Trailer Lights
 

In the process of trouble shooting a trailer light problem I realized that the brake lights (without the trailer connected) on the vehicle do not work when the 4 way flashers are on? The dealer tells me this is normal - that the 3rd brake light (cab) is there for that purpose. So if I'm driving with the flashers on I have no brake lights?

Also - when the trailer is connected the brake lights (and flashers) on the trailer fail to operate when the parking lights are on, only when they are off?

This trailer/truck combo has been working properly for almost 5 years so I'm stumped. FYI - I ran separate ground wire to all lights on the trailer.

2007 f150 Lariat Crew Cab

I'm no automotive wiring guru, but I've learned that the first thing to do when having trailer wiring problems is to test the 7-pin RV plug on the back of the tow vehicle using a circuit tester. Here's my circuit tester:
https://www.etrailer.com/Accessories...nsha/6562.html


I often discover a fuse problem when using that circuit tester. If all circuits work as designed, then you know the problem is in the trailer wiring, not the truck wiring.


The brake light and the emergency flashers use the same bulb. So if the brake lights work, the flashers should work also, and vice versa. Ignore that idiot that said the CHML replaces the brake lights on the back of your pickup.



Makes sense. The brake light and the flasher light and the turn signal light is the same filament in the same bulb. The flashers override the brake pedal light switch, so when the flashers are on, the brake lights are already on, but flashing. The turn signals do not override the flashers either. When the flashers are on, that's both turn signals flashing at the same time. If the brake light switch overroad the flashers, then the flashers would not flash when the brake pedal was pressed.


On my F-150, the brake lights and the flashers and the CHML and the turn signals will all work without the engine running or the key on. I'll bet yours are designed the same way.

Hey Smokey, thanks for the reply. That's a nifty tester, if I had trailers with brakes I'd be using one. As it is I have a little flat four tester with LEDs for each light.

Per your comment:

If the brake light switch overroad the flashers, then the flashers would not flash when the brake pedal was pressed.

My understanding is that on some vehicles the brake lights DO override the flashers, but not the turn signals. The reason is as I mentioned: if you're driving with the 4 ways on then you still have brake lights.

The other issue - no 4 ways or brake lights on the trailer when parking lights are on has me stumped. Not sure where to go with that. The trailer works fine on my other vehicle.

Thanks!

Greg

from the Adirondack Mountains

I haven't checked my F-150 so can't say for sure how it works but I have older vehicles where the brake light function overrides the emergency flasher. This is done in the event you are driving the vehicles behind you know that you are slowing down or stopping.

In any event I think behavior is going to depend upon the trailer setup. For those where the turn signal shares the same lights as the brakes I would not expect the brakes to override the emergency signal. If it did it would result in the brake lights overriding the turn signal. I'm no trailer expert but believe the two bulb/filament setup is the most common one on trailers which is what acts this way. If it by chance had a three bulb (or filament) setup then I would expect it to allow the emergency flashers to be overridden by the brakes since they share the same bulb but the turn signal does not.

If you have flashers and brake lights, then the brake lights should go off when the 4 ways are on.
If you have integrated stop/turn/signal then your brake lights won't (or shouldn't anyway) override your 4 ways.
The same way your brake lights wouldn't (or shouldn't) override your turn signal if you were indicating a turn and braking.

As far as the brake lights/ turn signals not functioning when you're driving with hazards on -- why are you driving with hazards on in the first place? In most states it's illegal to do so, since the use of hazards it to indicate a stopped vehicle. If the vehicle is stopped, no need to have brake lights or turn signals override that.
That being said, some manufacturers do it differently. My old S10 would override the hazards with the brake lights. Some vehicles also have dedicated turn signal bulbs in the rear, as opposed to the F150's with the combined stop/turn lamps, much like every trailer out there.


For the trailer issue: The lights may have been wired such that the brake/turn signals are wired to the tail light wire (brown), and the tail lights are wired to the brake/turn wire. This would result in bright tail lights, and then dimmer turn/brake lights. Easy enough to check - Just plug it into your truck and see if it's brighter when you turn on your headlights or when you step on the brake.

Interstate highways REQUIRE vehicles traveling below 45 MPH to use hazard lights. Read the signs.

IF you read my complete posts I said the trailer works fine on my other vehicle - so the trailer side plug is not the issue. Also said that this truck/trailer combo has worked fine for 5 years and nothing has been re-wired. Parking and brake/turn signal wires are NOT reversed. The vehicle plug tests show a problem - something has changed in the vehicle.

Forgive me, but from where I'm from, the minimum speed limit sign doesn't say anything about using hazards less than the sign posted. If you're travelling below 45 on an interstate, you need to not be on the interstate.

You mentioned you have a 4 way tester. What are the results from testing using that?

Tyler - sorry if I sound testy about this - the 4 way tester shows that when I turn on the parking lights both the 4 ways and the brake lights stop working WITHOUT the trailer connected. The problem is on the truck side.

As far as driving with flashers on (and this will be my last comment about that!) you are absolutely correct - you should not be driving on any high speed limited access thorough fair at 45 or less... if you have a choice. Suppose I get on a highway and the next exit is 20-30 or more miles away (you must have those in OK) and 10 miles down the road I get a flat. So I put on my doughnut spare WITH A 45 MPH SPEED LIMIT and continue on to the next exit. Now a short distance later a deer runs out on the road and I need to slow down quickly - only the tailgater behind me (who's busy texting anyway) DOESN'T SEE MY BRAKE LIGHTS - BECAUSE I HAVE NONE. And the rest, as they say, is history.

I posted a question here so I might get some help finding the problem, not a critique on my driving skills or state traffic laws. BTW - in PA we have signs that require flashers below 45 mph - on Interstate 84, 80, 81, 78, etc.

Did this problem get resolved? I know this has been a while. but my 2007 just developed this issue.