trailer wiring problem

[pcdreams]

I've got a 78 c-20 pu I'm trying to get hooked up to a camp trailer. all the lights on the truck work. everything but the brake lights on the trailer work (when headlights are on). brake lights on trailer work when headlights are off.

Any ideas? I'm thinking I likely have a wire backward somewhere but I'm not sure.. thinking this truck might be one that needs one of those converter things.

[Bearfootfarm]

Check your bulbs and make sure they have 2 elements each.
One should burn when the lights are on, and the 2nd is for the brake lights

[Hooligan]

Do the trailer running lights go off when you hit the brakes while the truck lights are on?

How do the turn signals work?

Bad grounds do funny things. Try running a ground wire from the ground connection on the trailer side of the connector to an known good ground on the truck side and see what happens.

Went through this last summer with my boat trailer. Turned out to be the ground connection on the truck frame.

In my case, with the truck lights off the brake lights worked and both turn signals and the taillights blinked all at the same time when the turn signals were turned on.

With the truck lights on the trailer running lights worked but went dark when the brakes were applied. The turn signals acted the same as in the first case.

[watcher]

I agree with Hooligan, start by making sure you have a good ground. After that its just following the wires to make sure everything is correct. A good tester is worth its weight in gold when trying to chase down wiring gremlins.

[wyld thang]

third ditto on the ground issue!!!

[fordy]

Originally Posted by pcdreams

I've got a 78 c-20 pu I'm trying to get hooked up to a camp trailer. all the lights on the truck work. everything but the brake lights on the trailer work (when headlights are on). brake lights on trailer work when headlights are off.

Any ideas? I'm thinking I likely have a wire backward somewhere but I'm not sure.. thinking this truck might be one that needs one of those converter things.

.................The brake lights and turn signals use the same extra bright bulb element , the brake lights is the same as both turn signals being activated at the same time . On your truck , there will probably be a ground wire just for the headlights somewhere on the radiator support , so it sounds like the tail lights are grounding back thru the headlights . Were it me , I'd run a 14 gauge dedicated ground wire from the front of the truck all the way back and tie it in too each trailer tail light . This way you will always have a complete circuit pathway with very low resistance . , fordy

[Hooligan]

Originally Posted by fordy

.................The brake lights and turn signals use the same extra bright bulb element , the brake lights is the same as both turn signals being activated at the same time . On your truck , there will probably be a ground wire just for the headlights somewhere on the radiator support , so it sounds like the tail lights are grounding back thru the headlights . Were it me , I'd run a 14 gauge dedicated ground wire from the front of the truck all the way back and tie it in too each trailer tail light . This way you will always have a complete circuit pathway with very low resistance . , fordy

I should have added to my post that this is more or less what I did too. I made sure I had a good ground connection from the trailer tongue to the truck frame and ran a ground wire to connect all four light fixtures. This is much better then relying on the contact of the fxtures to the trailer frame to provide the ground.

[pcdreams]

[QUOTE=Hooligan;4486128]Do the trailer running lights go off when you hit the brakes while the truck lights are on?

If the truck lights are on the the tail lights on the trailer work ok. turn signals (either side) both flash (on truck as well)

brakes.. all lights on rear of trailer go out

with the lights off on the truck everything works fine.

Ground is something to look at. I did put a new ground in to each light from frame straight up to light on both sides. but I did notice there is not a ground from truck to trailer..

someplace to start anyway

[CIW]

To save yourself some trial and error. Run one side of a set of jumper cables connecting it from your paickup frame to your trailer frame. See if that solves your problem. If it does there is one connection on a 7 blade trailer plug that can be dedicated for a frame ground. Even if it doesn't you should have it hooked up.
It can be found using a continuity (ohm) setting on your volt meter and testing to ground. If you get more than one wire showing continuity to ground, then you know that you have a frame short.

[arabian knight]

What year vehicle are we talking about?
Do you have Separate Turn signals? Meaning are the turn signals on the vehicle Amber?
Or are Both Turn and Tail lights in the same (red) light fixture on the vehicle?
It does make a difference in how you wire the trailer if Amber is your turn signal on the towing vehicle.
If it is a Amber Bulb just for the turn signal, then the Brake and Tail light wires are in the Red Inclosure.
If Amber is a separate light area for turn signals, it would then take 4 wires back to the trailer wiring. Not 3
One for Left turn, one for right turn, One for the Brake Lights , One for the Tail Lights.
There are adapters made, just for this purpose.

[pcdreams]

Originally Posted by arabian knight

What year vehicle are we talking about?
Do you have Separate Turn signals? Meaning are the turn signals on the vehicle Amber?
Or are Both Turn and Tail lights in the same (red) light fixture on the vehicle?
It does make a difference in how you wire the trailer if Amber is your turn signal on the towing vehicle.
If it is a Amber Bulb just for the turn signal, then the Brake and Tail light wires are in the Red Inclosure. It would then take 4 wires back to the trailer wiring. Not 3
One for Left turn, one for right turn, One for Brake LIghts , One for the Tail Lights.
There are adapters made, just for this purpose.

78 chevy c-20
both are red on the truck. I've got 3 wires tail/rt/lt

[arabian knight]

Put a Heavy Duty Flasher in for your blinkers~! Standard duty flashers will not handle the extra currant that the trailer lights will draw.

[Wis Bang 2]

Some trailer tail lights can be wired backwards and feed back thru the markers when the brake/turn is wired to the tail light, I had this problem on a boat trailer that was acting like your description.

A 78 pickup won't need a converter...that is for a truck w/ amber turn signals separate from the brake lamps.

[arabian knight]

Originally Posted by Wis Bang 2

Some trailer tail lights can be wired backwards and feed back thru the markers when the brake/turn is wired to the tail light, I had this problem on a boat trailer that was acting like your description.

A 78 pickup won't need a converter...that is for a truck w/ amber turn signals separate from the brake lamps.

True but a weak or standard duty flasher sure will mess with how the lights look and act.

[pcdreams]

Originally Posted by Wis Bang 2

Some trailer tail lights can be wired backwards and feed back thru the markers when the brake/turn is wired to the tail light, I had this problem on a boat trailer that was acting like your description.

hmm. Well I know originally the clearance lights were tied into the tail lights.. wonder if I should run a separate circuity for them..

[plowjockey]

Originally Posted by pcdreams

thinking this truck might be one that needs one of those converter things.

The only vehicles I have ever seen that need converters, are Japanese vehicles or Japanese heritage vehicles, like the Ford Ranger.

78 Chevy should be good to go.

Sound's like you have a short somewhere.

I have gotten so frustrated troubleshooting old trailer wires and light sockets, that I'd just spend the $15 for a new set of lights (if the just the basics), rip out the old lights/harness and replace with all new. Money well spent.

[fordy]

[QUOTE=pcdreams;4487712]

Originally Posted by Hooligan

Do the trailer running lights go off when you hit the brakes while the truck lights are on?

If the truck lights are on the the tail lights on the trailer work ok. turn signals (either side) both flash (on truck as well)

brakes.. all lights on rear of trailer go out

with the lights off on the truck everything works fine.

Ground is something to look at. I did put a new ground in to each light from frame straight up to light on both sides. but I did notice there is not a ground from truck to trailer..

someplace to start anyway

..................Actually , the metal to metal contact between the ball and the trailer coupler is a ...Ground.....on some trailers it works OK , on others there is no conductivity , at all ! , fordy

[pcdreams]

thanks guys. I ran a ground to the frame from the truck and everything is working right.. Lots brighter to boot.

Thanks.