1966 D100 tail light problem

Wiring, lights, heater controls, anything electrical..
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gtxgts1969
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1966 D100 tail light problem

Post by gtxgts1969 »

I'm having an issue with my passenger tail light (brake and turn signal). I'm 99% sure it's a wiring problem because i swapped the passenger tail light housing to the driver side and works like a champ.

When i turn on the parking lights, the passenger side comes on but is dim (compared to driver side). The turn signal, parking and brake light works fine on the driver side but a no go on the passenger side.

Both front turn signal/parking lights work. License plate light works

A wiring diagram would help me figure it out (I hope!!!) because the harness has been "modified" by the previous owner and quite a few wires are not color coded.

Any help will be greatly appreciated
Last edited by gtxgts1969 on Thu Apr 14, 2011 1:46 pm, edited 1 time in total.

EGroscup
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Re: tail light problem

Post by EGroscup »

It really sounds like the ground is bad on the passenger side. If that is not it get a test light out and start working your way forward. The bulkhead connector in the engine compartment where the electrics enter the cab can be fussy on these also. Erik

gtxgts1969
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Re: tail light problem

Post by gtxgts1969 »

Should I ground the passenger tail light housing to the frame or are you saying it's a wiring issue further down the line (towards the cab)?

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Re: tail light problem

Post by WD »

Start by grounding the metal housing. It can only help, never harm.

Mine had the same issues. I finally got fed up and mounted a second set of lights (on the bumper). Uppers are now run/turn, lowers are brake/run. Wiring on mine is a joke, the rear signals are piggybacked into the front signal leads, the side marker lights are run through the old fog light switch, the brake lights are always on (and I mean always, so I disconnected them...), the horn harness is new, but doesn't work, headlights are wired through the domelight... :thinking A lot of it started when the turn signal switch upper cam broke, they aren't any heavier duty than a paper clip and a couple tiny decorative brass rivets, mounted in cheap plastic. Once a lead in there gets bent or broken, all kinds of electrical glitches pop up.

Most of it on mine is that the firewall junction/circuit box is a corroded mess. Nothing a full rewire won't fix. I have a new harness, new fuse box, some new switches and relays, but, I have 5 trucks to play with and/or work on, 2 are good roadworthy ones, so it can wait.

Screwy wiring seems to be the biggest issues with old Dodges, regardless of year. The vast majority were work trucks, patched up and put back to work.

gtxgts1969
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Re: 1966 D100 tail light problem

Post by gtxgts1969 »

I found a schematic and i should have 4 wires coming from firewall to back of truck (lt blue - gas tank, dk green - passenger side directional & stop light, yellow - driver side directional & stop light and brown - tail light (for both tail lights and license plate).

After i uncovered the harness at the back of the truck, i had a yellow, brown, lt blue and white wire. **I have no clue what the white wire goes to.

I disconnected the yellow, green and brown wires at the firewall to do a continuity check. the yellow and brown wires check good but the green didn't.
My gas gauge works so i assume the lt blue wire at the back of the truck was spliced in to the green wire at the firewall sometime in the past?

I did a power check with the brown (tail light) wire with parking lights on and it checked good at wire at back. I also did a power check with yellow wire (directional & stop light) and it checked good at the back.

Since the green wire at firewall had no continuity to the back, i decided to run a new and seperate wire to the back and it checks good.

My problem now is when i attempt to hook up the new green wire to the tail light, i'm blowing the flashers at the fuse block and i don't know what to do now.

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Re: 1966 D100 tail light problem

Post by gtxgts1969 »

Here are 2 pictures:

First picture is a pictre of my bulkhead connector: I did a power check on the upper right and middle right connectors with turn signals operating and they worked (left side upper and middle were not working). What i want to know, is this the correct location for the REAR stop light and directional connections (yellow and dk green wires)?

Second picture is my wires at the rear of the harness: I have a yellow, brown, lt blue and white wire. I have no idea what the white wire is for but i assume the lt blue wire is a splice for the green wire.

Any help will be greatly appreciated.
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gtxgts1969
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Re: 1966 D100 tail light problem

Post by gtxgts1969 »

Update:

I removed the harness from the firewall to back of truck and stripped the tape. I didn't see anything that would cause a short, just some crappy splices.

I found that the white wire was junk and must have been used at one time for a lighted license plate.

The lt blue wire, as i suspected, was spliced into the original green wire.

Since i had the entire rear harness out, i thought i would put in a new flasher and try out the front blinkers and they work like a champ. So i'm guessing the bulkhead is good?

I haven't put the rear harness back in to re-check it but are there any other checks i can perform to verify other parts are not the problem (turn signal assembly, etc.)?

Thanks in advance!

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Re: 1966 D100 tail light problem

Post by EGroscup »

You are 2/3 of the way there. Do you have power for the green wire stop/turn at the bulkhead? if so your new wire should work. I am wondering if the light housing on the passenger side has an internal short. With the housing off the truck and the bulb out there should be no continuity between either wire and ground or between the two wires. Good luck, Erik

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Re: 1966 D100 tail light problem

Post by junior »

Not a good enough of a ground?

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Re: 1966 D100 tail light problem

Post by behindthebadge »

I am having a very similar issue with my '67 D100 passenger side tail light brake light. I am guessing that the 66 and 67 are not different. Thanks for the color code breakdown on the wiring... I am going to take this info and start trying to figure mine out today.

thanks, Randall

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Re: 1966 D100 tail light problem

Post by gtxgts1969 »

I built a completely new harness from firewall to tail lights and grounded tail lights to frame and works like a champ. It's a pain but i know it's all new and dependable.

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Re: 1966 D100 tail light problem

Post by RoukoSadra »

I took the old wire harness on my truck (only three wires and they were weather cracked) the blue wire gas gauge was ran through the cab somehow...? So I got a 10ft. 4wire harness and ran the brown(running lights), green(R/turn), yellow (L/turn) and the ground (white) from the firewall to the lights on the rear of my truck and they work like it was new.

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Re: 1966 D100 tail light problem

Post by marklein »

The best way I have found to correct lighting issues on this old stuff is to solder a ground wire to the light housing and run it directly to the frame. Fixes them right up along with scotchbriting and dielectric grease on the bulbs. I went so far on my 70 as to putting 3157 pigtails in the front signal housings by making the holes bigger, they have their own ground wire and solve a lot of issues, then added led bulbs and a grounded flasher to make them work.

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