On my 1999 Superduty the trailer battery charging wire does not charge. Have checked all the fuses and relays, all good. No visibly damaged wires. May have been this way for some time but never noticed it until I got a camper. Is this a commom issue?

 

I wouldn't say it's common but it's not surprising, your truck is 25 years old, who knows what that wiring harness has been through over the years.

I would start at the front of the truck, check to make sure you've got voltage at every fuse/relay for the power wire, and work your way to the back of the truck. Chances are you're going to find a rotted/broken wire somewhere due to age. It's possible that it's even inside the plug, the plug is NOT water tight usually, so the wires may have corroded off in there.

 

What do you mean by you "have checked all the fuses and relays, all good."

Only two fuses, and two relays, matter.

And if those two fuses and two relays are missing altogether, then how can they be checked? Especially and most likely the two relays that are located in a separate box all to themselves, and are NOT in the main fuse box. That's why it doesn't matter whether all of the other fuses or relays are good.

It would help others help you if you posted a photo of the relays that you checked, or indicate that you understand the exact location of the fuses and relays that enable the trailer battery charge circuit, since Ford never installed those fuses and relays in production, and instead put them in a bag in the glove box (if truck was optioned with trailer towing wiring provisions), leaving it up to the customer to install the relays and fuses. In 25 years time, that bag of relays and fuses might be MIA.

If you know the relays and fuses that I am talking about, then something else might be at issue. It is just that the most "common issue" related to this concern is that folks didn't realize that extra relays and fuses needed to be installed, and that Ford never installed them, even if Ford provided them in the glove box.

 

It will only have power with the key on. Does the relay and the fuse have power?