I have on 02 FX4 with a factory towing package with 4-pin and 7-pin trailer connections. I've only pulled a trailer one time, about a year ago, using the 4-pin connection, and even though the wires were really torn up right behind the hitch end plug on the truck, all the trailer lights worked.

Things didn't work out so well today when I needed to tow another trailer. On the trailer I have no brake lights and when I turn on either turn signal, both turn signals flash. I know it isn't an issue with the trailer, because in addition checking all connections and wire continuity, I hooked-up another trailer and had the same issues.

I've checked the fuses and replaced the 4-pin plug (and removed the 7-pin while I was at it). It was suggested that I somehow connected the left and right brake/turn signal wires when I spliced-in the new plug. That wasn't an issue however because I soldered and shrink tubed all four connections. Just to make sure I pulled out the multi-tester and checked continuity across the left and right brake/turn signal contacts in the plug and got a positive continuity beep. Next I disconnected the harness at the frame rail by the spare tire and tried it again. There was no continuity in the trailer harness. So I started checking continuity at all the connections towards the front of the truck and got to the fuse box under the hood.

Apparently there is continuity of these two circuits beyond the fuses and fuse box. And this is truly where my knowledge ends. I don't know where the power sources come from that go into the fuse box, and I'm not even sure if that is the issue anyway.

Any suggestions are greatly appreciated. I'm worn out. Sorry for the long winded post.

Chris

 

Have you checked the bulbs on the trailer and your truck?

 

On the 4pin you should have a ground, running light, left blinker,and right blinker. If you switched one of the blinkers with the running lights, that could be the problem.

 

The trailer has brand new light fixtures. Bulbs on the truck are good but it shouldn't matter because they're wired independently of the truck lights.

 

I have now triple checked my connections in the hitch harness. Green to green, yellow to yellow, brown to brown, white to white.

 

At this point, I'd be ringing out the 4-wire, on the truck, with the trailer disconnected, to see what signals you really have there. One of the old, light-bulb type checkers would be handy, or an old, analog voltmeter.

First, ohm out the ground wire to be sure it's at ground. Then, with a blinker on, make sure you can see one of the wires pulsating with respect to the ground wire. Do the next one for the other blinker, then do the running lights. If all three check out, then the problem is in the trailer wires.

On the trailer, you should be able to apply 12V to each of the three leads (with respect to the ground line) and see that the correct trailer lights come on.

That is, when we can't figure out what's wrong, try to figure out what is working correctly so it can be ruled out. Then focus on what's left.

I hope you find this helpful. I'm afraid it comes across as condescending, but I assure you it's not intended that way.

Doug

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When things get hopelessly confusing it's time to remove what you have and start over again.
Testing each one as you go to be sure it's correct as well as inspecting for wire damage and crosses.
The auto stores have a connector tester you can plug in and see what you have.
When all is correct, to test trailer lights turn on the parking lights and 4 way flasher then walk around the trailer to see what you have.
Bulbs out,one circuit open, poor ground when some lights dim etc or all is ok.
Do this test every time you get ready to tow.
I can't tell how many times I follow trailers with no lights, one light out, no turn signal light etc.
No excuse for starting out without checking lights or repairing when known trouble is present.
Good luck.

 

So everything checks out when I test voltage on truck plug:

brake on = 12volts @ left turn and right turn contacts (yellow and green wires)
head lights on = 12volts @ tail light contact (brown wire)
left turn on = 11volts @ left turn contact (yellow wire)
right turn on = 11volts @ right turn contact (green wire)


Ohms is a different story:

brake on = beeps but no numeric value @ left turn and right turn contacts (yellow and green wires)
head lights on = beep but no numeric value @ tail light contact (brown wire) and readings of 150ohm @ left and right turn contacts
left turn on = 500ish ohm @ left turn contact (yellow wire) and 150ish ohm @ right turn contact (green wire)
right turn on = 500ish ohm @ right turn contact (green wire) and 150ish ohm @ left turn contact (yellow wire)


Can anyone make sense of this?

 

Checking resistances has no benefit, don't worry about it..
Doing so either shows an open, a short or good continuity depending on what your checking for.
If your sure you have everything correct at the plug on the truck, there is a problem with the trailer wiring.
Can't be anything else.
Don't forget that in a dual filament bulb that works as a tail and stop function each filament shares a common ground. (a bulb with two contacts plus it's shell)
When that ground has a problem it can show up as some funny actions.
Good luck.

 

The trailer doesn't have a ground wire, and I will be adding one. It just doesn't make sense though, that I noticed the same problems when I hooked up a different trailer, one that does have a ground.

I guess I will give it another shot tomorrow. I'm going to install a ground wire, and then test the trailer by taking the battery our of my truck. If it tests ok and I am still having problems with the truck side of the equation I suppose I can tap into the tail light wiring on the truck to power the trailer plug. Do you see any problems with that?

 

Sounds like a bad ground.......

 

I would not advise tapping into existing rear area wiring.
The flash rate will change, a problem on the trailer will take out the truck lighting, and the wire size is not large enough.
You already have an engineered system for the purpose.
If you must go around it, you would need to reproduce what you already have.
That requires a fused heavy lead from the battery to the rear area.
Then you need to wire in three relays that are driven from the truck wiring that will pass the new power through to the functions they are connected to.
One relay will power the tail lights the other two left and right turn, stop and 4way functions.
Better to get some local help to find what your missing.
If you ever want to Tow a trailer with electric brakes the new layout won't do for using a controller.
Good luck.

 

Can't check resistance on a powered circuit. Has to be completely off

Also checking resistance isn't a tell all test, you can have 0 resistance through a single strand, but it can't carry the voltage load.

You need to do voltage drop test to get a better idea of what's going on in the circuit, and help diagnose any issues

 

That's true, strands sever or burn apart inside the insulation, over time, - specially at the bends.

 

Well, problem solved! Lack of ground on the trailer was the kicker. The other trailer must have a bad ground too.

Thanks for all your help guys!

I learned a few good lessons out of all this:
*Like how resistance in auto circuits isn't the final word. (Sure am glad I spent all day yesterday tracking that positive continuity all the way into the auxiliary fuse box and then into the main fuse box and up to the blinker switch. UGH!)
*Or the importance of grounding
*And the importance of having plenty of cord length while rewiring trailer plugs