I figured I had a loose connection where the trailer harness hooks up to the truck. I checked last night in the dark by shaking and wiggling everything at the harness...and it definitely is not that. I did notice however that the jarring made while closing the truck door will cause the trailer lights to flash. So....it seems there's a loose connection closer to the door of the truck. I'm not certain but it sure looks like the trailer wiring is factory.

There are no issues with the lights on the truck.

I have about 96,000 miles on the truck. It's a 1995 F150 XLT, 4x4 Super Cab, 5.8L, auto trans.

Are there any common problem areas with the wiring in these trucks that could cause my trailer lights to flash when hitting bumps in the road like this?

 

May have a look at the ground wire and related connections.

When this has a problem, lights will usually still kinda work at a standstill, with the grounding going through the trailer hitch which does not provide a consistent, low-impedence electrical connection.

 

Thanks! Makes sense. Is there a usual spot for the ground in these cases...or am I going to have to trace it from the hitch forward?

 

No idea how the ground wire is routed or where it ends up on the truck.

Unless this flashing occurs across multiple trailers, suggest chances are better than average, although not guaranteed, that the problem is somewhere from the harness connector back to the trailer - have seen the ground wire pinched along its run, and even intentionally cut off at the connector.

Perhaps with the trailer unhitched and harness unplugged, use an ohmmeter or continuity tester between the respective harness pin (white wire, if I recall the color codes for 4-wire correctly ??) and a good grounding point on the truck and then on the trailer, with appropriate wire wiggling and frame jarring / banging / bouncing to simulate road conditions to see what you've got as far as a stable reading or chirping on the meter, hoping something shows as to which direction you need to be looking - towards the truck or the trailer.

My experience with this problem has usually been that there will be no continuity from the trailer pin to the frame, confirming the ground is occurring only through the hitch connection, so may test the trailer first in the interests of time.

Good luck and keep us posted.

 

You didn't describe or show the bumps, so we can't know what "like this" is.Did you shake & wiggle it as violently as the bumps in the road do? Same amount of noise from the trailer suspension?Did you try thumping the trailer relays under the hood? They're near enough to the door for its jarring to vibrate them.If you had posted pics of it, we could give you more opinions.You should put ALL the truck's details & history (as much as you know) into your signature so it shows with each post, as this page explains:

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Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location in your profile & upload an avatar of the truck. "Auto trans" isn't specific enough - which model? And is the engine MAP or MAF? Which type of EGR system does it use - sonic, PFE, or DPFE? Which vapor system - CANP or VMV? You should also add the miles, mods, damage, repairs, tires, bed size, complete list of options... The more pics you post of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you.Yes, Ford published this TSB:

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I always run a separate ground to the trailer and never rely on the hitch-to-tow-vehicle ground.

 

All trailer connectors include a ground, as does the factory Ford trailer option. And most PnP aftermarket trailer wiring adapters for tow vehicles automatically connect the trailer ground wire to the vehicle's return wire circuit &/or ground.

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Sometimes there is not a ground in the trailer connector, and in these cases it works better to have a little pigtail to the tow vehicle for ground only. This is not very common any more, but still some small utility trailers are this way.