Brake lights won't work when headlights/running lights are on

[jvm]

I just installed a brand new light set and wiring harness on an old boat trailer. All lights work fine: turns, brakes, running lights, hazard lights. But when the running lights are on and you hit the brakes, the trailer lights go out instead of get brighter.

Towing with a 2000 Chevy Silverado pickup with a round factory plug and a round-to-flat plug adapter plugged in for the trailer harness flat plug to attach to. I grounded a white wire from the trailer harness to bare metal on the trailer and made sure the mounting bolts for the trailer lights were attached to clean bare metal on the trailer.

Note that the truck has separate amber lights for turn signals. Thanks for any help.

 

[alldodge]

Did the trailer lights ever work correctly on your truck prior to changing to new lights?

 

[mike_i]

I have a 2002 silverado and just replaced all lights and wiring on my trailer. Exactly which trailer lights go out when you hit the brakes?

 

[jvm]

AllDodge, the old lights did work OK at one time but they were ancient and rusted and I couldn't find lenses to replace the old ones (1 cracked & 1 missing) so I bought a new set of lights and wiring.

Mike_i, when you hit the brakes when the trailer running lights are off (truck headlights/running lights off), then the tail lights of the trailer light up.

When you hit the brakes when the trailer running lights are on (truck headlights on) instead of the tail lights of the trailer getting brighter, they both go out. I checked the bulbs on the new trailer tail lights and both filaments in each bulb are fine.

Thanks.

 

[alldodge]

Since it worked fine before I'm thinking you have the tail lights connected for brake and brake for tail lights. Check the color of your wires
Brown parking lights
Yellow left turn
Green right turn

 

[mike_i]

Is you trailer wired as in this diagram?

 

[bruceb58]

Bad ground. When the headlights are off, the brake lights use the filament of the running lights as a sneak path to ground. When the running lights are on, that sneak path disappears.

Take a jumper cable and attach to your vehicle. Then connect the other end to bare metal of your trailer and see if they work properly. If they do, you have a bad ground between the vehicle and the trailer. You are wiring the ground through your connector to the trailer right? Not relying on the hitch?

 

[lckstckn2smknbrls]

Is it a tilting trailer?

 

[jvm]

Hi all & thanks for the replies. The trailer lights are wired as AllDodge described & like the diagram that was posted by Mike_i. Yes it is a tilting trailer.

Bruceb58, yes we grounded a white wire from the connector harness to bare metal on the trailer. We tried a ground jumper wire from truck to trailer as you described, last fall but had the same problem with the brake lights. I think we'll take your advice and try again next weekend.

The Silverado is my brother's truck and it is very rusty. The factory trailer harness rusted away from the frame of the truck and was hanging by the wiring. He now has it held to the bumper with a cable tie so we'll have to try to clean some rust away and attach it better. If the mount of that factory harness is supposed to provide a ground, it is definitely not right now.

Thanks again.

 

[lckstckn2smknbrls]

Sometimes it's difficult to use the trailer frame as the ground when it's a tilting trailer. Try running a white ground wire to each light.

 

[josh_booth]

Had a ground wire go bad once on truck side, caused a similar problem. Ran a new ground wire to the plug on the truck and fixed the problem.

Also agree tilt trailers may need more grounding, maybe a heavy duty ground strap between main trailer frame and tilt frame at the hinge point would be easier than running to each light.

 

[bruceb58]

Also agree tilt trailers may need more grounding, maybe a heavy duty ground strap between main trailer frame and tilt frame at the hinge point would be easier than running to each light.

+++++1

 

[ahicks]

So, 2 filaments in each bulb. A bright one and a dim one. This makes it possible to wire the lights up using the wrong filaments. If the dim filament is wired to the brake/turn signal circuit, they'll work perfectly, but then be completely overwhelmed when the bright filaments are on - when you turn the running lights on. Simple fix, all you need to do is switch the bulb wires at each of the 2 lights.

 

[bruceb58]

So, 2 filaments in each bulb. A bright one and a dim one. This makes it possible to wire the lights up using the wrong filaments. If the dim filament is wired to the brake/turn signal circuit, they'll work perfectly, but then be completely overwhelmed when the bright filaments are on - when you turn the running lights on. Simple fix, all you need to do is switch the bulb wires at each of the 2 lights.

You missed the part where the lights completely go out when the brakes are applied.

 

[72fj40]

I think ahicks is on the right track, somewhat. The OP, rewired the trailer and now he has issues when the running lights on, he also states he had the same problem last yr, prior to rewiring, except it happened when the brakes were applied. The OP mentions, that the truck has separate yellow turn signals. When the turn signals are separate from the brake lights a trailer tail light converter is required, or the trailer has to have separate turn and brake lights. Using just a round to flat plug adaptor is not enuff to make the lights work correctly. He also states the original truck wiring is suspect too. He needs to confirm that the truck has a converter, if not he needs one. His symptoms are consistent with not having a converter, except usually it's the application brakes lights that stop the turns from blinking, so his rewiring is suspect too.

 

[72fj40]

At a minimum, the OP needs a continuity tester to id the pins in the round plug. Also, plug the round to flat adaptor and check the functions of flat plug pins.

 

[ahicks]

You missed the part where the lights completely go out when the brakes are applied.

My bad....

 

[bruceb58]

He needs to confirm that the truck has a converter, if not he needs one. His symptoms are consistent with not having a converter, except usually it's the application brakes lights that stop the turns from blinking, so his rewiring is suspect too.

Only problem is that a convertor has nothing to do with running lights. It only is used for brake and turn. His symptom is that when running are on, putting on the brake lights causes everything to go out.

With a Silverado, there likely isn't a convertor anyway. Would be interested in finding out what year the OP's Silverado is.

 

[jvm]

Thanks for the suggestions everyone. I will be heading to the lake tomorrow and will do further testing this weekend.

Note the truck is a 2000 Silverado.

I should have made it clear that we installed the new lights & wiring harness last fall & had this problem from the start. We gave up on it for the winter and now we're back testing again to figure it out. So we didn't have this problem prior to the new lights being installed.

Last fall we made sure all our grounds were OK and we did test all the points on the flat plug and round connector with one of the old lights and some jumper wires and everything worked as it should but the trailer hooked up still wouldn't work.

When I searched this forum before posting this topic, I saw a few mentions of a converter for a vehicle with separate amber turn signals but assumed his truck would have this converter since it has the factory trailering package?

 

[frobotz]

Sometimes it's difficult to use the trailer frame as the ground when it's a tilting trailer. Try running a white ground wire to each light.

Exactly my thoughts..