Tail Light Challenges

[supplyguy]

Tail lights all worked fine before removing for painting. Upon reassembly I had one left tail light, no brake lights, no turn signals. Reworked the grounds and connections, and the right light worked but left stopped completely. REALLY cleaned the connections and added dialectic grease and now:

1) Right tail light assembly works fine despite NO ground to the car frame.
2) Left tail light assembly has brake light and turn signal but no tail light.
3) With flashers on, left and right light alternate - they do not blink in unison.
4) When left turn signal blinks, right tail light dims, so looks almost like alternating flashers.
5) Found a mystery black wire not in the bundle of wires into the trunk, capped and hot with lights on. No idea what this powers
6) Trunk light on the right works....found a ground wire with a female spade connector running to the left side, unconnected. For a 71...is this a normal ground wire for trunk light?

Appreciate any advice. Two days in the garage fiddling I am out of ideas and patience is wearing thin.

[wizard124]

Dielectric grease [edit]
Dielectric grease is electrically insulating and does not break down when high voltage is applied. It is often applied to electrical connectors, particularly those containing rubber gaskets, as a means of lubricating and sealing rubber portions of the connector without arcing.
A common use of dielectric grease is in high-voltage connections associated with gasoline engine spark plugs. The grease is applied to the rubber boot of the plug wire. This helps the rubber boot slide onto the ceramic insulator of the plug. The grease also acts to seal the rubber boot, while at the same time preventing the rubber from becoming stuck to the ceramic. Generally spark plugs are located in areas of high temperature, and the grease is formulated to withstand the temperature range expected. It can be applied to the actual contact as well, because the contact pressure is sufficient to penetrate the grease. Doing so on such high pressure contact surfaces has the advantage of sealing the contact area against corrosion.
Another common use of dielectric grease is on the rubber mating surfaces or gaskets of multi-pin electrical connectors used in automotive and marine engines. The grease again acts as a lubricant and a sealant on the nonconductive mating surfaces of the connector. It is not recommended to be applied to the actual electrical conductive contacts of the connector because it could interfere with the electrical signals passing through the connector in cases where the contact pressure is very low. Products designed as electronic connector lubricants, on the other hand, should be applied to such connector contacts and can dramatically extend their useful life. Polyphenyl Ether, rather than silicone grease, is the active ingredient in some such connector lubricants.

Above is from Wikipedia which defines dialectric grease as an insulator. Spray clean all the connections, double check all leads and grounds and see what you get.

[azruss]

all your light issues are symptoms of bad or no grounds. sounds like one of your stray wires is a ground. the one stray that is hot may be for your trunk light.

[seabeelt]

The mystery black wire, assuming its on the left side of the trunk is a ground wire. Try connecting that and see if it solves your problems. Sounds similar to an issue my son had with his tail lights. Ended up being that particular wire was loose.
R/

[toplessexpat]

I'd just finished cleaning the SUV yesterday morning, pulling out of the wash bay there was a yellow 1979 passing me, with some very interesting tail-light clusters. We exchanged a few words at the next couple of stop lights - he'd created them himself using round LED truck lights and what looked like a fibreglass frame. They were quite nice, albeit modern. A little italian-supercar like

[supplyguy]

Thanks for the info. Two minor points of clarification....mystery wire reads hot when lights are on and I probe with grounded circuit tester. Limited experience here made me think it was meant to power something, but everything I know of is powered through the harness and this is a stray. The trunk light works when I ground the separate unconnected line under the upper rear trunk....has a spade terminal but no visible partner ground spade on the frame.

[4uall]

these videos may help

https://www.youtube.com/watch?v=7YxW6bc_gk8

https://www.youtube.com/watch?v=EgwR9sYN5f0

[azruss]

If the stray black wire is supposed to be ground and is unconnected, it could easily read 12v, although i would expect it to read something more like 9 volts. think if it this way 12V+ from the battery to light bulbs to black wire. If you place the meter between chassis ground and black wire, you should read 12v minus the voltage drop from the resistance of the bulbs. If you want to safely test the theory, but an in-line fuse between the black wire and ground. If the black wire turns out to be a dead short, the fuse will blow and you wont cook your wire.

[supplyguy]

Jay - The videos made me giddy. That's the kind of fundamental stuff I needed. Had no idea that symbol on the wiring diagram meant a ground! It helps if forum members remember that some of us are really starting with limited experience. I have read many posts suggesting we noobs just get the wiring diagram, which I have, and yet sometimes that's not unlike suggesting the sick read Gray's anatomy to figure out what's ailing. Those two videos were a fine start. BTW - searched all over Advance Auto and Home Depot for star washers, unsuccessfully, until an old wise assistant at Home Depot got it - I wanted external lock washers! Never underestimate how new we can be.

Azruss - thanks for the tip. Had presumed since my circuit tester lit up it was a power line. Will experiment with fuses tonight.

Will give this one more shot tonight!

[supplyguy]

Surrender. Installed the inline fuse on the mystery line, touched the bare wire to metal, and fuse did not blow. It is also only powered when key is on. Shrug. Cleaned all grounds and connectors and added star washers and dialectic grease and all for naught. Sometimes I have brake lights sometimes I don't. Tomorrow the car goes to a professional. I have futzed arund way too long on this with no progress.

[azruss]

I just got finished getting my taillights working last night. If you have a black wire coming out of the harness from the front just before the taillight clip, that is a ground. Even hooked to the taillight bolt, it can still be bad. The issue is trying to get it tight enough for a decent connection. I moved mine to the stud holding the spring hinge bracket for the gas lid. Okay, on to the important stuff. I found i had poor continuity between the board and the bulb sockets and center connector. These are rivited to the board. You can test them by jumping across the circuit to bypass the rivit. Easy enough to follow on the board. I had to solder 4 joints on the left light and 3 on the right. The black wire in the center of the clip is a ground. All the other wires are 12V, so the bulb sockets are 12v+ and the center connector is ground. These can be a real headache chasing down. If your mechanic has no experience with these, he will chase the gremlins for hours on your dime. He will check the voltage at the clip and if all that is good, he will tell you to get new boards. Not a bad idea as mine looked very pristine, but had lots of issues. On my 80, with the key off, and light switch off, there is no power to the back. With the key off and the switch in the middle position, you should have taillights and side lights only. When the key is on, and switch is off, you should have brake lights, backup lights, and turn signals. With the key on and switch all the way down, you should have everything.

[seabeelt]

71s typically don't have boards. 79 and on have them I think.
Do you have access to a wiring diagram? If not, let me know and I will send you a copy of mine

Black wire coming from tail lights is a ground wire and should be attached from the lights to the body.

Light blue wire should run to the directionals - outermost light
Red wire goes to the stop lights - second light in from the outside
Yellow wire goes to the left rear running light and the right number plate light in the bumper
Yellow/black wire goes to the right rear running light and the left number plate light in the bumper and the trunk light
White wire goes to the back up lights- inner most light

Hope that helps
R/

[supplyguy]

Thanks for help. I rearranged some wires (hint - the colors in Italian on my tail lights do NOT correspond to the wires that ended up working) and got brake lights, one tail light, and reverse lights. One tail light wouldn't cooperate. Was turning the car in anyway and will have the mechanic doublecheck, and see if the stray power wire can be id'd.

[canadan]

Give up its easier. Was out jetskiing the last few days. Beats fixing fiats.

[spider2081]

I have been involved in a customer service business that involves electrical troubleshooting for years. It is very common for people to think what appears to be a single problem is actually caused by a combination of faults. Keeping an open mind to the fact that more than one thing can be causing a single appearing problem is important.