Tail Lights, Turn Signals, Brake Lights - Don't work when headlights are on....

[VegasRed]

Hey Everyone,
This is my first post, so I hope to add enough detail for thoughtful support. Also this is my first Bronco, 1970, about 8 months in and love every minute of it.

Scenario
Day Time Driving -
Brake Lights, Hazards, Turn Signals ect... All work fine.

Night Time -
When I turn on my headlights, all lighting comes on EXCEPT the tail lights. This also compounds as for some reason the Blinkers, Hazards, and Brake Lights all fail to work as well.

As soon as I turn the headlights off all the functionality comes back.

What could cause functionality to fail by turning on the headlights? I am not sure if there is a switch associated with the tail lights where this all inner connects or if perhaps there is a fuse that is also the central point of failure. Being a new owner, I thought asking here first before starting to dig through variables may be the best option.

I appreciate any and all feedback

 

[DirtDonk]

The only two things I can think of off-hand, are a bad headlight switch OR (and far more likely) a bad ground at one or more of the lights.
And not just the rear...

Has the Bronco been re-painted? Does it have rust anywhere? If so, either one or both can effect grounds. Negatively, pun intended.
Has it been re-wired, or is it rocking the original harness still?

Check these things:

1. Does the battery have a dedicated ground wire to the body?
2. Are the front and rear running lamp sockets clean, tight, and not damaged in any way at the points that the bulbs contact the sockets?
3. Do your front lights have dedicated grounding points to the radiator core support?

Check and clean all grounds for the lamps. For the rear, these points are the four holes (two per lamp) where the lights attach to the body. The housings themselves do not have to touch the body. In fact, they did not from the factory. There were small plastic-ish tab thingies (insulators) that were between the housings and the paint. The ground was through the screws.
There is a separate ground wire, but it's for the backup lamps only. It certainly helps the rest of the rear lights, but only a little to none. The backup lamps were an add-on after '66 and were insulated from the rest of the housing to keep water out. So they did not get a good ground (always) through the connection. Hence the separate ground wire.

The front running lamps got their ground through different points, depending on the year, and whether or not the housings have been replaced with aftermarket.
So just follow the wires and make sure that you clean the mating surfaces and tighten the screws.

In other words, detail and "blueprint" all of the lighting components to make sure they are working as expected.
If your lamp sockets have been replaced, there is better than an 80% chance they were wired backwards. Running lamps bright, brake and turn lamps dim. So you need to verify that each wire goes to the correct filament on the bulb.

Fun stuff, right?

Paul

 

[DirtDonk]

Oh, and welcome to classicbroncos!

Paul

 

[VegasRed]

Oh, and welcome to classicbroncos!

Paul

LOL Thanks for the indoctrination to the group. Sounds like I should start with the switch and then go through the wires and grounds. Thanks for the help and guidance!

 

[Viperwolf1]

Bad grounds at the rear light buckets.

 

[Steve83]

Sounds like I should start with the switch and then go through the wires and grounds.

No, the headlight switch has no effect on brake/turn/hazard lights; so you can go directly to the taillight grounds. Make sure the reflector stampings are electrically grounded to the body, or just pull a new return wire from each taillight (like a trailer) up to the main body ground near the battery (which also needs to be clean & tight).

You should put ALL your truck's details & history (as much as you know) into your signature(<-click that) so it shows with each post. Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location (nearest city) in your profile &/or signature. Not all of the truck's details are relevant to these issues, but you don't necessarily know which ones are relevant, so just put everything in now. The more pics you post (NOT in your sig) of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you.

 

[nutter3]

Yes, bad grounds. I just went through this with my 78 bronco. Then I compounded problems by putting in new bulb sockets, and wiring one wrong. I still have a problem with my reverse lights are always on when the key is on-which I believe to be either the NSS or a a direct short after the NSS.
To fix the problem with the headlights affecting the turn signal, I Cleaned and replaced my grounds, new light sockets at the rear, and added a ground from the frame to tailight housing area.