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Old 03-18-2023, 12:34 PM   #7
JRTJH
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Join Date: Mar 2011
Location: Gaylord
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On F150's around 2010-2015 (maybe even today's models), Ford included a plastic bag in the glove compartment with a fuse and a relay to "enable trailer battery charging capability"... Some dealers installed the parts and threw the bag away, some left the bag in the glove box and left it up to the owner to install them, some took the bag and put it in the parts department with intent to sell it to the owner if they wanted the parts... So, no idea what the status of your parts might be.

That said, if the fuse and relay are not installed, your truck will not charge a trailer battery.

Here's what to look for: Under the hood is a "high voltage power distribution box"... Open that box and check for a relay in space #9 and a fuse in space #21 (not sure on your truck if it's space 21A or 21B).

Part numbers for the 2013 F150:
Relay: 8L8T-14B192-AA
Fuse: 6E5T-14A094-BA And these may have changed for different year trucks. The fuse is a 30 amp and the relay is available at AutoZone and NAPA as well as any Ford dealer.

If those are present and working, with the truck running, you should have 12 volt power to the 7 pin umbilical connector on the truck. (measure for 12 VDC between pins 1 and 4) Here is a diagram of the trailer end and the truck end of the 7 pin umbilical. Some trucks have a "interlock" to prevent trailer power with the truck in park. So you may have to put your truck in gear with the engine running to get power to the umbilical connector.

The wire to the umbilical Pins 1 and 4 are 16 Ga, so you won't get a lot of amperage to the battery. If the battery is "nearly dead" then after towing an hour or so, you're still going to have a "uncharged trailer battery"... It isn't going to be a "high capacity charging system" but better described as a "efficient trickle charger"... All day towing (8-10 hours) may show a significant charge increase, but you'll hardly notice a measurable change after a 1-2 hour tow.
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