

So what Idid on my '76 is rewired it with a fresh trailer wiring harness from the auto parts store. Now I know that all of the wires are new and good.

I started at the 4-plug socket where it comes out of the firewall and ran the wires all the way back to the light. The trailer harness is nice because the wires are already lable with left/right/brake and the white on is the ground.

I did that and haven't had any problems with my light working.

Mark
 
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Its strange. There is no ground wire to each light housing. There is two wires to each lamp and my guess is the ground is where the body of the light is touching metal. Im going to try screwing a wire into the body and onto the frame.
 

LEB Ben

Arrogant A-hole At-Large
34,919
1,124
outside your house
IIRC...the ground wire should come out of the socket and there should be a hole in the sheetmetal behind the light that the groundwire gets screwed to.
 
The ground wire comes out of the Bulb socket not the TL housing
 

LEB Ben

Arrogant A-hole At-Large
34,919
1,124
outside your house
The ground wire comes out of the Bulb socket not the TL housing

Am I missing something?

IIRC...the ground wire should come out of the socket...


and then the wire has an eyelet at the other end and gets screwed into the sheet metal behind the light...right?
 

smokey

Hitech hillbilly
Staff member
This on a flat bed truck it is not using the normal sockets or harness.
It was a hack job by a previous owner.
The lights are aftermarket housings on this truck.
 

73F100Shortbed

That's how we roll!
5,937
320
NJ
If it is some hacked up mess, just rewire everything. That's my opinion. But it sounds like a bad ground like suggested.
 
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Just to mention, Yes it is a flatbed so it does not have factory lights. The body (77' 1/2 ton that is in excellent condition) was placed on a 76' 3/4 ton running gear. I dont normally agree with "hacking" up a classic but my hat is off for the person who put it together. The truck is in great shape. The truck is unaware it is hacked up, shhhh.

My friend is coming over today to look at the light. We briefly looked at it over a few beers and it looks like the ground is part of the light housing. What we are thinking is drill a hole in the back of the housing and screw a wire into it and onto the frame. It just seems strange that it doesnt have that yet the other light works perfect.
 

fatherdoug

Tonto Papadapolous
Do the left rear lights work ok, Signal, tail, and brake? The wiring on my '78 runs along the left frame rail(driver's side) to the back of the truck, then crosses over to the right side. If the left side works ok, then the problem is between the left and right side somewhere. Do you have license plate lights? If one or both of those work, then the problem is narrowed down to somewhere between the license plate light and the right rear light.
 

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