Brake Lights

[lwestbro]

This may be a stupid question. When I apply the brakes, the brake lights come on. When I turn the headlights on, no brake lights. Any idea?

[Libero]

Was everything working OK and this situation just started without any changing circumstance?

Do you have 5 to 4 wire converter for your hitch connector circuit?

Do your signal lights work?

[lwestbro]

It's been going on for some time, just now got my attention.

Don't know, I'll look.

Signals work. Brake lights work with headlights off. Turn headlights on, rear lights work, just no brake lights.

[smlranger]

Check the ground/s. Faulty grounds can make lights do weird things.

[Libero]

You said signal lights are working OK but not the brake lights when the tail lights are turned on. This would imply your signal lights are on a seperate circuit from your brake lights. Usually signal lights are amber if the lights are seperate and this would be a 5 wire system.

If you do not have seperate signal lights from brake lights , then the same filliments are most often used for signal and brake lights but seperate from the tail light filiament and often there is a third single mounted brake light. Do you have one of these seperate brake lights and does that light work OK?

If your toad or trailer are 4 wire, then you would have 5 to 4 converter just before the rear receptical. These converters can go bad and really mess up the lighting system. I have those converters go bad twice (on different vehicles,) This converter module is the only place where tail, signal and brake lights join in a single component. But without knowing more specifics, I cannot say for certain this is the culprit.

Someone noted a ground as a possible cause, but it has been my observation that grounds are picked up per light assembly so all bulbs in that assembly would be effected so while the problem could be a bad ground it would have to be in several places at once thus has a higher probability of that not being the cause if all 2 or 3 brake lights are not coming on when the head and tail lights are turned on.

Having said that, I would check grounds anyway.

[FIRE UP]

Originally Posted by smlranger

Check the ground/s. Faulty grounds can make lights do weird things.

I'd have to agree 100% here. We've all seen what happens to tail lights when a bad ground is present. Turn the tail lights on then, put a turn signal on and, watch the tail light go out when the turn signal flashes or, all the lights on the rear dim when the turn signal is used and more. So, I'd definitely do a ground test before going anywhere else.

It's not hard to do the test. Just clamp a wire onto a known good ground and run it to the tail light housing or, the ground on the bulb if you can access it. Then, apply the brake lights again both with the tail lights on and off. Let us know what the results are so that our assumptions prove good or, no good.
Scott

[Sammie]

Originally Posted by lwestbro

This may be a stupid question. When I apply the brakes, the brake lights come on. When I turn the headlights on, no brake lights. Any idea?

lwestbro,

I had a similar problem several years ago on my 2000 Winnebago Adventurer. I can't remember what exactly the problem was, but the cure ended up being a shorted wire in the trailer plug connection on the rear of my motorhome. Take the trailer plug apart and look for a stray wire touching another wire. It only takes one strand of stray wire to screw things up.

Sammie

[cardiffman]

I have wired a few cars over the 50 years of RVing but the quickest, least troublesome was the most recent. I bought a new GMC Terrain this year and after looking things over, I decided to bypass all of the factory wiring.

I bought a couple of GM type light sockets at the local auto store and installed them by drilling holes in the rear lamp housings. By running the wires underneath attached by wire ties, the wiring was a snap. Of course you have to run an independent ground wire along but that is no problem.

Good luck.

[FIRE UP]

Originally Posted by cardiffman

I have wired a few cars over the 50 years of RVing but the quickest, least troublesome was the most recent. I bought a new GMC Terrain this year and after looking things over, I decided to bypass all of the factory wiring.

I bought a couple of GM type light sockets at the local auto store and installed them by drilling holes in the rear lamp housings. By running the wires underneath attached by wire ties, the wiring was a snap. Of course you have to run an independent ground wire along but that is no problem.

Good luck.

cardiffman,
Just wondering Sir, how this ties into the OPs problem? He really didn't mention anything about a toad. He's got a problem solely on his RV. It could be a ground problem or, as others have stated, it could be a single strand of wire touching a point it's not supposed to in the trailer plug. Untill he gets back to us with the remedy, we'll not know what it is.
Scott

[cardiffman]

Woops! I mistook this web site as a source of information sharing, not nit-picking. I'm outta here.

[OldChief7155]

If they use the same bulb they should have dual filaments. Maybe somebody replaced them with the wrong bulbs.

[FIRE UP]

OldChief,
I don't have much experience with today's newer based bulbs but, in the old days, a "Stop/turn/tail" bulb was termed, an 1157. That bulb has two contacts on the bottom and, a 15 mm base and off-set pins so the bulb cannot be installed incorrectly.

The bulbs used for "back-up light" bulbs were known as, 1156 bulbs. Those had the same size, 15 mm base but, the pins were parallel and, there was/is a single contact in the bottom. Now, for the most part, you couldn't install those two bulbs in the wrong sockets. Now there's always some "gorilla grip" strong guy that will force them in, whether they're supposed to go or not.

But, today's newer "wedge base" or push in style are totally different. As stated, I've not had too much experience with them. The OPs got what's most likely a ground problem. A weak ground may work for one application of lights but, add another application, compounding the load, and the ground goes nuts or, flat out disappears. So, as others have stated, I'd start out by making sure all grounds are good and clean.
Scott