Need help troubleshooting trailer light issues

I borrowed a trailer today and the owner of the trailer had it hooked to a Tacoma and the lights on the trailer work great (headlights on or off). Hook up to my Sport Trac and I get the following:

Truck Headlights off:
Trailer - brake lights work
Trailer - left or right blinker causes both trailer lights to flash

Truck Headlights on:
Trailer - nothing works

I purchased a tester for the flat connection and everything tested out okay with the headlights on or off.

I have to pull a load of furniture tomorrow evening but I don't know where to go next. Wiring looks good on the truck and test out okay. New wiring on the trailer that works on other vehicles but not hooked to the Sport Trac. Any ideas?

 

[thebrakeman]

If the 4-flat on the trailer works correctly with the 4-flat on Tacoma, but does not work with the 4-flat on your SportTrac, the issue with with the Ford. You say that everything test out OK, but you must be missing something. Did you add the wiring, or was it from the factory?

It sounds like a system that has tapped into the wrong running/signal wires. Can't think of why they would completely stop doing anything with the lights on. Stock or aftermarket, I think you need to start tracing wires.

 

[thebrakeman]

PS - Do not do you hauling without lights working tonight. If you have to, just use your buddy's Tacoma (and trailer), and let him use the Explorer. He'll have bragging rights until you get the wiring sorted out.

 

[Wimberley]

Thanks Brakeman. I'm going to take off work early and move during daylight hours. The 4 flat tester showed the LT, RT, Brake, and Running lights were all working properly regardless of headlights being on. I believe this is a factory install of the light wiring. If not, someone did a very good job. I purchased this truck used about 4 months ago.

 

[thebrakeman]

Were you testing with a multi-meter, or one of those actual trailer-simulator deals (with lights, etc)?

 

[Wimberley]

trailer-simulator

 

[thebrakeman]

IIRC, all SportTrac came with the HD towing package, including the 7/4-way combo connector. Is that the case...do you also have the 7-pin?

That's what I have, and I've noticed I have some issues when using my 4-way (probably corrosion from getting left open, but never used), but the 7-pin works fine. I found a cheap 7-to-4-way adaptor, and all is well.