98 Expedition -trailer running lights fuse blows

[kdwolfe]

On my 98 expedition the trailer running light fuse blows only when I turn on my headlights. My brake lights and turn signals work fine. I hooked my trailer to another vehicle and the lights on the trailer work fine.
It's not a wiring problem on the trailer.
I checked the voltage at the trailer harness and it looks ok.
Any ideas???

[ctesla]

make sure up under the rear of the vehicle, where the splice (or plug-in connector) from tail lights to trailer power is grounded.
only other item might be if Ford has a 10-20amp fuse, and the other vehicle you tested the trailer on has a 30?
if so, try going up 5-10amps in fuse rating, but no more than 5-10, otherwise you might be smoking wires.
but it sounds like a faulty ground to chassis/body.

hope this helps

[kdwolfe]

Originally Posted by ctesla

make sure up under the rear of the vehicle, where the splice (or plug-in connector) from tail lights to trailer power is grounded.
only other item might be if Ford has a 10-20amp fuse, and the other vehicle you tested the trailer on has a 30?
if so, try going up 5-10amps in fuse rating, but no more than 5-10, otherwise you might be smoking wires.
but it sounds like a faulty ground to chassis/body.

hope this helps

Thanks for the suggestions. The fuse calls for 20amps. I tried 30amps and it blew. The ground looks good near the connector. There is no corrision or loose wires. (The blinkers and brake light work fine)
The light switch for the running lights and headlights feed this fuse.
Do you think it could be a bad switch? The headlights work fine.

[fleettech]

if it is blowing the fuse it is shorting to ground somewhere does it do it with a trailer hooked up?

[kdwolfe]

Originally Posted by fleettech

if it is blowing the fuse it is shorting to ground somewhere does it do it with a trailer hooked up?

No, Only when the trailer is hooked up. When I have my other vehicle hooked to the trailer everything works fine. I figured the trailer must be
ok.

The strange thing is that the turn signals and brake lights work fine, only the running lights blow a fuse. That's why I'm wondering if the switch could be bad?