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GT/CS Taillight Harness I.D. and Restoration

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PNewitt

Guest
I thought it was important, with all the previous discussion about taillight wiring, to provide this diagram, and we could have a detailed discussion about how this works--and how to restore the original brightness "the original way". I think Steve (RV Trash) could/should chime in here, too.

Here is the basic way this harness works:

- There are two white plastic connectors that connect to the 14401 Under Dash Harness (just like one in a non-GT/CS '68 Mustang), that provide the power to the taillights. This harness also provides the power to the backup lights, and the license plate light. It runs along the rocker panels of your GT/CS, through the interior quarter panels, and come out into the trunk on the driver's side.

These connectors include two sets of wires (look at the far left of the drawing):
-#5 Orange with a blue stripe, for the RIGHT Brake and Turn Signal Lights

-#9, Green with an orange stripe, for the LEFT Brake and Turn Signal Lights

As you look at these wires, note that they each are split into three parts, for the three (1157) Lamps. A non-GT/CS '68 Mustang uses only one wire here, not split into three separate wires. The added amperage of two more 1157 bulbs can be handled by the gauge of the wire in this harness (higher amperage is provided by the HD Alternator and a bigger under-hood stock type relay).

-#14 Black is the "Parking Elements" wire, that is on when you turn on your parking or headlights. Note that this same wire powers up the license plate light at the same time.

-#140 Black with a red stripe, is ONLY there to power the backup lights, when you engage the shifter switch, whether auto or manual. This wire has nothing to do with the brightness quality of your taillights.

GROUND WIRES : Just right of the Lic. Plate light diagram, you'll see "GND" with three little lines. This indicates the ground wire for the backup lights harness. Next, note at the top of the diagram, the connectors with a wire that comes off the top, with the letters "GND". This indicates the ground wires that are part of the separate Thunderbird taillight harness used for the GT/CS. This harness includes the sockets for the bulbs, AND it has it's own ground wire (THIS can be the culprit for poor lamp brightness, since corrosion and "crud" can get in between the bolt and eyelet and the surface of the metal body that it is secured to).

If you follow the wires "57B", and "57C", you'll see how the ground wire for the backup lights, AND the ground wires off the T-Bird harnesses are common (57 A-B-C all work together, and are black). When you plug in the T-Bird harness to the main harness, you're providing the (negative) common ground for both the backup lights AND the taillights.

Note that the connectors, as drawn are placed like they're pages in a book. If you were to connect them, the left diagram would fold over to connect to the right diagram to match the contacts.

We'll get into the specifics of testing and contact problems next, but first, here is the identification of your harness.

Questions? Comments?

Next, we'll talk about wire gauge, and how to test the continuity of these wires.

Paul N.
 

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PNewitt

Guest
And here is the bulb I.D., so you know when you look into a socket, you know which pin is for either the brake/signal part of the 1157 bulb, and which pin is for the parking/headlight circuit.

Paul N.
 

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rvrtrash

Well-known member
Joined
Apr 25, 2003
Messages
3,649
Does this help?

http://www.californiaspecial.com/forums/showthread.php?t=4381

If anyone has any specific questions, it's probably easier to email me, 'cause I've been busy and only check in here every couple of days or so lately.

Steve

I should add, I bead blasted the insides of my buckets-sanding would accomplish the same thing only slower- and then painted them with "Chrome" paint. It really makes the bulbs reflect and appear brighter than they are.
 
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PNewitt

Guest
I went to the site that Steve referred us to, and I'll add some things; some of which Steve has already stated (and are paraphrased (I hope this is with Steve's blessing)). The diagram of the GT/CS taillight wiring is my own, drawn for my book (of my own GT/CS's wiring), back in 1989, and not a Ford diagram, but thanks for the compliment. This diagram and the info here will be included in my upcoming book.

IF you are having poor light levels with your taillights, there are some things you can do. We'll assume that your alternator and voltage regulator are in good shape, right down to the proper tension on the alternator pulley belt, and that the fuses for these items are in good shape, and the fuse contacts are clean, too. I recommend that you get a copy of the Ford Shop Manual for the 1968 Mustang; the size of a phone book (see the pages in Group 15). It's worth it's weight in gold.

Let's start with the under-dash wiring, and if the taillight wiring harness has any problems, before we go any farther. At this point, you won't need to dissassemble your taillight assembly (yet). You'll need some wiring saavy to get into this, and if you don't, you can explain this to your local auto electrical repair person. Before you begin, I recommend that you first properly & safely disconnect the battery.

On the driver's side, attached to the 14401 Under Dash Harness, are two milky white plugs, which are shown at the far left in my diagram. Disconnect these plugs, and inspect them for any corrosion, dirt, oxidation, etc. You'll need a Multi-meter that you can get at a home store for about $15. Learn how it works, and use the continuity check, which will let you know if the wires will properly carry power from one end to the other. As a first check, you'll need to unplug the T-Bird taillight harnesses (those big black plugs in the trunk), to test. You'll need to get some insulated wire (about 18-20 gauge) to attach to & extend one of the test leads of the Multi-meter to do this check from the dash to the trunk (or for those so inclined, remove the rear seatback to run the test wires back there). By using my diagram as a reference, check the first two wires that directly go to the taillights (remember, you're testing the half of the milky white plugs that are part of the taillight harness, NOT the dash harness side):

Test the #5, Orange with the blue stripe wire--with the three sockets (1-2-3) of the RIGHT (passenger) side taillight harness plug. This involves one test probe of the meter placed on the #5 socket pin at the dash end (milky white plug), and the other test probe (extended with wire) on each of the three contacts in the trunk plug. If the meter needle moves to indicate full continuity, you're OK. If the needle it isn't quite moving as far as it should, you have a resistance problem (which we'll get to later).

Next, do the same test for the #9 Green w/orange stripe wire, and the left side socket on the driver's side.

Then, on the second under-dash milky-white plug's wires, do a continuity check on the #14 Black pin along with the "fourth" pin on each of the black plugs, AND with the plug of the license plate light (which you'll have to remove to get to). This wire is the one that lights up your taillights (and lic. plate lamp) when you turn on the headlight switch to the parking and headlights position. Presumably, a bad licence plate lamp could dim the right taillight, since they're all wired in series on the #14 wire.

The second wire of the second white plug under the dash is #140, black with a red stripe. This is the ground wire. Test one probe at the dash end (milky white plug), and then place the other probe on any clean unpainted metal part of the car, such as the floor. You should get a good reading. If not, try the second probe right on the ground eyelet that is screwed onto the trunk floor, bumper, or right behind the filler neck in the trunk. I would then test one probe on the eyelet, and the other on the body metal next to it, to see if the ground wire is clean, and is making a good contact. If you wish to go farther, test the first probe on #140 pin (under the dash), and the second probe on the potmetal reflector housing to see how well the continuity is there.

A weak spot with this setup is that T-bird harness attaches the ground wire to the taillight bezel, which means that the bulb sockets' grounding is dependant on how well they make contact to the pot metal reflectors, since they are pressed into place. Everything from the ground wire, to how the (3) sockets contact the reflector must be clean and free of oxidation and "crud" to allow the lights to work as bright as they should when they were new.

IF you're getting a bad reading in the continuity, it's either some oxidation on the plug's pins and sockets; OR a frayed wire somewhere along the taillight harness that is touching the metal of the car, grounding out. I've seen this happen a lot. The constant rubbing of the insulation in tight spaces for over 20-30 years or more wears down the insulation. You might need to pull out the whole harness from the dash to the trunk, and inspect it closely. I've seen exposed wires, still inside the web mesh sheath worn by years of use, exposure, etc.

More solutions to follow.

Paul N.
 

GTCSMustang

Well-known member
Joined
Mar 16, 2003
Messages
719
Paul,
Did you ever check with Ford to see if they had a wiring diagram for the GT/CS taillight harness?
Scott
 
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PNewitt

Guest
Paul,
Did you ever check with Ford to see if they had a wiring diagram for the GT/CS taillight harness?
Scott

Well...yeah..they did, that is, Shelby Automotive did, and I have a copy of that blueprint. However, it's in electrical "shorthand", that no one, 'cept for an electrical engineer could understand. Just plugs with parentheses and numbers and lines. So--I did my own version to help folks out. I'm sure a Shelby version would be helpful to '67-'68 owners, too.
 
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PNewitt

Guest
First, to answer GTBandit's question, yes, wires 5 and 9 come from the turn signal switch in the steering column. Refer to the Ford Shop Manual (for the '68 Mustang), and look at page 19-27 at the back of the book. What that does, is when you stop, the flasher interrupts the brake (filiment) signal to flash, even though they are on the same wire (either #5 or #9).

In part three of this procedure to get better light out of your taillights, I'll walk you though some more options. This is something that needs photos to go with it, but those that understand doing these types of things--here it is:

Previously, I commented on if you have a frayed wire, and/or poor connectors that they need to be fixed. (as per Steve:) You would repair any breaks in the wires using the same soldering techniques. If you have a bare wire, I prefer to wrap it with elec. tape, melt the tape and seal with silicone. When you are all finished, check each of your contacts with a digital volt meter or ohm meter. (as per Paul) I would use heat shrink tubing, but it isn't always easy to put in place with other wires and connectors in the way.

Here, I'll talk about how to clean and fix the harness from the dash end to the other.

After all the many miles and time, it would be a good idea to give all the contacts of your connectors a good cleaning. I know it would be a week-long project, so, we'll concentrate on the taillight harness (no. 14405) only. You'll need some small wire brushes, which I recommend the Dremel type. See: http://www.mytoolstore.com/dremel/brushes.html

Using a new "Stylus" rechargable Dremel Tool might be a good investment. You can also use some electrical contact cleaner, like WD-40 (warning: it's flammable), or TV Tuner contact cleaner. Carefully pull apart the plugs, not breaking the clips on the sides. Spray a bit of cleaner, and then use the wire wheel. Wipe the excess off. Before attaching the plugs, apply some contact grease, Ford # F8AZ-19G208 (to reduce the possibility of moisture corrosion at the terminals) OR # ESA-M1C150-A ,or ask for the best electrical switch grease that Ford or Motorcraft offers at your local Ford dealer (This is good for the bulbs, too.)

If the whole harness is in good shape (no bare wires, etc.), proceed to the trunk area. Unlock the trunk, and you'll see where the bumper bolts are. Unbolt the bumper, and you'll have to unscrew the license plate light wire assembly. Also remove the gas cap. Then, you'll see the special screws (with washer-type heads) that run along the top of the fiberglass back panel--and the few screws that are below the taillights (behind where the bumper was). Remove them, too. With all the screws removed, carefully hold the whole back panel and remove it from the rear of the car. Make sure you've removed the rubber grommets that hold the T-Bird taillight harness plugs, which will need to come through the holes of the large black metal backing plate (a unique GT/CS and Shelby part).

With the assembly on the ground, if this has never been off the car, you'll see an amazing amount of road grime and dust on it (and wondering how anything could still work in there! LOL!). Keep track of the assembly with digital photos as you go along, so you have a reference to put it back together!!

Remove the potmetal T-Bird reflector assemblies, and take off the chrome bezels and lenses (keep track of all the screws in a zip lock bag). The bare reflectors will each have the three bulb sockets in place. Remove the bulbs, then remove the sockets--which may take some doing; but do not bend the sockets "out of round" with a wrench when doing this.

Make sure the contact springs are in good shape and the contacts move freely in the socket. If the socket needs cleaning, use an aerosol electrical contact cleaner, followed by light rubbing with very fine steel wool (or that Dremel brush) until the contact heads and socket body are shiny. If the socket needs replaced due to damage or excessive corrosion to the body, you will have to find a similar socket to replace it (I know that Mike Jewell knows that part number...<??> )

if you can't restore the original socket, aftermarket sockets seem to have a slightly smaller outside diameter (source: RV Trash; Steve), so to keep the socket tight in the tail light bucket and provide a proper ground, you will have to scuff the outside base of the new socket with sandpaper and apply a thin layer of solder. If you build the base of the socket up a little, you can file the solder down gradually until the socket fits snuggly into the bucket.

(as per Steve) If the contacts or contact springs are damaged but the socket body is fine, find a new socket with the correct type of contacts (I used NAPA #LS6228), remove the new contacts, springs and wire leads from the new socket and install them into your old socket. Whether you are replacing the entire socket or just the contacts, you will have to cut the wiring harness and splice in the new leads. However--you will need to get each wire of the new socket replaced in the exact same location as original, because if you don't, your lights won't work, and you'll be scratching your head....I recommend soldering and using a waterproof heat shrink tubing (available at electronics stores. It's heat shrink with glue inside of it.--PN). I'd avoid just using vinyl electrical tape, because it won't last as long as we'd like it to here.

(as per Steve) If soldering, I like to put the wires end to end and push them together, making a kind of braid where the individual strands are meshed with each other. Another method would be to lay one wire end on top of the other and twist them together, but either way, the wire should look slightly larger diameter when you are done, without anything sticking out at a 90 deg. angle. For those adept as soldering, heat the twisted wires together with the iron, then apply the solder, and allow it to flow into the wires. Let cool and slide your heat shrink tubing over the splice, and heat it with a heat gun (or use the shaft of the soldering iron).

(as per Steve) You would repair any breaks in the wires using the same soldering techniques. If you have a bare wire, I prefer to wrap it with elec. tape, melt the tape and seal with silicone. When you are all finished, check each of your contacts with a digital volt meter or ohm meter. You should have no more than .3 ohms of resistance between the contacts and the corresponding terminal in the (black) plug.

(as per Steve) Finally, remove the screw holding the ground wire to the tail light bucket, and scuff the terminal lug and mounting surface of the bucket with steel wool or sandpaper to remove oxidation. Replace the ground wire and screw and then check for .3 ohms or less between each socket body and the ground terminal in the six prong plug. If all sockets are higher, check the ground wire and it's connection to the bucket. If only one/some of the sockets are higher, you need to remove those sockets, clean the mounting surfaces and try again. When you are sure all readings are correct, make sure the bulbs will fit snugly in the sockets and measure between the ground terminal of the six prong plug and the other terminals. All reading should be the same as measuring just the bulb itself. Once you are sure everything is correct, reassemble the tail light panel in the reverse order and you are done.

So--this is basically a "ONE END TO THE OTHER" thorough cleaning of the taillight harness plug pins and sockets, and the bulb sockets, spring-loaded contacts. it's also a check of how well the sockets make contact to the potmetal housings. As Steve mentioned, your reflectors are probably poor in reflectivity--so have them bead blasted, and polished on the inside (or wet sanded with 600--then 1500-2000 grit sandpaper)--then spray on some high quality chrome paint. That alone will make a huge difference in how bright the lights are. Be sure to include some new housing gaskets for the fiberglass; and for the lenses, too.

Paul N. (with help from Steve) (right, Steve?)
 

rvrtrash

Well-known member
Joined
Apr 25, 2003
Messages
3,649
Just a few things. Using heat shink tubing is the best choice, but don't cut a wire just to use it. I use the tape method when I can't get heat shink tubing to where I need it. I wouldn't use WD-40 as a contact cleaner because of the residue it leaves (Yes, I know it works but it attracts dust and causes a grime buildup over time) and would really suggest the electrical contact cleaner. It's pretty cheap and dries clean. If you are going to use a dremel type tool, I wouldn't use the wire wheel but would use one of the felt pads instead. Even small wire wheels can cause damage. The contact grease is a good idea. That looks like it pretty much covers it.

Steve
 
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