2021 F150 all factory trailer wiring. Last fall on final trip every now and then the trailer brakes would pulsate like ABS but not applying the brakes inside the truck. Can't do much in winter but spring has arrived. did have a chance to hook up another trailer and same thing happens.
Bought one of those cheap 7 pin trailer testers every light works brake light shows a small flicker - was told normal. However with it plugged in shows trailer not attached.
Next I bought some cheap trailer lights - still shows no trailer attached. Now I bought electric. brake magnets and temporary wired them up. My ideas was to temporary run a light bulb inside the cab connected to the brake magnets wire running from the 7 pin connector. if the bulb would flash - I could make changes and would know if I resolved the issue

All wired up -my fake trailer. Touchscreen I go to towing and make my trailer active - shows active. cluster shows cross traffic disabled trailer attached. Soon as I touch the gain control - no trailer attached message appears
What the heck?

Why all the trouble - it's a random issue. If I take it to a dealer i would bet they couldn't make trailer brakes pulsate - even me I could drive 40 miles with no issue then it will start acting up for 10 - 15 seconds and go away

 

If it was me, I'd lay on ground under rear of truck, unplug every single electrical connector, spray with electric cleaner, and reassemble. Even better to reassemble with some dielectric grease. My guess you have a poor connection. Check the ground back there as well. Good luck.

 

There is a Compliance Recall Notice 26C10 / NHTSA Recall 26V104 for the TRM (Trailer Module) for 21-26 models. www.ford.com/support/recalls That would be good to check into.

 

There is another trailer light recall that was just released for 21-26 F150s relating to non functioning trailer brakes and lights. Perhaps you need that recall completed.....

 

Was hoping they would allow the update to be OTA -

Have my fake trailer attached and get the message no trailer attached - but a second later a warning cross traffic assist not available trailer attached. Screen shows active trailer and i can adjust gain