Bad Ground....but, where?
 

I have a common problem I believe but didn't have time to fully investigate it last night. Hoping you guys can save me a lot of time.

American Auto Wire harness on a 71 C10. Everything is new in the way of lights and wires. Turn signals work without headlights on as they should. When I turn on headlights the side markers will still flash but the bumper lights will not. It appears to be a ground issue just not sure where.

One thing I did find is that I can twist the wires on the backside of the parking lights and if I get them in just the right position the turn signal works. Could both of the lights be bad (LMC specials)?

I did take one of the lights off the bumper and the low side continued to flash and the high side went dead when it lost its ground to the bumper.

Looking for ideas on where to begin to find the issue. Thanks in advance.

Re: Bad Ground....but, where?
 

To get you started, here's a thread that shows all of the stock ground locations: https://67-72chevytrucks.com/vboard/...d.php?t=422234

Re: Bad Ground....but, where?
 

Thanks 68. I have all of my ground straps in place.

Re: Bad Ground....but, where?
 

I have the same problem with one of my bumper marker/turn signal lights. It appears that the contact between the bulb and socket terminals/ground is wonky. I ordered a new one to see if it helps. Twisting the wires moves the contact pad relative to the bulb and makes it work....not a ground issue.

Re: Bad Ground....but, where?
 

IIRC there should be grounds on the engine side of the core support, very close to the headlight buckets, one on each side of the truck. The ground fittings comes out of my original wiring harness and merely bolts to the core support.

Again, IIRC, my side markers would not flash IF the parking lights were not hooked up to the wiring harness AND grounded at the bumper.

Re: Bad Ground....but, where?
 

That sounds more like your problem, now that I understand what you meant by twisting the wires together.

Re: Bad Ground....but, where?
 

Get you a long jumper wire that will reach from the neg post on your battery to anywhere on the truck with alligator clips both ends then you can test each problem area to see if it is in fact a ground.
If no place to hook the clip I push a small safety pin thru the ground wire and hook the clip to that.

Re: Bad Ground....but, where?
 

I think you are right and it is the light itself. Seems like the bulb socket is not grounding to the light housing. I will run a ground from the battery to the back of the light socket and see if that gives me what I'm after.

Re: Bad Ground....but, where?
 

Also, to test, remove the lens and wiggle the bulb/lamp. If all starts to work properly, then again it is not a ground issue but more a socket or bulb compatibility issue. Try a different bulb/brand. Try a bulb with a copper base in lieu of nickel (colored?).

I can pull my turn signal assembly connect it to a power supply and it behaves the same way. So, it cannot be a ground issue.

Incidentally, the side marker on the side that is giving me trouble does not flash.

Re: Bad Ground....but, where?
 

Messed with it some over my lunch hour and discovered that the low filament is flashing with the lights on. You just can't see it over the high filament burning. If I move the wires just right on the back of the light housing I can get them to work. When I let go they stop. It is something in the socket. I'll try a different bulb and see.

Re: Bad Ground....but, where?
 

Yep, that is how mine is behaving. As a temporary fix, I can position the bulb so it is almost about to let go of the retaining nubs and it stays working. So, it is not the wires, it is the socket. When you wiggle the wires, you are moving the contact plate in the socket.

Re: Bad Ground....but, where?
 

Did you purchase yours at LMC as well? Wondering if there is a fix short of ordering new ones from another vendor.

Re: Bad Ground....but, where?
 

I did as LMC was the only vendor that had it in stock. The part is due at my door this Friday, so I will test over weekend and report back. I am also going to try a copper based lamp.

Re: Bad Ground....but, where?
 

Found it!! Flipped the wires where they connect at the firewall and they worked. The LMC lights are opposite of what the AAW kit shows in the brown and blue wires.
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Re: Bad Ground....but, where?
 

Uhm, wait. So your problem was with the LMC replacement and not the original assembly? If so, dang, sorry. Good to know about the LMC assembly hitting my doorstep tomorrow.

Re: Bad Ground....but, where?
 

Yes. Discovered that the blue wire on the LMC part goes to brown on the AAW harness and not blue to blue. Flipped the wires on both sides and they work as expected.

Re: Bad Ground....but, where?
 

best advise right there.
RRM

Re: Bad Ground....but, where?
 

I agree with this.
I would also add, If one ground is bad, others are probably bad or weak.
I recommend systematically removing, cleaning and reinstalling all the grounds on the truck. Bonus points if you replace all the star washers and apply a dab of di-electric grease. Don't be afraid to replace wires or even add extra grounds (the more the merrier).
I have fixed all kinds of electrical gremlins, simply by redoing all the grounds.

Re: Bad Ground....but, where?
 

I can confirm that the LMC 71/72 front left turn signal is wired backwards. Probably the right is the same. I had to cut wires and swap before splicing back together. After doing so the assembly worked properly. Otherwise, the assembly is a nice reproduction.

Re: Bad Ground....but, where?
 

Dielectric grease is an electrical insulator, not a conductor.

Re: Bad Ground....but, where?
 

Yes, it is insulator. It insulates the connection from moisture and corrosion.
If the connection is solid, the grease gets pushed out of the contact points, especially when you are using a star washer.
It is not completely necessary but it helps to prevent corrosion, that's why I use it for grounds that are exposed to the elements.
I've been doing it for years, it works great. Opinions vary, I guess.

Re: Bad Ground....but, where?
 

I agree. I has to be non-conductive or it could short out electrical connectors.

Re: Bad Ground....but, where?
 

Glad you found the same thing and we have them working.

Re: Bad Ground....but, where?
 

Love that dielectric grease!